Effect of biochar and its combined fertilizers on the dynamics of soil nitrogen supply in tea plantation

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However, there is the potential for considerable nitrogen loss to occur. This study assesses the nitrogen retention of acidic tea plantation’s soil and the role of biochar in improving nitrogen dynamics, highlighting the need for innovative technologies to streamline and enhance nitrogen supply management. Methods: Adopting a modified two-week aerobic incubation and ion-exchange membrane technology, this research offers a novel approach to evaluate soil nitrogen supply and to monitor the nitrogen dynamics of tea plantation soil following early-summer supplementary fertilization. Results: The study revealed that the surface soil of tea plantation had the ability to provide 48 mg N·kg -1 soil as inorganic nitrogen for 130 days. The utilization of a small amount of biochar (10 t·ha -1 ) had no impact on the soil's effective nitrogen availability. Nonetheless, the application of biochar at rates of 20 and 30 t·ha -1 resulted in a significant enhancement in soil effective nitrogen availability as measured using ion exchange membranes, with an increase of 65%–81%. Furthermore, the utilization of biochar-based organic fertilizers, when used at appropriate rates, has the potential to enhance the availability of nitrogen in the soil, thereby increasing its effectiveness. Conclusion: The study's findings underscore the efficacy of the employed methodologies in capturing the nuanced impact of biochar on nitrogen retention and availability in tea plantation soils. The use of aerobic incubation and ion-exchange membrane technology has proven effective in elucidating the potential of biochar to significantly improve nitrogen dynamics. Biochar Nitrogen Dynamics Tea Plantation Soil Nitrogen Supply Nitrogen exposure Biochar-Based Organic Fertilizer. Figures Figure 1 Figure 2 Introduction Nitrogen (N) is a vital nutrient necessary for the growth of tea trees ( Camellia sinensis L.), since it has a direct role in crucial physiological processes including chlorophyll production, hence influencing plant growth and development (Li et al., 2016). Increased nitrogen supply has also been shown to increase amino acid concentrations and decrease polyphenol concentrations (Rebello et al., 2022 ). High rates of N fertilizer application have been shown to favour shoot growth over root growth and as a result increase the susceptibility of the crop to drought (Cheruiyot et al., 2009 ). Tea plants receive a substantial quantity of N fertilizer annually. The quality of green tea increases with increasing N rate, while the quality of black tea decreases with N additions greater than 200 kg N·ha -1 (Rebello et al., 2022 ). In China, for instance, the average quantity of N fertilizer that is applied to tea plantations is between 300 and 450 kg N·ha -1 , and the average annual harvest of tea leaves is around 800 kg·ha -1 (Chen & Lin, 2016 ; Xiao et al., 2018 ). The N concentration of tea is around 4–6% (Chen & Li, 2016), which means that only 30–50 kg N is removed in the harvested leaves and approximately 90% of the applied N fertilizer is either lost or retained in the soil. Tea plantations are commonly associated with acidic soils that receive considerable rainfall, which result in high potential for loss of nutrients (Yan et al., 2018 ). The primary kind of fertilizer utilized in tea plantations is compound fertilizer containing multiple nutrients, accounting for 70% of overall application. This compound fertilizer is enriched with various nutrients, and ammonium (NH 4 + ) serves as the principal N source (Venkatesan et al., 2004 ). In addition, soil N mineralization is a significant source of N supply for plants but is frequently not considered when calculating the amount of N fertilizer to apply. Biochar is a carbon-rich product produced by the transformation of organic materials through the pyrolysis process, which produces a product that has a porous structure, large specific surface area and strong adsorption capacity (Chen et al., 2019 ). The application of biochar in agricultural production has attracted much attention. Research has demonstrated that the use of biochar has the potential to improve soil physical and chemical characteristics, soil microbial communities, soil fertility, and stimulate crop development (Singh et al., 2022 ). It plays an important role in regulating soil N transformation, e.g., N mineralization and nitrification, via its significant cation exchange capacity (CEC) and adsorption capabilities, which contribute to the improved retention of organic matter and NH 4 + in soils. This enhanced retention subsequently leads to increased availability of NH 4 + for plant uptake while simultaneously lowering N losses (Gai et al., 2014 ; Prayogo et al., 2014 ). It is important to note that tea plants have a greater ability to take up NH 4 + relative to nitrate (NO 3 - , Ruan et al., 2007 ). The impact of biochar on the composition of soil microbial communities and the processes of N transformation, especially N mineralization, can be attributed, in part, to its ability to elevate soil pH and boost soil carbon content (Prayogo et al., 2014 ; Palansooriya et al., 2019 ). In a study conducted by Cen et al. ( 2021 ), it was shown that the utilization of biochar-based fertilizers resulted in a higher retention rate of NH 4 + in the soil, as compared to ammonium sulphate applied alone. Specifically, the retention rate increased from 41–66%. Soil N exposure expresses the presence of various N forms in soil over time (Burton et al., 2008b ; Burton & Zebarth, 2014 ). It quantifies the presence of soil mineral N (NH 4 + and/or NO 3 - ) over time compared to instantaneous measures of soil mineral N concentration (Burton & Zebarth, 2014 ). It reflects potential risks, particularly in the context of NO 3 - loss through N 2 O emissions. Previous studies reported that soil N exposure is correlated with cumulative N 2 O emissions, whereas instantaneous measures of soil mineral N seldom are (Burton et al., 2008a ; Burton et al., 2008b ; Maharjan & Venterea, 2013 ; Pelster et al., 2013 ). Nitrogen exposure can be calculated by integrating conventional soil sampling methods over time, but this approach is time-consuming, labour intensive, and discontinuous. The emergence of ion-exchange membrane technology addresses these limitations, offering a more efficient approach for the continuous measurement of soil N exposure (Harrison & Maynard, 2014 ; León Castro & Whalen, 2016 ). This innovative technique holds the promise of being more convenient and accurate, with potential widespread application in future research and practical applications. Soil N supply to the plant is a result of the combination of carryover of inorganic N from the previous growing season and the mineralization organic N over the growing season (Zebarth et al., 2005 ; Nyiraneza et al., 2012 ). Measuring or predicting the N mineralization of soil has proven to be challenging because it is driven by soil biological processes. Traditional methods for determining soil N mineralization potential involve chemical extractions or long-term aerobic incubation, these approaches have practical drawbacks, such as time consuming and the exclusion of biological factors (Luce et al., 2011 ; Ros et al., 2011 ). These measures of the N mineralization potential are generally well correlated (Sharifi et al., 2007a ; Sharifi et al., 2007b ; Dessureault-Rompré et al., 2010 ) and been applied to predicting growing season N mineralization (Dessureault-Rompré et al., 2012 ; Laurence et al., 2024). Biological N Availability (BNA), the mineralization of N occurring over a 14-day aerobic incubation, corresponds to the soil labile organic N pool, or pool I as defined by Sharifi et al. ( 2007b ) and is used to quantify N mineralization potential. Dessureault-Rompré et al. ( 2015 ) used BNA (Pool I) in combination with pedotransfer functions to reflect climate to predict growing season N mineralization. Despite the increased interest in biochar in agriculture, most studies have concentrated on common crops like corn, wheat, and vegetables. The main response variables measured have been crop growth and soil microbial communities (Vaccari et al., 2011 ; Yu et al., 2019 ; Ogura et al., 2021 ; Wang et al., 2023 ). The majority of soil-related biochar research has focused on soil improvement and soil remediation (Sohi et al., 2011; Zhu et al., 2017 ). Few studies have examined the impact of biochar on N cycling processes in the acidic soils of tea plantations, particularly soil N supply in these areas and particular the period following early-summer supplementary fertilization where N loss potential is high. Therefore, in this study, we monitored soil N mineralization potential and soil N dynamics using a 2-week aerobic incubation method and ion exchange membrane (IEM) technology to examine the influence of various biochar application rates and fertilizer combinations on soil N dynamics during this summer period. Materials and Methods Site Characteristics The research was conducted in a tea plantation located in Youxi County, China (26°2'35" N, 118°7'36" E). The study region experiences a humid subtropical climate (Köppen Cfa), which is distinguished by warm and humid summers, as well as moderate winters. Spring and summer are characterized by a significant amount of precipitation with an average annual precipitation of 1600 millimetres, accompanied by a frequency of 194 wet days over a year. The long-term average temperature in January is 9.3°C and in July is 27.9°C. The tea plantation primarily cultivates the Fuyun No. 6 variety. The soil in the area is classified as a Ultisol, characterized by a pH level of 3.8. All tea plants were grown in terraces at uniform heights using conventional cultivation practices. The spring rainy season spans from February to June, and results in approximately 60% of the total annual precipitation. The months of May and June exhibit the highest levels of precipitation, accounting for approximately 34% of the total annual rainfall. The period from July to September constitutes the summer storm and typhoon season, and results in approximately 27% of the total annual precipitation. Experimental Design and Treatments The research was set up in December 2022 as a completely randomized design, consisting of two parallel studies, encompassing a total of eight treatments. Each treatment was replicated four times. The first study, conducted from late April to late August 2023, examined the N status of soils in tea plantations as impacted level of biochar applied in December 2022. The experiment comprised of five treatments: no biochar addition (B00, control), 10 t biochar·ha -1 (B10), 20 t biochar·ha -1 (B20), 30 t biochar·ha -1 (B30), and 40 t biochar·ha -1 (B40). Synthetic fertilizer (22-9-9), containing ammonium nitrate (NH 4 NO 3 ), was applied in a rate of 165 kg N·ha -1 in December of 2022, as well as all biochar. The application of fertilizer and implementation of agricultural practices in the experimental area adhered to the conventional tea plantation management practices commonly employed in the local region. A second study examined the impact of various combinations of soil amendments on soil N dynamics beginning with the addition of supplemental fertilizer in late April 2023. Three treatments were considered: i . 70% synthetic fertilizers + 30% organic fertilizers (OF), ii . 70% synthetic fertilizers + 30% biochar-based organic fertilizer (BF), and iii . 