Seasonal compound climate extremes restructure microbial drivers of wetland litter decomposition

Seasonal compound climate extremes restructure microbial drivers of wetland litter decomposition | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 16 October 2025 V1 Latest version Share on Seasonal compound climate extremes restructure microbial drivers of wetland litter decomposition Authors : Huizhu Li , Zhenyu Wang , Xueke Wang , Wei Liu , Jiamin Shi , Ming Jiang , Guangxuan Han , Liming Yan 0009-0003-7712-1863 [email protected] , and Jianyang Xia 0000-0001-5923-6665 Authors Info & Affiliations https://doi.org/10.22541/au.176063642.23613716/v1 201 views 121 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Seasonal compound climate extremes (SCEs), marked by asynchronous warming and drying, are emerging as novel drivers of litter decomposition in coastal wetlands. We hypothesize that SCEs alter this multi-phase process, which spans rapid leaching over weeks, microbial processing over months, and refractory breakdown over years, by disrupting associated microbial dynamics. Through a 730-day decomposition experiment embedded in a seven-year SCEs experiment, we found that winter–spring warming and summer–autumn rainfall reduction slowed winter–spring decay (absolute change: -7.13% and -3.58%, respectively) but enhanced summer–autumn decay (+7.54% and +5.77%, respectively), yielding seasonal compensation with no net annual mass loss. This reversal was driven by microbial functional succession restructuring in which bacterial complementarity disruption initially slowed winter-spring decay, whereas salinity-tolerant fungal recruitment later stimulated summer-autumn decomposition. The resulting functional succession redistributed carbon turnover temporally without altering annual loss, demonstrating a compensatory mechanism that stabilizes coastal carbon cycling under SCEs. Seasonal compound climate extremes restructure microbial drivers of wetland litter decomposition Huizhu Li 1 , Zhenyu Wang 1 , Xueke Wang 1 , Wei Liu 1 , Jiamin Shi 1 , Ming Jiang 1 , Guangxuan Han 2 , Liming Yan 1,* , Jianyang Xia 1,* 1 Research Center for Global Change and Ecological Forecasting, School of Ecological and Environmental Sciences, Institute of Eco-Chongming, East China Normal University, Shanghai 200241, China 2 Yellow River Delta Field Observation and Research Station of Coastal Wetland Ecosystem, Chinese Academy of Sciences, Yantai 264000, China * Corresponding authors Email: [email protected] , [email protected] Article type: Research Article Authur contributions: J.X. and G.H. initiated the idea of the study; Y.L., Z.W. and M.J. designed the study; Z.W., H.L., X.W., W.L. and J.S. collected the data; H.L. performed statistical analysis; J.X. and Y.L. contributed to concept polishing and critical revision of the manuscript; H.L. wrote the first draft and figures and all authors contributed substantially to the revisions. Abstract Seasonal compound climate extremes (SCEs), marked by asynchronous warming and drying, are emerging as novel drivers of litter decomposition in coastal wetlands. We hypothesize that SCEs alter this multi-phase process, which spans rapid leaching over weeks, microbial processing over months, and refractory breakdown over years, by disrupting associated microbial dynamics. Through a 730-day decomposition experiment embedded in a seven-year SCEs experiment, we found that winter–spring warming and summer–autumn rainfall reduction slowed winter–spring decay (absolute change: -7.13% and -3.58%, respectively) but enhanced summer–autumn decay (+7.54% and +5.77%, respectively), yielding seasonal compensation with no net annual mass loss. This reversal was driven by microbial functional succession restructuring in which bacterial complementarity disruption initially slowed winter-spring decay, whereas salinity-tolerant fungal recruitment later stimulated summer-autumn decomposition. The resulting functional succession redistributed carbon turnover temporally without altering annual loss