70% synthetic fertilizers + biochar (Bio, at a rate of 7.5 t·ha -1 ). A cumulative amount of 495 kg N·ha -1 was applied throughout the year, comprising 148.5 kg N·ha -1 from organic or biochar-based organic fertilizer, and 346.5 kg N·ha -1 from synthetic fertilizers (split into 3 applications), containing NH 4 NO 3 . In accordance with the conventional tea plantation management practices in the local area, the application of organic fertilizers, biochar-based organic fertilizers, and biochar took place in December 2022. Synthetic fertilizer was applied in December 2022 and April 2023 in an equal rate of 115.5 kg N·ha -1 , with one more scheduled in late August 2023. All soil amendments were broadcasted in the alleyway close to the plants and not incorporated into the soil. The biochar used in the experiment was produced from maize stover (Henan Sanli New Energy Co., Ltd., China), and its properties were described by Hu et al. ( 2023 ). Briefly, dried maize stover was pyrolyzed at around 450°C for 4 hours, yielding biochar with 56.7% C, 3.2% H, 3.6% N, and at pH 8.9. The organic fertilizer utilized in this study was derived from sheep manure sourced from Wofuwo Fertilizer Industry Co., Ltd. in China. The manure had a N content of 0.98%, with a C:N ratio of 16.7. The synthetic fertilizer (22-9-9, Holitech Technology Co., Ltd., China) employs NH 4 NO 3 as its N source, H 2 PO 4 as P source, and KCl as K sources. The biochar-based organic fertilizer was formulated following the guidelines outlined in the Chinese standard [NY/T 3618 − 2020], where a ratio of 10:1 (w/w) organic fertilizer to biochar was employed during the mixing process. Biochar was not considered as a N source. Soil Sampling and the Determination of Soil Nitrogen Supply Samples were collected from two soil depths (0–20 & 20–40 cm) from each experimental plot prior to fertilization in April 2023. The soil samples were air-dried and sieved using a 2-mm sieve. The measurement of BNA was conducted using the methodology described for Pool I by Sharifi et al. (2007). A total of 30 grams of soil was combined with 30 grams of Ottawa sand and placed in a detachable 55 mm Büchner funnel. The funnel was equipped with glass fibre filter paper. The Büchner funnel, containing the soil sample mixture, was affixed onto a 500 mL evacuation flask and linked to a vacuum system. The soil samples were leached with 200 mL of a 0.05 M calcium chloride (CaCl 2 ) solution, and the leachate collected. Following leaching the soil samples, together with the detachable Büchner funnel's upper structure, were removed and sealed with parafilm at both ends. Subsequently, these samples were placed in an incubator set at a temperature of 25°C for a duration of 14 days. Following the completion of the incubation, the soil samples were reintroduced into the vacuum filtering system and leached again with 200 mL of a 0.05 M CaCl 2 solution. The concentration of NH 4 + -N and NO 3 - -N in the leachates was determined using the Skalar San + + CFA analyzer (Skalar Analytical B.V., Netherlands). The measurement of NH 4 + -N and NO 3 - -N concentrations in the first leachate was employed as an indicator to assess the quantity of mineral N contained in the soil at the time of sampling. The concentration of mineral N in the second leachate is as a result of N mineralization of organic N during the two-week period and is used as a measure of BNA (mg N·kg -1 soil). Soil N mineralization (N min , unit: mg N·kg -1 soil) was estimated for a 130-day (t) growing period, according to the equation described by Dessureault-Rompré et al. ( 2015 ): $${N}_{min}=(0.123*TN+0.00312*BNA)*t+BNA*\left(1-{e}^{-{k}_{L}*t}\right)$$ where TN was the soil total N content (g N·kg -1 soil) and have a mean value of 1.776 ± 0.157 g N·kg -1 soil at the research site, and the value of k L = 0.12 d -1 as suggested by Dessureault-Rompré et al. ( 2015 ). Monitoring Soil Inorganic Nitrogen Dynamic Using Ion Exchange Membranes The anion/cation exchange membrane (AEM/CEM) technique was used to monitor soil inorganic N fluxes during late April to mid-August 2023, as described by Nyiraneza et al. ( 2021 ). Membranes were cut to 6 × 5.2 cm 2 for AEMs and 7 × 4.5 cm 2 for CEMs and were then acid washed and regenerated by soaking and shaking in 0.5 M HCl solution for 30 min followed by 1 M NaCl solution for 2h prior to installation into the soil. Saturated membranes were inserted 10–15 cm into the soil and compacted to ensure full contact between soil and membrane. The membranes were buried in the soil for 1–3 weeks and were removed and replaced with a second membrane in the same position to ensure sequential collection from the same soil interface. This process was continued over the monitoring period to provide a continuous measure of NH 4 + and NO 3 - flux (µg N·cm -2 ·d -1 ). Following removal from the soil, membranes were returned to lab and shaken with 40 ml of 1 M KCl solution for 1h to extract NH 4 + and NO 3 - . The extracts were filtered using Whatman ® #5 filter paper (Whatman plc, United Kingdom) and stored frozen until subsequent analysis. The concentrations of NH 4 + -N and NO 3 - -N in the extracts were analyzed using a Skalar San + + CFA analyzer (Skalar Analytical B.V., Netherlands). The flux of NH 4 + -N to the CEM and NO 3 - -N to the AEM were expressed as µg NH 4 + -N/NO 3 - -N·cm -2 ·d -1 according to equation: $$N flux=\left(C*V\right)/\left(A*T\right)$$ where the C is the concertation of NH 4 + -N or NO 3 - -N in unit of µg N·ml -1 , the V is the volume of 1M KCl used to extract minerals N (ml), A is the area of the membrane (cm 2 ), and the T is the number of days the membrane remained in soil. The sum of NH 4 + -N flux and NO 3 - -N flux over a given period (111d, April 26 to August 15) were calculated and presented as NH 4 + -N exposure (AE) and NO 3 - -N exposure (NE), respectively. Statistical analysis The each of the two experiments was setup as a completely randomized design with four treatments and four replications. Data were presented as mean ± standard error. Minitab ® software (Minitab, LLC., U.S.) was used for statistical analysis. One-way analysis of variance (ANOVA) was used to test for significant influence of treatment. Post-hoc means comparisons were calculated when significant treatment effects were evident (p ≤ 0.05) using the Fisher's Least Significant Difference (LSD) method. Data were visualized by drawing graphs using OriginPro 2021 (OriginLab Corp., U.S.). Results Soil nitrogen mineralization and soil mineral nitrogen Nitrogen mineralization in tea plantation soils was assessed through a two-week aerobic incubation (Table 1 ). The findings revealed that when averaged across all treatments, the estimated N mineralization for 130-days in the surface (0–20 cm) soil was 47.95 ± 1.2 mg N·kg -1 soil, significantly greater that in the soil collected from 20–40 cm which averaged 44.08 ± 0.63 mg·kg -1 soil. In contrast to the soil at a depth of 20–40 cm, the surface soil demonstrated greater variability in N mineralization for 130-days. The estimated N mineralization in the surface soil ranged from 34.30 to 60.35 mg N·kg -1 , with a coefficient of variation (CV) of 14.56%. Conversely, the N mineralization in the 20–40 cm soil spanned from 38.03 to 51.79 mg N·kg -1 soil, with a CV of 8.54%. Nevertheless, both studies revealed a lack of statistically significant variance in the mean values across each treatment. The N mineralization for 130-days spanned from 44.05 to 49.52 mg N·kg -1 soil in response to varying levels of biochar addition, with no statistically significant differences observed (Table 2 ). Similarly, the combination of different soil amendments did not yield a significant difference in N mineralization, falling within the range of 48.25 to 51.03 mg N·kg -1 soil. Table 1 Estimated soil nitrogen mineralization for a 130-day growing period and soil mineral nitrogen prior to fertilization from 2 depths (Unit: mg N·kg -1 soil). Soil nitrogen mineralization for 130-days estimated using a two-week aerobic incubation. KCl extractable soil mineral nitrogen prior to fertilization – NH 4 + -N & NO 3 - -N. The average values presented in the table represent means ± standard error of the mean. Variables Depth Average Min Max CV N Min 130d ** 0–20 cm 47.95 ± 1.20 34.30 60.35 14.56 20–40 cm 44.08 ± 0.63 38.03 51.79 8.54 NH 4 + -N ** 0–20 cm 4.64 ± 0.22 2.49 8.63 26.84 20–40 cm 3.47 ± 0.17 1.77 6.59 28.80 NO 3 − -N n.s . 0–20 cm 10.60 ± 0.94 3.78 22.08 52.42 20–40 cm 9.17 ± 0.95 1.30 24.91 60.14 ** denotes significance at the at p-value ≤ 0.01 level n.s. indicates no significant difference between the two depths of the variable. Mineral N concentration was measured in experimental soil samples prior to fertilizer application (Table 1 ). The findings indicated that the NH 4 + -N concentration in the surface soil was 4.64 ± 0.22 mg N·kg -1 soil, a significantly greater value compared to the deeper soil (3.47 ± 0.17 mg N·kg -1 soil on average), representing an increase of 33.72%. The NO 3 - -N content of the surface soil (ave. 10.60 ± 0.94 mg N·kg -1 soil) was 2.28 times higher than that of NH 4 + -N, while the NO 3 - -N content of the deeper soil (ave. 9.16 ± 0.95 mg N·kg -1 soil) was slightly lower than that of the surface soil, but the difference between the two was not statistically significant. Soil NO 3 - -N content was more variable than NH 4 + -N, with the coefficients of variation for NO 3 - -N in the surface and deeper soils being 52.42% and 60.14%, respectively, versus 26.84% and 28.80% for NH 4 + -N, respectively. Table 2 Response of estimated soil nitrogen mineralization for 130-days to soil amendments (Unit: mg N·kg -1 soil). Soil nitrogen mineralization as impacted level of biochar application: no biochar addition (B00), 10 t biochar·ha -1 (B10), 20 t biochar·ha -1 (B20), 30 t biochar·ha -1 (B30), and 40 t biochar·ha -1 (B40). Response of soil nitrogen mineralization to various combinations of soil amendments: 70% synthetic fertilizers + 30% organic fertilizers (OF), 70% synthetic fertilizers + 30% biochar-based organic fertilizer (BF), and 70% synthetic fertilizers + biochar (Bio). The average values presented in the table represent means ± standard error of the mean. CV refers to the coefficient of variation. Treatments Average Min Max CV B00 44.05 ± 3.91 34.30 53.08 17.76 B10 39.97 ± 0.81 38.37 40.98 3.50 B20 50.81 ± 2.23 47.21 57.33 8.80 B30 45.93 ± 0.44 45.33 46.80 1.68 B40 51.03 ± 4.18 39.50 57.72 16.37 BF 48.25 ± 4.19 40.52 57.89 17.38 Bio 50.92 ± 2.45 43.90 55.24 9.62 OF 47.76 ± 4.68 40.14 60.35 19.58 Soil mineral nitrogen exposure The determination of soil mineral N exposure was conducted using the IEM approach (Fig. 1 ). The study on the effect of different biochar application rates on the soil's effective N dynamics revealed that AE increased with biochar rate and plateaued at higher applications (≥ 20 t biochar·ha -1 , Fig. 1 a). The AE within 111 days was 0.33 ± 0.03 mg N·cm -2 in the absence of biochar (B00). The introduction of biochar resulted in elevated AE, with a corresponding rise of 21.17% and 47.26% in the B10 and B20 treatments, respectively. The AE in B20, B30, and B40 treatments was measured 0.48 ± 0.04, 0.53 ± 0.04, and 0.46 ± 0.05 mg N·cm -2 , respectively, with no statistically significant differences. In the treatment absent biochar (B00), the exposure to NO 3 - -N was measured 3.24 ± 0.26 mg N·cm -2 , 8.90 times higher than AE (Fig. 1 b). Overall, NE exceeded AE by 8.59 times, ranging from 4.89 to 10.26 times. Compared with B00, the NE of B10 was only 2.33 ± 0.15 mg N·cm -2 , whereas the difference was not significant. However, the application of increased biochar rates resulted in a significant elevation in the NE of B20, reaching a value of 5.38 ± 0.52 mg N·cm -2 . This measurement was 66.26% greater than the NE seen in B00, which amounted to 2.15 mg N·cm -2 . The highest NE occurred at B30, with a value of 5.93 ± 0.66 mg N·cm -2 , 83.06% greater than B00. Nevertheless, B40 experienced a significant 23.6% reduction in NE (1.40 mg N·cm -2 ) compared to B30. Biological nitrogen availability A two-week aerobic incubation was employed to measure BNA. Figure 1 c illustrates the variation in BNA to various biochar application rates. The findings indicated that BNA levels ranged from 8.23 to 16.1 mg N·kg -1 soil (ave. 13.05 mg N·kg -1 soil, CV = 36%). No significant statistical differences in BNA were observed as a result of the different biochar rates on BNA. Various combinations of soil amendments led to varying levels of soil N exposure after early-summer supplementary fertilization (Fig. 1 d & 1 e). The AE of the biochar treatment (Bio) at a rate of 7.5 t·ha -1 , where the biochar was partially replaced the synthetic fertilizer, exhibited the highest value of 3.22 ± 0.16 mg N·cm -2 . This value significantly exceeded the AE in the biochar-based organic fertilizer treatment (1.51 ± 0.35 mg N·cm -2 ). The AE resulting from the application of organic fertilizer treatment was found to be 0.78 ± 0.05 mg N·cm -2 , representing only 24.19% of the AE observed in the Bio treatment. Conversely, NE showed independent responses, with concentrations of 9.75 ± 4.79, 14.30 ± 3.38, and 8.84 ± 9.37 mg N·cm -2 in the Bio, BF, and OF treatments, respectively. However, no statistically significant differences were identified across these three treatments. It is noteworthy that both CVs for NH 4 + -N and NEs were relatively high for BF at 45.95% and 47.27%, respectively. Likewise, there was no significant influence of distinct soil amendment combinations on BNA (Fig. 1 f). The BNA values for Bio, BF, and OF treatments were 16.02, 14.13, and 13.78 mg N·kg -1 soil, respectively. Importantly, these treatments did not differ significantly from the BNA observed in the absence of both N addition and biochar (B00). Soil ammonium and nitrate fluxes Figure 2 portrays the temporal patter in the flux of NH 4 + -N and NO 3 - -N to the ion exchange membrane, offering insight into the influence of different combinations of soil amendments on the availability of soil mineral N following early-summer supplementary fertilization. Although the patterns of NH 4 + -N and NO 3 - -N fluxes were similar, distinct differences in the magnitude of flux among treatments was apparent (Fig. 2 ). Divergence in NH 4 + -N fluxes became evident one week after fertilizer application (Fig. 2 a). The treatment employing a combination of synthetic fertilizer and biochar exhibited NH 4 + -N fluxes reaching 112.92 µg N·cm -2 ·d -1 in the initial week after early-summer supplemental fertilization, a significantly higher value compared to biochar-based organic fertilizer (BF, 26.28 µg N·cm -2 ·d -1 ) and organic fertilizer (OF, 5.88 µg N·cm -2 ·d -1 ) treatments. The NH 4 + -N fluxes across all treatments peaked in week 2, with the Bio treatment reaching 204.89 µg N·cm -2 ·d -1 , representing a 2.17-fold increase compared to BF and a 7.42-fold increase compared to OF. Subsequently, there was a significant decline in NH 4 + -N fluxes. By Week 3–4, Bio, BF, and OF all experienced 80–85% reductions NH 4 + -N availability. After 6 weeks, NH 4 + -N fluxes for each treatment exhibited minor fluctuations, following a gradual decrease until the study's conclusion. The average NH 4 + -N flux at the experiment's end was 1.39 ± 0.26 µg N·cm -2 ·d -1 , with no statistical differences among treatments 2 months after fertilizer application. The fluxes of NO 3 - -N displayed comparable temporal patterns to NH 4 + -N, but with no significant differences between treatments (Fig. 2 b). NO 3 - -N fluxes rose within a week after fertilizer application, reaching a maximum of 322.64 µg N·cm -2 ·d -1 during the second week. Similar to NH 4 + -N fluxes, NO 3 - -N fluxes declined after two weeks, reaching a minimum of 58.42 µg N·cm -2 ·d -1 in the sixth week. This contrasts with NH 4 + -N fluxes, which reached their lowest point after four weeks of fertilization. Subsequently, NO 3 - -N fluxes stabilized after a minor peak at 93.06 µg N·cm -2 ·d -1 . Discussion Soil nitrogen supply capacity in tea plantation Soil organic N mineralization is an important source of plant available nitrogen. Quantifying soil N mineralization functions as an important component in assessing soil N supply and provides valuable insights for making fertilizer recommendations. This study evaluated the summer N supply ability of soils in tea plantations using a two-week aerobic incubation method. Over a period of 130 days, the topsoil was able to provide 47.95 mg N·kg -1 of mineralized N, which is equivalent to 125 kg·ha -1 in the top 15 cm, assuming a bulk density of 1.3 Mg·m -3 . Soil N supply decreased by 8% in the 15–30 cm soil layer, supplying and additional 114 kg N·ha -1 . This observation is consistent with the soil N capacity of 96–120 kg N·ha -1 in the top 15 cm typically observed in eastern Canadian agricultural soils throughout the growing season, as reported by Wu et al. ( 2008 ). It slightly exceeds central Chinese corn field rates of 26–42 mg N·kg -1 (Maitlo et al., 2002). This variation may be ascribed to disparities in soil TN content. A meta-analysis of global N mineralization conducted by Li et al. ( 2019 ) demonstrated that soil TN has a significant impact on soil N mineralization via exerting influence on microbial biomass. The soil TN content at this research site was measured to be 1.77 ± 0.157 g·kg -1 . This value is within the established range of TN values for agricultural soils in eastern Canada, which is reported to be between 1-1.95 g·kg -1 (Wu et al., 2008 ). Also, the TN content at this site was found to be greater than the range of TN values reported for maize land soils in central China, which ranged between 0.62–1.09 g·kg -1 as documented by Maitlo et al. (2002). The response of N mineralization to different biochar application rates as well as different soil amendment combinations was not significant. This suggests that the organic nitrogen in biochar is sufficiently stable as to not affect soil N mineralization in the short term (6 months, December 2022 to April 2023). Our findings align with those of Dempster et al. ( 2012 ). Indeed, the relationship between biochar and soil N mineralization is intricate. Past studies have documented instances of either stimulation, reduction, or no substantial effect on nitrogen mineralization (Kolb, 2009; Dempster et al. 2012 ; Tammeorg et al., 2012 ; Lentz et al., 2014 ). Biochar-induced inhibition of N mineralization has been attributed to the high C:N ratio of biochar, leading to N immobilization by soil microbes (Tammeorg et al., 2012 ). On the other hand, the enhancement of N mineralization by biochar has been linked to the adsorption of phenolics from the soil environment by the biochar (Kolb, 2009). Effect of biochar application on soil nitrogen exposure Soil N exposure, in the form of NH 4 + -N and NO 3 - -N, represents the available N content in the soil that could be readily taken up by plants and/or be lost to the environment (Burton et al., 2008b ; Burton & Zebarth, 2014 ). Hence, N exposure serves as an indicator of soil fertility and soil health. Soil N exposure can be employed to indicate the quantity of available N supplied by the soil over time, reflecting the potential for plant uptake of N and/or the susceptibility of N losses to the environment. Compared to the absence of biochar, the AE rose by 0.07 mg N·cm -2 , whereas NE fell by 0.90 mg N·cm -2 in the low biochar treatment (B10). This implies the reduction in soil NO 3 - availability in the low biochar treatment and a reduced risk of NO 3 - loss. The observation aligns with the research findings reported by Hu et al. ( 2023 ). A low corn stover biochar rate has been shown partially offset the loss of NO 3 - by inhibiting nitrification (Hu et al., 2023 ). In a pot experiment, Yao et al. ( 2022 ) revealed that low biochar rate decreased nitrification by suppressing the growth and reproduction of ammonia-oxidizing archaea and bacteria (AOA & AOB). The conclusion was substantiated by the observation that applying a low biochar rate resulted in a decrease in the soil NO 3 - -N to NH 4 + -N ratio from 9.9 to 5.89. The present study observed that the application of biochar at moderate rates (20 and 30 t·ha -1 ) resulted in a simultaneous elevation AE and NE, hence enhancing soil N availability. These findings are consistent with previous investigations conducted by Clough and Condron ( 2010 ) as well as Nguyen et al. ( 2017 ). The altered soil physicochemical qualities and N cycle may be attributed to the distinctive features of biochar (Liu et al., 2018 ; Ibrahim et al., 2023 ). According to Nartey and Zhao ( 2014 ), biochar has a substantial surface area and possesses a notable adsorption capability. The use of NH 4 + adsorbed by biochar has been found to be beneficial for plants, resulting from a delayed production of NO 3 - and mitigating leaching loss (Zheng et al., 2013 ; Wang et al., 2023 ). Furthermore, the application of alkaline biochar has been found to have the potential to elevate the pH levels and enhance the moisture content of acidic soil. Additionally, it has been seen to stimulate N mineralization and facilitate various N cycle activities within the soil (Wang et al., 2017 ), although this was not observed in the current study. Upon reaching an application rate of 40 t·ha -1 , the NE in the soil declined, resulting in a reduction in the availability of soil effective N. The findings indicated the presence of a threshold in the impact of biochar application, beyond which the promotion of soil N supply ceased to exhibit further enhancement. The explanation is that excessive biochar application reduces soil NO 3 - due to a higher C:N ratio which shifts the soil microbial activity from N mineralization to N immobilization (Kelly et al., 2015 ). Additionally, the larger amount of biochar would lead led to a greater adsorption of NH 4 + , hence diminishing the mobility of inorganic N in the soil (Clough et al., 2013 ). Based on the findings, varying rates of biochar application did not have a statistically significant impact soil N mineralization and BNA in the first year of study. This result suggests that the increased soil N exposure resulting from biochar application does not originate from organic N mineralization. Instead, it is ascribed to the adsorption characteristics of biochar, which enhance the retention of NH 4 + . Effect of soil amendment on the temporal pattern in soil nitrogen dynamics Tea plantations commonly adopt a split fertilizer application strategy, with the practice of early-summer supplementary fertilization being widely adopted. This study presents findings on the temporal pattern of soil mineral N flux in relation to various fertilizer combination schemes after the application of supplementary fertilization throughout the summer season. Biochar exhibits a greater ability to sorb positively charged NH 4 + -N than negatively charged NO 3 - -N (Zhang et al., 2020 ). This explains the significant variation seen in AE across various fertilizer combinations, while NE was not significantly impacted. The AE tended to increase with the increase of biochar application (Bio > BF > OF). In comparison to the treatment with solely organic fertilizer, the AE was 313% and 94% higher in the treatments receiving biochar alone or a biochar-based organic fertilizer, respectively. It is noteworthy to mention that there was no significant difference observed in the AE between B00 (no biochar & no fertilizer) and OF, suggesting NH 4 + -N loss occurred. The application of biochar has been shown to be an effective strategy for mitigating the loss of NH 4 + -N and promoting its retention within the soil (Cai et al., 2016 ). Furthermore, despite the absence of a statistical difference in soil NE across the various treatments, the biochar-based organic fertilizer treatments exhibited a wide range of NO 3 - -N levels, spanning from 7.74 to 21.45 mg N·cm -2 with a CV of 47.27%. This suggests the potential of applying biochar-based organic fertilizer as an approach to improve soil available N. Soil available N peaked in the second week after fertilizer application. During week 3&4, the NH 4 + -N flux was seen to decrease by over 80% as a result of processes such as plant uptake, nitrification, and losses. Similarly, NO 3 - -N flux was reduced by an average of 82% after week 2. Nevertheless, the decrease in NO 3 - -N exhibited a noticeable delay and did not reach its low point until the sixth week after the application of fertilizer. The observed variations were ascribed to the adsorption features of biochar. It was shown that the NH 4 + -N flux of all treatments exhibited a reduction of over 80% within 2–4 weeks after fertilization. However, the decrease of NO 3 - -N flux lagged behind NH 4 + -N by about 2 weeks, indicating that NH 4 + -N adsorbed by the biochar could still be converted via nitrification. Unfortunately, the available evidence does not provide a conclusive explanation for the observed phenomenon of NH 4 + -N and NO 3 - -N fluxes experiencing a slight rebound subsequent to the low point. Consequently, further studies are required to investigate this matter thoroughly. Conclusion The two-week aerobic incubation method provides a straightforward and efficient means of evaluating tea plantation soil N supply capacity, and the IEM technique successfully monitored soil N dynamics. The acidic topsoil of tea plantations in Fujian Province could provide 48 mg N·kg -1 of soil inorganic N for a 130-days growing period, and the N supply capacity of the deeper soil was only 8% lower than that of the topsoil. The use of biochar has led to a notable increase in the availability of N in soil. However, this increase is not linear, as it first rises and afterwards declines. After careful analysis, it has been determined that the optimal amount of biochar to be applied is 20 t·ha -1 . This decision takes into consideration both the soil available N and the associated cost of application. Furthermore, the utilization of biochar has been shown to be a viable strategy for mitigating the loss of inorganic fertilizer and enhancing its retention capacity. The utilization of biochar-based organic fertilizer has promising prospects in enhancing the availability of N in soil. The primary impact of the supplementary fertilizers applied in the summer season was predominantly observed within a period of 6–8 weeks after application, with particular emphasis on the second week. The study was unable to distinguish between different pathways of soil N outputs and did not explore the impact of different fertilizer combinations on crop development due to constraints imposed by the experimental circumstances. Further exploration of these two features can be undertaken in future research endeavours. Abbreviations AE Ammonium Exposure AEM Anion Exchange Membrane ANOVA Analysis of Variance BF Biochar-Based Organic Fertilizer CEC Cation Exchange Capacity CEM Cation Exchange Membrane CV Coefficient of Variation IEM Ion Exchange Membrane N Nitrogen NE Nitrate Exposure NH 4 + Ammonium NH 4 NO 3 Ammonium Nitrate NO 3 - Nitrate OF Organic Fertilizers Declarations Fundings This work was financially supported by Central Government Guided Local Science and Technology Development projects under Grant No. 2021L3021, and Science and Technology Project of Fujian Academy of Agricultural Sciences under Grant No. XTCXGC2021010. The work was also supported by a Canadian Natural Sciences and Engineering Council CREATE-CSS grant. Competing Interests All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript. Author Contributions All authors contributed to the study conception and design. Material preparation and data collection were performed by Zhang B, Liu C, Li Q, Ye J, and Lin Y. Data analysis was carried out by Zhang B and Liu C. The first draft of the manuscript was written by Zhang B. Wang Y and Burton D provided technical support and supervision throughout the research process. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgments This work was financially supported by Central Government Guided Local Science and Technology Development projects under Grant No. 2021L3021, and Science and Technology Project of Fujian Academy of Agricultural Sciences under Grant No. XTCXGC2021010. The work was also supported by a Canadian Natural Sciences and Engineering Council CREATE-CSS grant. References Burton DL, & Zebarth BJ (2014) Nitrate Exposure: A metric to describe the influence of soil NO 3 - on N 2 O emissions. 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Environmental pollution 227:98-115. https://doi.org/10.1016/j.envpol.2017.04.032 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3991015","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":281820539,"identity":"f54eca75-2bda-4b0b-a6d9-9547a5406195","order_by":0,"name":"Bangwei Zhang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Bangwei","middleName":"","lastName":"Zhang","suffix":""},{"id":281820540,"identity":"1845e583-9af4-44b4-8da9-3583da085bd7","order_by":1,"name":"Cenwei Liu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Cenwei","middleName":"","lastName":"Liu","suffix":""},{"id":281820541,"identity":"53fe8839-b4bd-4b7d-ba63-8fd2ff4549c9","order_by":2,"name":"Qiang Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Qiang","middleName":"","lastName":"Li","suffix":""},{"id":281820542,"identity":"cee90df5-4dc1-42d2-aa71-fb8c57f7c233","order_by":3,"name":"Jing Ye","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Ye","suffix":""},{"id":281820544,"identity":"359efa46-11f4-4756-b656-f3d021a44269","order_by":4,"name":"Yi Lin","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yi","middleName":"","lastName":"Lin","suffix":""},{"id":281820545,"identity":"c0a40046-313d-4442-9eb4-e2fc05db52fa","order_by":5,"name":"Yixiang Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxElEQVRIiWNgGAWjYFCCBCAuAGL2xsaHH4jXYgDEPIebjSVI0yKR3ibAQ4wG+fYENokPBjZ58pEP2xgkGOzkdBsIaDE484BNcoZBWrHh7cS2BwUMycZmBwhpkUhgu81jcDhx4+zEdgMJhgOJ2whpkZ8B0zLzYJsEDzFaGG5AtcyXYCRSC9Av7D+BfkncwJMIDGQDIvwCDDFmgw8VNonz248/fPihwk6OoBYGBn5IlBuAVRoQVI5sXQMpqkfBKBgFo2BEAQDbFkJr9oPt3AAAAABJRU5ErkJggg==","orcid":"","institution":"Fujian Academy of Agricultural Sciences","correspondingAuthor":true,"prefix":"","firstName":"Yixiang","middleName":"","lastName":"Wang","suffix":""},{"id":281820547,"identity":"41a51e4d-7177-4a1d-b0cc-a6db99766d12","order_by":6,"name":"David L Burton","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"David","middleName":"L","lastName":"Burton","suffix":""}],"badges":[],"createdAt":"2024-02-26 13:19:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3991015/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3991015/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":53257248,"identity":"ba5a446d-45e8-4173-acfd-05c1b70e9699","added_by":"auto","created_at":"2024-03-22 13:47:40","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":61345,"visible":true,"origin":"","legend":"\u003cp\u003eSoil mineral nitrogen exposure in 111 days and biological nitrogen availability (BNA) at tea plantation. The effect of various levels of biochar application: no biochar addition (B00), 10 t biochar·ha\u003csup\u003e-1\u003c/sup\u003e (B10), 20 t biochar·ha\u003csup\u003e-1\u003c/sup\u003e (B20), 30 t biochar·ha\u003csup\u003e-1\u003c/sup\u003e (B30), and 40 t biochar·ha\u003csup\u003e-1\u003c/sup\u003e (B40), on \u003cstrong\u003e(a)\u003c/strong\u003e NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N exposures, \u003cstrong\u003e(b) \u003c/strong\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N exposures, and \u003cstrong\u003e(c)\u003c/strong\u003e BNA. The responses of \u003cstrong\u003e(d)\u003c/strong\u003e NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N exposures, \u003cstrong\u003e(e)\u003c/strong\u003e NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N exposures, and \u003cstrong\u003e(f)\u003c/strong\u003e BNA to various combinations of soil amendments: 70% synthetic fertilizers + 30% organic fertilizers (OF), 70% synthetic fertilizers + 30% biochar-based organic fertilizer (BF), and 70% synthetic fertilizers + biochar (Bio). The X in the figures indicates the mean value of each treatment. The mean values with different letters indicate a significant difference between treatments (P ≤ 0.05). The symbol of n.s. in the graphs indicates no significant difference between means (P \u0026gt; 0.05)\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3991015/v1/4a7b9f736c3337fcbc3d181b.jpg"},{"id":53257250,"identity":"a77d8f5f-6a8c-4ed3-9cbe-43d646190f81","added_by":"auto","created_at":"2024-03-22 13:47:41","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":38208,"visible":true,"origin":"","legend":"\u003cp\u003eSoil NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N \u003cstrong\u003e(a)\u003c/strong\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N \u003cstrong\u003e(b)\u003c/strong\u003e fluxes in response to various combinations of soil amendments: 70% synthetic fertilizers + 30% organic fertilizers (OF), 70% synthetic fertilizers + 30% biochar-based organic fertilizer (BF), and 70% synthetic fertilizers + biochar (Bio). The symbols of ** and * in the figures indicate the significant diffidence between means at p-value ≤ 0.01 and 0.05, respectively. Mean values of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes are given due to non-significant differences among treatments (red markers and line)\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3991015/v1/8a08e225ff441ac568819b36.jpg"},{"id":66494842,"identity":"cbcf543e-94d3-4b13-9c5f-57af1170e1b7","added_by":"auto","created_at":"2024-10-13 11:37:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":851684,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3991015/v1/f07a127b-8d7d-4d02-a778-13bc8f9ca26a.pdf"}],"financialInterests":"","formattedTitle":"Effect of biochar and its combined fertilizers on the dynamics of soil nitrogen supply in tea plantation","fulltext":[{"header":"Introduction","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eNitrogen (N) is a vital nutrient necessary for the growth of tea trees (\u003cem\u003eCamellia sinensis\u003c/em\u003e L.), since it has a direct role in crucial physiological processes including chlorophyll production, hence influencing plant growth and development (Li et al., 2016). Increased nitrogen supply has also been shown to increase amino acid concentrations and decrease polyphenol concentrations (Rebello et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). High rates of N fertilizer application have been shown to favour shoot growth over root growth and as a result increase the susceptibility of the crop to drought (Cheruiyot et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Tea plants receive a substantial quantity of N fertilizer annually. The quality of green tea increases with increasing N rate, while the quality of black tea decreases with N additions greater than 200 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (Rebello et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In China, for instance, the average quantity of N fertilizer that is applied to tea plantations is between 300 and 450 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e, and the average annual harvest of tea leaves is around 800 kg\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (Chen \u0026amp; Lin, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Xiao et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The N concentration of tea is around 4\u0026ndash;6% (Chen \u0026amp; Li, 2016), which means that only 30\u0026ndash;50 kg N is removed in the harvested leaves and approximately 90% of the applied N fertilizer is either lost or retained in the soil. Tea plantations are commonly associated with acidic soils that receive considerable rainfall, which result in high potential for loss of nutrients (Yan et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The primary kind of fertilizer utilized in tea plantations is compound fertilizer containing multiple nutrients, accounting for 70% of overall application. This compound fertilizer is enriched with various nutrients, and ammonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) serves as the principal N source (Venkatesan et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). In addition, soil N mineralization is a significant source of N supply for plants but is frequently not considered when calculating the amount of N fertilizer to apply.\u003c/p\u003e \u003cp\u003eBiochar is a carbon-rich product produced by the transformation of organic materials through the pyrolysis process, which produces a product that has a porous structure, large specific surface area and strong adsorption capacity (Chen et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The application of biochar in agricultural production has attracted much attention. Research has demonstrated that the use of biochar has the potential to improve soil physical and chemical characteristics, soil microbial communities, soil fertility, and stimulate crop development (Singh et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). It plays an important role in regulating soil N transformation, e.g., N mineralization and nitrification, via its significant cation exchange capacity (CEC) and adsorption capabilities, which contribute to the improved retention of organic matter and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in soils. This enhanced retention subsequently leads to increased availability of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e for plant uptake while simultaneously lowering N losses (Gai et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Prayogo et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). It is important to note that tea plants have a greater ability to take up NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003erelative to nitrate (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, Ruan et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The impact of biochar on the composition of soil microbial communities and the processes of N transformation, especially N mineralization, can be attributed, in part, to its ability to elevate soil pH and boost soil carbon content (Prayogo et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Palansooriya et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In a study conducted by Cen et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), it was shown that the utilization of biochar-based fertilizers resulted in a higher retention rate of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in the soil, as compared to ammonium sulphate applied alone. Specifically, the retention rate increased from 41\u0026ndash;66%.\u003c/p\u003e \u003cp\u003eSoil N exposure expresses the presence of various N forms in soil over time (Burton et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2008b\u003c/span\u003e; Burton \u0026amp; Zebarth, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). It quantifies the presence of soil mineral N (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and/or NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) over time compared to instantaneous measures of soil mineral N concentration (Burton \u0026amp; Zebarth, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). It reflects potential risks, particularly in the context of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e loss through N\u003csub\u003e2\u003c/sub\u003eO emissions. Previous studies reported that soil N exposure is correlated with cumulative N\u003csub\u003e2\u003c/sub\u003eO emissions, whereas instantaneous measures of soil mineral N seldom are (Burton et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2008a\u003c/span\u003e; Burton et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2008b\u003c/span\u003e; Maharjan \u0026amp; Venterea, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Pelster et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Nitrogen exposure can be calculated by integrating conventional soil sampling methods over time, but this approach is time-consuming, labour intensive, and discontinuous. The emergence of ion-exchange membrane technology addresses these limitations, offering a more efficient approach for the continuous measurement of soil N exposure (Harrison \u0026amp; Maynard, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Le\u0026oacute;n Castro \u0026amp; Whalen, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This innovative technique holds the promise of being more convenient and accurate, with potential widespread application in future research and practical applications.\u003c/p\u003e \u003cp\u003eSoil N supply to the plant is a result of the combination of carryover of inorganic N from the previous growing season and the mineralization organic N over the growing season (Zebarth et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Nyiraneza et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Measuring or predicting the N mineralization of soil has proven to be challenging because it is driven by soil biological processes. Traditional methods for determining soil N mineralization potential involve chemical extractions or long-term aerobic incubation, these approaches have practical drawbacks, such as time consuming and the exclusion of biological factors (Luce et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Ros et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). These measures of the N mineralization potential are generally well correlated (Sharifi et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2007a\u003c/span\u003e; Sharifi et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2007b\u003c/span\u003e; Dessureault-Rompr\u0026eacute; et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and been applied to predicting growing season N mineralization (Dessureault-Rompr\u0026eacute; et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Laurence et al., 2024). Biological N Availability (BNA), the mineralization of N occurring over a 14-day aerobic incubation, corresponds to the soil labile organic N pool, or pool I as defined by Sharifi et al. (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2007b\u003c/span\u003e) and is used to quantify N mineralization potential. Dessureault-Rompr\u0026eacute; et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) used BNA (Pool I) in combination with pedotransfer functions to reflect climate to predict growing season N mineralization.\u003c/p\u003e \u003cp\u003eDespite the increased interest in biochar in agriculture, most studies have concentrated on common crops like corn, wheat, and vegetables. The main response variables measured have been crop growth and soil microbial communities (Vaccari et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Yu et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Ogura et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The majority of soil-related biochar research has focused on soil improvement and soil remediation (Sohi et al., 2011; Zhu et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Few studies have examined the impact of biochar on N cycling processes in the acidic soils of tea plantations, particularly soil N supply in these areas and particular the period following early-summer supplementary fertilization where N loss potential is high. Therefore, in this study, we monitored soil N mineralization potential and soil N dynamics using a 2-week aerobic incubation method and ion exchange membrane (IEM) technology to examine the influence of various biochar application rates and fertilizer combinations on soil N dynamics during this summer period.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSite Characteristics\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe research was conducted in a tea plantation located in Youxi County, China (26\u0026deg;2'35\" N, 118\u0026deg;7'36\" E). The study region experiences a humid subtropical climate (K\u0026ouml;ppen Cfa), which is distinguished by warm and humid summers, as well as moderate winters. Spring and summer are characterized by a significant amount of precipitation with an average annual precipitation of 1600 millimetres, accompanied by a frequency of 194 wet days over a year. The long-term average temperature in January is 9.3\u0026deg;C and in July is 27.9\u0026deg;C. The tea plantation primarily cultivates the Fuyun No. 6 variety. The soil in the area is classified as a Ultisol, characterized by a pH level of 3.8. All tea plants were grown in terraces at uniform heights using conventional cultivation practices. The spring rainy season spans from February to June, and results in approximately 60% of the total annual precipitation. The months of May and June exhibit the highest levels of precipitation, accounting for approximately 34% of the total annual rainfall. The period from July to September constitutes the summer storm and typhoon season, and results in approximately 27% of the total annual precipitation.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eExperimental Design and Treatments\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe research was set up in December 2022 as a completely randomized design, consisting of two parallel studies, encompassing a total of eight treatments. Each treatment was replicated four times.\u003c/p\u003e \u003cp\u003eThe first study, conducted from late April to late August 2023, examined the N status of soils in tea plantations as impacted level of biochar applied in December 2022. The experiment comprised of five treatments: no biochar addition (B00, control), 10 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B10), 20 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B20), 30 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B30), and 40 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B40). Synthetic fertilizer (22-9-9), containing ammonium nitrate (NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e), was applied in a rate of 165 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e in December of 2022, as well as all biochar. The application of fertilizer and implementation of agricultural practices in the experimental area adhered to the conventional tea plantation management practices commonly employed in the local region.\u003c/p\u003e \u003cp\u003eA second study examined the impact of various combinations of soil amendments on soil N dynamics beginning with the addition of supplemental fertilizer in late April 2023. Three treatments were considered: \u003cem\u003ei\u003c/em\u003e. 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;30% organic fertilizers (OF), \u003cem\u003eii\u003c/em\u003e. 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;30% biochar-based organic fertilizer (BF), and \u003cem\u003eiii\u003c/em\u003e. 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;biochar (Bio, at a rate of 7.5 t\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e). A cumulative amount of 495 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e was applied throughout the year, comprising 148.5 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e from organic or biochar-based organic fertilizer, and 346.5 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e from synthetic fertilizers (split into 3 applications), containing NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e. In accordance with the conventional tea plantation management practices in the local area, the application of organic fertilizers, biochar-based organic fertilizers, and biochar took place in December 2022. Synthetic fertilizer was applied in December 2022 and April 2023 in an equal rate of 115.5 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e, with one more scheduled in late August 2023.\u003c/p\u003e \u003cp\u003eAll soil amendments were broadcasted in the alleyway close to the plants and not incorporated into the soil. The biochar used in the experiment was produced from maize stover (Henan Sanli New Energy Co., Ltd., China), and its properties were described by Hu et al. (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Briefly, dried maize stover was pyrolyzed at around 450\u0026deg;C for 4 hours, yielding biochar with 56.7% C, 3.2% H, 3.6% N, and at pH 8.9. The organic fertilizer utilized in this study was derived from sheep manure sourced from Wofuwo Fertilizer Industry Co., Ltd. in China. The manure had a N content of 0.98%, with a C:N ratio of 16.7. The synthetic fertilizer (22-9-9, Holitech Technology Co., Ltd., China) employs NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e as its N source, H\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e as P source, and KCl as K sources. The biochar-based organic fertilizer was formulated following the guidelines outlined in the Chinese standard [NY/T 3618\u0026thinsp;\u0026minus;\u0026thinsp;2020], where a ratio of 10:1 (w/w) organic fertilizer to biochar was employed during the mixing process. Biochar was not considered as a N source.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eSoil Sampling and the Determination of Soil Nitrogen Supply\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eSamples were collected from two soil depths (0\u0026ndash;20 \u0026amp; 20\u0026ndash;40 cm) from each experimental plot prior to fertilization in April 2023. The soil samples were air-dried and sieved using a 2-mm sieve.\u003c/p\u003e \u003cp\u003eThe measurement of BNA was conducted using the methodology described for Pool I by Sharifi et al. (2007). A total of 30 grams of soil was combined with 30 grams of Ottawa sand and placed in a detachable 55 mm B\u0026uuml;chner funnel. The funnel was equipped with glass fibre filter paper. The B\u0026uuml;chner funnel, containing the soil sample mixture, was affixed onto a 500 mL evacuation flask and linked to a vacuum system. The soil samples were leached with 200 mL of a 0.05 \u003cem\u003eM\u003c/em\u003e calcium chloride (CaCl\u003csub\u003e2\u003c/sub\u003e) solution, and the leachate collected. Following leaching the soil samples, together with the detachable B\u0026uuml;chner funnel's upper structure, were removed and sealed with parafilm at both ends. Subsequently, these samples were placed in an incubator set at a temperature of 25\u0026deg;C for a duration of 14 days. Following the completion of the incubation, the soil samples were reintroduced into the vacuum filtering system and leached again with 200 mL of a 0.05 \u003cem\u003eM\u003c/em\u003e CaCl\u003csub\u003e2\u003c/sub\u003e solution.\u003c/p\u003e \u003cp\u003eThe concentration of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N in the leachates was determined using the Skalar San\u0026thinsp;+\u0026thinsp;+\u0026thinsp;CFA analyzer (Skalar Analytical B.V., Netherlands). The measurement of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N concentrations in the first leachate was employed as an indicator to assess the quantity of mineral N contained in the soil at the time of sampling. The concentration of mineral N in the second leachate is as a result of N mineralization of organic N during the two-week period and is used as a measure of BNA (mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil). Soil N mineralization (N\u003csub\u003emin\u003c/sub\u003e, unit: mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil) was estimated for a 130-day (t) growing period, according to the equation described by Dessureault-Rompr\u0026eacute; et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e):\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Equa\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${N}_{min}=(0.123*TN+0.00312*BNA)*t+BNA*\\left(1-{e}^{-{k}_{L}*t}\\right)$$\u003c/div\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003ewhere TN was the soil total N content (g N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil) and have a mean value of 1.776\u0026thinsp;\u0026plusmn;\u0026thinsp;0.157 g N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil at the research site, and the value of k\u003csub\u003eL\u003c/sub\u003e = 0.12 d\u003csup\u003e-1\u003c/sup\u003e as suggested by Dessureault-Rompr\u0026eacute; et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eMonitoring Soil Inorganic Nitrogen Dynamic Using Ion Exchange Membranes\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe anion/cation exchange membrane (AEM/CEM) technique was used to monitor soil inorganic N fluxes during late April to mid-August 2023, as described by Nyiraneza et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Membranes were cut to 6 \u0026times; 5.2 cm\u003csup\u003e2\u003c/sup\u003e for AEMs and 7 \u0026times; 4.5 cm\u003csup\u003e2\u003c/sup\u003e for CEMs and were then acid washed and regenerated by soaking and shaking in 0.5 \u003cem\u003eM\u003c/em\u003e HCl solution for 30 min followed by 1 \u003cem\u003eM\u003c/em\u003e NaCl solution for 2h prior to installation into the soil. Saturated membranes were inserted 10\u0026ndash;15 cm into the soil and compacted to ensure full contact between soil and membrane. The membranes were buried in the soil for 1\u0026ndash;3 weeks and were removed and replaced with a second membrane in the same position to ensure sequential collection from the same soil interface. This process was continued over the monitoring period to provide a continuous measure of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e flux (\u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e). Following removal from the soil, membranes were returned to lab and shaken with 40 ml of 1 \u003cem\u003eM\u003c/em\u003e KCl solution for 1h to extract NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. The extracts were filtered using Whatman\u003csup\u003e\u0026reg;\u003c/sup\u003e #5 filter paper (Whatman plc, United Kingdom) and stored frozen until subsequent analysis.\u003c/p\u003e \u003cp\u003eThe concentrations of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N in the extracts were analyzed using a Skalar San\u0026thinsp;+\u0026thinsp;+\u0026thinsp;CFA analyzer (Skalar Analytical B.V., Netherlands). The flux of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N to the CEM and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N to the AEM were expressed as \u0026micro;g NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N/NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e according to equation:\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Equb\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$N flux=\\left(C*V\\right)/\\left(A*T\\right)$$\u003c/div\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003ewhere the \u003cem\u003eC\u003c/em\u003e is the concertation of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N or NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N in unit of \u0026micro;g N\u0026middot;ml\u003csup\u003e-1\u003c/sup\u003e, the \u003cem\u003eV\u003c/em\u003e is the volume of 1M KCl used to extract minerals N (ml), \u003cem\u003eA\u003c/em\u003e is the area of the membrane (cm\u003csup\u003e2\u003c/sup\u003e), and the \u003cem\u003eT\u003c/em\u003e is the number of days the membrane remained in soil.\u003c/p\u003e \u003cp\u003eThe sum of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N flux and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N flux over a given period (111d, April 26 to August 15) were calculated and presented as NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N exposure (AE) and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N exposure (NE), respectively.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe each of the two experiments was setup as a completely randomized design with four treatments and four replications. Data were presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard error. Minitab\u003csup\u003e\u0026reg;\u003c/sup\u003e software (Minitab, LLC., U.S.) was used for statistical analysis. One-way analysis of variance (ANOVA) was used to test for significant influence of treatment. Post-hoc means comparisons were calculated when significant treatment effects were evident (p\u0026thinsp;\u0026le;\u0026thinsp;0.05) using the Fisher's Least Significant Difference (LSD) method. Data were visualized by drawing graphs using OriginPro 2021 (OriginLab Corp., U.S.).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eSoil nitrogen mineralization and soil mineral nitrogen\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eNitrogen mineralization in tea plantation soils was assessed through a two-week aerobic incubation (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The findings revealed that when averaged across all treatments, the estimated N mineralization for 130-days in the surface (0\u0026ndash;20 cm) soil was 47.95\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil, significantly greater that in the soil collected from 20\u0026ndash;40 cm which averaged 44.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.63 mg\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil. In contrast to the soil at a depth of 20\u0026ndash;40 cm, the surface soil demonstrated greater variability in N mineralization for 130-days. The estimated N mineralization in the surface soil ranged from 34.30 to 60.35 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e, with a coefficient of variation (CV) of 14.56%. Conversely, the N mineralization in the 20\u0026ndash;40 cm soil spanned from 38.03 to 51.79 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil, with a CV of 8.54%. Nevertheless, both studies revealed a lack of statistically significant variance in the mean values across each treatment. The N mineralization for 130-days spanned from 44.05 to 49.52 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil in response to varying levels of biochar addition, with no statistically significant differences observed (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Similarly, the combination of different soil amendments did not yield a significant difference in N mineralization, falling within the range of 48.25 to 51.03 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEstimated soil nitrogen mineralization for a 130-day growing period and soil mineral nitrogen prior to fertilization from 2 depths (Unit: mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003esoil). Soil nitrogen mineralization for 130-days estimated using a two-week aerobic incubation. KCl extractable soil mineral nitrogen prior to fertilization \u0026ndash; NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N \u0026amp; NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N. The average values presented in the table represent means\u0026thinsp;\u0026plusmn;\u0026thinsp;standard error of the mean.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eVariables\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDepth\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAverage\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCV\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eN Min 130d \u003csup\u003e**\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u0026ndash;20 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e47.95\u0026thinsp;\u0026plusmn;\u0026thinsp;1.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e34.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e60.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e14.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u0026ndash;40 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e44.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e38.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e51.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N \u003csup\u003e**\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u0026ndash;20 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.64\u0026thinsp;\u0026plusmn;\u0026thinsp;0.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e26.84\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u0026ndash;40 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e28.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-N \u003csup\u003en.s\u003c/sup\u003e.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u0026ndash;20 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e22.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e52.42\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e20\u0026ndash;40 cm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e24.91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"6\" nameend=\"c6\" namest=\"c1\"\u003e \u003cp\u003e** denotes significance at the at p-value\u0026thinsp;\u0026le;\u0026thinsp;0.01 level\u003c/p\u003e \u003cp\u003en.s. indicates no significant difference between the two depths of the variable.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eMineral N concentration was measured in experimental soil samples prior to fertilizer application (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The findings indicated that the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N concentration in the surface soil was 4.64\u0026thinsp;\u0026plusmn;\u0026thinsp;0.22 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil, a significantly greater value compared to the deeper soil (3.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil on average), representing an increase of 33.72%. The NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N content of the surface soil (ave. 10.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.94 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil) was 2.28 times higher than that of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, while the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N content of the deeper soil (ave. 9.16\u0026thinsp;\u0026plusmn;\u0026thinsp;0.95 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil) was slightly lower than that of the surface soil, but the difference between the two was not statistically significant. Soil NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N content was more variable than NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, with the coefficients of variation for NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N in the surface and deeper soils being 52.42% and 60.14%, respectively, versus 26.84% and 28.80% for NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, respectively.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResponse of estimated soil nitrogen mineralization for 130-days to soil amendments (Unit: mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003esoil). Soil nitrogen mineralization as impacted level of biochar application: no biochar addition (B00), 10 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B10), 20 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B20), 30 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B30), and 40 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e (B40). Response of soil nitrogen mineralization to various combinations of soil amendments: 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;30% organic fertilizers (OF), 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;30% biochar-based organic fertilizer (BF), and 70% synthetic fertilizers\u0026thinsp;+\u0026thinsp;biochar (Bio). The average values presented in the table represent means\u0026thinsp;\u0026plusmn;\u0026thinsp;standard error of the mean. CV refers to the coefficient of variation.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTreatments\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAverage\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCV\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e44.05\u0026thinsp;\u0026plusmn;\u0026thinsp;3.91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e34.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e53.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e17.76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e39.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e38.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.50\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50.81\u0026thinsp;\u0026plusmn;\u0026thinsp;2.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e47.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e57.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e8.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e45.93\u0026thinsp;\u0026plusmn;\u0026thinsp;0.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e45.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e46.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.68\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e51.03\u0026thinsp;\u0026plusmn;\u0026thinsp;4.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e39.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e57.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e16.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e48.25\u0026thinsp;\u0026plusmn;\u0026thinsp;4.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e57.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e17.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50.92\u0026thinsp;\u0026plusmn;\u0026thinsp;2.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e43.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e55.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e9.62\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e47.76\u0026thinsp;\u0026plusmn;\u0026thinsp;4.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e40.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e60.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e19.58\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eSoil mineral nitrogen exposure\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe determination of soil mineral N exposure was conducted using the IEM approach (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The study on the effect of different biochar application rates on the soil's effective N dynamics revealed that AE increased with biochar rate and plateaued at higher applications (\u0026ge;\u0026thinsp;20 t biochar\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The AE within 111 days was 0.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e in the absence of biochar (B00). The introduction of biochar resulted in elevated AE, with a corresponding rise of 21.17% and 47.26% in the B10 and B20 treatments, respectively. The AE in B20, B30, and B40 treatments was measured 0.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04, 0.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04, and 0.46\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, respectively, with no statistically significant differences.\u003c/p\u003e \u003cp\u003eIn the treatment absent biochar (B00), the exposure to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N was measured 3.24\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, 8.90 times higher than AE (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Overall, NE exceeded AE by 8.59 times, ranging from 4.89 to 10.26 times. Compared with B00, the NE of B10 was only 2.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, whereas the difference was not significant. However, the application of increased biochar rates resulted in a significant elevation in the NE of B20, reaching a value of 5.38\u0026thinsp;\u0026plusmn;\u0026thinsp;0.52 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e. This measurement was 66.26% greater than the NE seen in B00, which amounted to 2.15 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e. The highest NE occurred at B30, with a value of 5.93\u0026thinsp;\u0026plusmn;\u0026thinsp;0.66 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, 83.06% greater than B00. Nevertheless, B40 experienced a significant 23.6% reduction in NE (1.40 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e) compared to B30.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eBiological nitrogen availability\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eA two-week aerobic incubation was employed to measure BNA. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec illustrates the variation in BNA to various biochar application rates. The findings indicated that BNA levels ranged from 8.23 to 16.1 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil (ave. 13.05 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil, CV\u0026thinsp;=\u0026thinsp;36%). No significant statistical differences in BNA were observed as a result of the different biochar rates on BNA.\u003c/p\u003e \u003cp\u003eVarious combinations of soil amendments led to varying levels of soil N exposure after early-summer supplementary fertilization (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed \u0026amp; \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). The AE of the biochar treatment (Bio) at a rate of 7.5 t\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e, where the biochar was partially replaced the synthetic fertilizer, exhibited the highest value of 3.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e. This value significantly exceeded the AE in the biochar-based organic fertilizer treatment (1.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.35 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e). The AE resulting from the application of organic fertilizer treatment was found to be 0.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, representing only 24.19% of the AE observed in the Bio treatment. Conversely, NE showed independent responses, with concentrations of 9.75\u0026thinsp;\u0026plusmn;\u0026thinsp;4.79, 14.30\u0026thinsp;\u0026plusmn;\u0026thinsp;3.38, and 8.84\u0026thinsp;\u0026plusmn;\u0026thinsp;9.37 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e in the Bio, BF, and OF treatments, respectively. However, no statistically significant differences were identified across these three treatments. It is noteworthy that both CVs for NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NEs were relatively high for BF at 45.95% and 47.27%, respectively.\u003c/p\u003e \u003cp\u003eLikewise, there was no significant influence of distinct soil amendment combinations on BNA (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef). The BNA values for Bio, BF, and OF treatments were 16.02, 14.13, and 13.78 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e soil, respectively. Importantly, these treatments did not differ significantly from the BNA observed in the absence of both N addition and biochar (B00).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eSoil ammonium and nitrate fluxes\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e portrays the temporal patter in the flux of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N to the ion exchange membrane, offering insight into the influence of different combinations of soil amendments on the availability of soil mineral N following early-summer supplementary fertilization. Although the patterns of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes were similar, distinct differences in the magnitude of flux among treatments was apparent (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDivergence in NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes became evident one week after fertilizer application (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The treatment employing a combination of synthetic fertilizer and biochar exhibited NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes reaching 112.92 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e in the initial week after early-summer supplemental fertilization, a significantly higher value compared to biochar-based organic fertilizer (BF, 26.28 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e) and organic fertilizer (OF, 5.88 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e) treatments. The NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes across all treatments peaked in week 2, with the Bio treatment reaching 204.89 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e, representing a 2.17-fold increase compared to BF and a 7.42-fold increase compared to OF. Subsequently, there was a significant decline in NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes. By Week 3\u0026ndash;4, Bio, BF, and OF all experienced 80\u0026ndash;85% reductions NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N availability. After 6 weeks, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes for each treatment exhibited minor fluctuations, following a gradual decrease until the study's conclusion. The average NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N flux at the experiment's end was 1.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.26 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e, with no statistical differences among treatments 2 months after fertilizer application.\u003c/p\u003e \u003cp\u003eThe fluxes of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N displayed comparable temporal patterns to NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N, but with no significant differences between treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes rose within a week after fertilizer application, reaching a maximum of 322.64 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e during the second week. Similar to NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes declined after two weeks, reaching a minimum of 58.42 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e in the sixth week. This contrasts with NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N fluxes, which reached their lowest point after four weeks of fertilization. Subsequently, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes stabilized after a minor peak at 93.06 \u0026micro;g N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e\u0026middot;d\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eSoil nitrogen supply capacity in tea plantation\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eSoil organic N mineralization is an important source of plant available nitrogen. Quantifying soil N mineralization functions as an important component in assessing soil N supply and provides valuable insights for making fertilizer recommendations. This study evaluated the summer N supply ability of soils in tea plantations using a two-week aerobic incubation method. Over a period of 130 days, the topsoil was able to provide 47.95 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e of mineralized N, which is equivalent to 125 kg\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e in the top 15 cm, assuming a bulk density of 1.3 Mg\u0026middot;m\u003csup\u003e-3\u003c/sup\u003e. Soil N supply decreased by 8% in the 15\u0026ndash;30 cm soil layer, supplying and additional 114 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e. This observation is consistent with the soil N capacity of 96\u0026ndash;120 kg N\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e in the top 15 cm typically observed in eastern Canadian agricultural soils throughout the growing season, as reported by Wu et al. (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). It slightly exceeds central Chinese corn field rates of 26\u0026ndash;42 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e (Maitlo et al., 2002). This variation may be ascribed to disparities in soil TN content. A meta-analysis of global N mineralization conducted by Li et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) demonstrated that soil TN has a significant impact on soil N mineralization via exerting influence on microbial biomass. The soil TN content at this research site was measured to be 1.77\u0026thinsp;\u0026plusmn;\u0026thinsp;0.157 g\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e. This value is within the established range of TN values for agricultural soils in eastern Canada, which is reported to be between 1-1.95 g\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e (Wu et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Also, the TN content at this site was found to be greater than the range of TN values reported for maize land soils in central China, which ranged between 0.62\u0026ndash;1.09 g\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e as documented by Maitlo et al. (2002).\u003c/p\u003e \u003cp\u003eThe response of N mineralization to different biochar application rates as well as different soil amendment combinations was not significant. This suggests that the organic nitrogen in biochar is sufficiently stable as to not affect soil N mineralization in the short term (6 months, December 2022 to April 2023). Our findings align with those of Dempster et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Indeed, the relationship between biochar and soil N mineralization is intricate. Past studies have documented instances of either stimulation, reduction, or no substantial effect on nitrogen mineralization (Kolb, 2009; Dempster et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Tammeorg et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Lentz et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Biochar-induced inhibition of N mineralization has been attributed to the high C:N ratio of biochar, leading to N immobilization by soil microbes (Tammeorg et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). On the other hand, the enhancement of N mineralization by biochar has been linked to the adsorption of phenolics from the soil environment by the biochar (Kolb, 2009).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eEffect of biochar application on soil nitrogen exposure\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eSoil N exposure, in the form of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N, represents the available N content in the soil that could be readily taken up by plants and/or be lost to the environment (Burton et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2008b\u003c/span\u003e; Burton \u0026amp; Zebarth, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Hence, N exposure serves as an indicator of soil fertility and soil health. Soil N exposure can be employed to indicate the quantity of available N supplied by the soil over time, reflecting the potential for plant uptake of N and/or the susceptibility of N losses to the environment.\u003c/p\u003e \u003cp\u003eCompared to the absence of biochar, the AE rose by 0.07 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, whereas NE fell by 0.90 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e in the low biochar treatment (B10). This implies the reduction in soil NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e availability in the low biochar treatment and a reduced risk of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e loss. The observation aligns with the research findings reported by Hu et al. (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). A low corn stover biochar rate has been shown partially offset the loss of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e by inhibiting nitrification (Hu et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In a pot experiment, Yao et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) revealed that low biochar rate decreased nitrification by suppressing the growth and reproduction of ammonia-oxidizing archaea and bacteria (AOA \u0026amp; AOB). The conclusion was substantiated by the observation that applying a low biochar rate resulted in a decrease in the soil NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N to NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N ratio from 9.9 to 5.89.\u003c/p\u003e \u003cp\u003eThe present study observed that the application of biochar at moderate rates (20 and 30 t\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e) resulted in a simultaneous elevation AE and NE, hence enhancing soil N availability. These findings are consistent with previous investigations conducted by Clough and Condron (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) as well as Nguyen et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The altered soil physicochemical qualities and N cycle may be attributed to the distinctive features of biochar (Liu et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Ibrahim et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). According to Nartey and Zhao (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), biochar has a substantial surface area and possesses a notable adsorption capability. The use of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e adsorbed by biochar has been found to be beneficial for plants, resulting from a delayed production of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and mitigating leaching loss (Zheng et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Furthermore, the application of alkaline biochar has been found to have the potential to elevate the pH levels and enhance the moisture content of acidic soil. Additionally, it has been seen to stimulate N mineralization and facilitate various N cycle activities within the soil (Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), although this was not observed in the current study.\u003c/p\u003e \u003cp\u003eUpon reaching an application rate of 40 t\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e, the NE in the soil declined, resulting in a reduction in the availability of soil effective N. The findings indicated the presence of a threshold in the impact of biochar application, beyond which the promotion of soil N supply ceased to exhibit further enhancement. The explanation is that excessive biochar application reduces soil NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e due to a higher C:N ratio which shifts the soil microbial activity from N mineralization to N immobilization (Kelly et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Additionally, the larger amount of biochar would lead led to a greater adsorption of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, hence diminishing the mobility of inorganic N in the soil (Clough et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eBased on the findings, varying rates of biochar application did not have a statistically significant impact soil N mineralization and BNA in the first year of study. This result suggests that the increased soil N exposure resulting from biochar application does not originate from organic N mineralization. Instead, it is ascribed to the adsorption characteristics of biochar, which enhance the retention of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eEffect of soil amendment on the temporal pattern in soil nitrogen dynamics\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eTea plantations commonly adopt a split fertilizer application strategy, with the practice of early-summer supplementary fertilization being widely adopted. This study presents findings on the temporal pattern of soil mineral N flux in relation to various fertilizer combination schemes after the application of supplementary fertilization throughout the summer season.\u003c/p\u003e \u003cp\u003eBiochar exhibits a greater ability to sorb positively charged NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N than negatively charged NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N (Zhang et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). This explains the significant variation seen in AE across various fertilizer combinations, while NE was not significantly impacted. The AE tended to increase with the increase of biochar application (Bio\u0026thinsp;\u0026gt;\u0026thinsp;BF\u0026thinsp;\u0026gt;\u0026thinsp;OF). In comparison to the treatment with solely organic fertilizer, the AE was 313% and 94% higher in the treatments receiving biochar alone or a biochar-based organic fertilizer, respectively. It is noteworthy to mention that there was no significant difference observed in the AE between B00 (no biochar \u0026amp; no fertilizer) and OF, suggesting NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N loss occurred. The application of biochar has been shown to be an effective strategy for mitigating the loss of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and promoting its retention within the soil (Cai et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Furthermore, despite the absence of a statistical difference in soil NE across the various treatments, the biochar-based organic fertilizer treatments exhibited a wide range of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N levels, spanning from 7.74 to 21.45 mg N\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e with a CV of 47.27%. This suggests the potential of applying biochar-based organic fertilizer as an approach to improve soil available N.\u003c/p\u003e \u003cp\u003eSoil available N peaked in the second week after fertilizer application. During week 3\u0026amp;4, the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N flux was seen to decrease by over 80% as a result of processes such as plant uptake, nitrification, and losses. Similarly, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N flux was reduced by an average of 82% after week 2. Nevertheless, the decrease in NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N exhibited a noticeable delay and did not reach its low point until the sixth week after the application of fertilizer. The observed variations were ascribed to the adsorption features of biochar. It was shown that the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N flux of all treatments exhibited a reduction of over 80% within 2\u0026ndash;4 weeks after fertilization. However, the decrease of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N flux lagged behind NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N by about 2 weeks, indicating that NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N adsorbed by the biochar could still be converted via nitrification. Unfortunately, the available evidence does not provide a conclusive explanation for the observed phenomenon of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e-N and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N fluxes experiencing a slight rebound subsequent to the low point. Consequently, further studies are required to investigate this matter thoroughly.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe two-week aerobic incubation method provides a straightforward and efficient means of evaluating tea plantation soil N supply capacity, and the IEM technique successfully monitored soil N dynamics. The acidic topsoil of tea plantations in Fujian Province could provide 48 mg N\u0026middot;kg\u003csup\u003e-1\u003c/sup\u003e of soil inorganic N for a 130-days growing period, and the N supply capacity of the deeper soil was only 8% lower than that of the topsoil. The use of biochar has led to a notable increase in the availability of N in soil. However, this increase is not linear, as it first rises and afterwards declines. After careful analysis, it has been determined that the optimal amount of biochar to be applied is 20 t\u0026middot;ha\u003csup\u003e-1\u003c/sup\u003e. This decision takes into consideration both the soil available N and the associated cost of application. Furthermore, the utilization of biochar has been shown to be a viable strategy for mitigating the loss of inorganic fertilizer and enhancing its retention capacity. The utilization of biochar-based organic fertilizer has promising prospects in enhancing the availability of N in soil. The primary impact of the supplementary fertilizers applied in the summer season was predominantly observed within a period of 6\u0026ndash;8 weeks after application, with particular emphasis on the second week.\u003c/p\u003e \u003cp\u003eThe study was unable to distinguish between different pathways of soil N outputs and did not explore the impact of different fertilizer combinations on crop development due to constraints imposed by the experimental circumstances. Further exploration of these two features can be undertaken in future research endeavours.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eAE Ammonium Exposure\u003c/p\u003e\n\u003cp\u003eAEM Anion Exchange Membrane\u003c/p\u003e\n\u003cp\u003eANOVA Analysis of Variance\u003c/p\u003e\n\u003cp\u003eBF Biochar-Based Organic Fertilizer\u003c/p\u003e\n\u003cp\u003eCEC Cation Exchange Capacity\u003c/p\u003e\n\u003cp\u003eCEM Cation Exchange Membrane\u003c/p\u003e\n\u003cp\u003eCV Coefficient of Variation\u003c/p\u003e\n\u003cp\u003eIEM Ion Exchange Membrane\u003c/p\u003e\n\u003cp\u003eN Nitrogen\u003c/p\u003e\n\u003cp\u003eNE Nitrate Exposure\u003c/p\u003e\n\u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e Ammonium\u003c/p\u003e\n\u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e Ammonium Nitrate\u003c/p\u003e\n\u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e Nitrate\u003c/p\u003e\n\u003cp\u003eOF Organic Fertilizers\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cem\u003eFundings\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by Central Government Guided Local Science and Technology Development projects under Grant No. 2021L3021, and Science and Technology Project of Fujian Academy of Agricultural Sciences under Grant No. XTCXGC2021010. The work was also supported by\u0026nbsp;a Canadian Natural Sciences and Engineering Council CREATE-CSS grant.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eCompeting Interests\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAll authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAuthor Contributions\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation and data collection were performed by Zhang B, Liu C, Li Q, Ye J, and Lin Y. Data analysis was carried out by Zhang B and Liu C. The first draft of the manuscript was written by Zhang B. Wang Y and Burton D provided technical support and supervision throughout the research process. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was financially supported by Central Government Guided Local Science and Technology Development projects under Grant No. 2021L3021, and Science and Technology Project of Fujian Academy of Agricultural Sciences under Grant No. XTCXGC2021010. The work was also supported by a Canadian Natural Sciences and Engineering Council CREATE-CSS grant.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBurton DL, \u0026amp; Zebarth BJ (2014) Nitrate Exposure: A metric to describe the influence of soil NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e on N\u003csub\u003e2\u003c/sub\u003eO emissions. In 20th World Congress of Soil Science 438-438. http://dx.doi.org/10.13140/2.1.2941.9200\u003c/li\u003e\n\u003cli\u003eBurton DL, Li X, \u0026amp; Grant CA (2008a) Influence of fertilizer nitrogen source and management practice on N\u003csub\u003e2\u003c/sub\u003eO emissions from two Black Chernozemic soils. Canadian Journal of Soil Science 88:219\u0026ndash;227. https://doi.org/10.4141/CJSS06020\u003c/li\u003e\n\u003cli\u003eBurton DL, Zebarth BJ, Gillam KM, \u0026amp; MacLeod JA (2008b) Effect of split application of fertilizer nitrogen on N\u003csub\u003e2\u003c/sub\u003eO emissions from potatoes. Canadian Journal of Soil Science 88:229-239. https://doi.org/10.4141/CJSS06007\u003c/li\u003e\n\u003cli\u003eCai Y, Qi H, Liu Y, \u0026amp; He X (2016) Sorption/desorption behavior and mechanism of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e by biochar as a nitrogen fertilizer sustained-release material. 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Environmental pollution 227:98-115. https://doi.org/10.1016/j.envpol.2017.04.032\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Biochar, Nitrogen Dynamics, Tea Plantation, Soil Nitrogen Supply, Nitrogen exposure, Biochar-Based Organic Fertilizer.","lastPublishedDoi":"10.21203/rs.3.rs-3991015/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3991015/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground and Aims:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTea plantations are frequently given substantial quantities of nitrogen fertilizers. However, there is the potential for considerable nitrogen loss to occur. This study assesses the nitrogen retention of acidic tea plantation’s soil and the role of biochar in improving nitrogen dynamics, highlighting the need for innovative technologies to streamline and enhance nitrogen supply management.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAdopting a modified two-week aerobic incubation and ion-exchange membrane technology, this research offers a novel approach to evaluate soil nitrogen supply and to monitor the nitrogen dynamics of tea plantation soil following early-summer supplementary fertilization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study revealed that the surface soil of tea plantation had the ability to provide 48 mg N·kg\u003csup\u003e-1\u003c/sup\u003e soil as inorganic nitrogen for 130 days. The utilization of a small amount of biochar (10 t·ha\u003csup\u003e-1\u003c/sup\u003e) had no impact on the soil's effective nitrogen availability. Nonetheless, the application of biochar at rates of 20 and 30 t·ha\u003csup\u003e-1\u003c/sup\u003e resulted in a significant enhancement in soil effective nitrogen availability as measured using ion exchange membranes, with an increase of 65%–81%. Furthermore, the utilization of biochar-based organic fertilizers, when used at appropriate rates, has the potential to enhance the availability of nitrogen in the soil, thereby increasing its effectiveness.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study's findings underscore the efficacy of the employed methodologies in capturing the nuanced impact of biochar on nitrogen retention and availability in tea plantation soils. The use of aerobic incubation and ion-exchange membrane technology has proven effective in elucidating the potential of biochar to significantly improve nitrogen dynamics.\u003c/p\u003e","manuscriptTitle":"Effect of biochar and its combined fertilizers on the dynamics of soil nitrogen supply in tea plantation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-22 13:47:36","doi":"10.21203/rs.3.rs-3991015/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"588376ca-fa8e-4890-bea3-85a29e132502","owner":[],"postedDate":"March 22nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-10-13T11:29:31+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-22 13:47:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3991015","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3991015","identity":"rs-3991015","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}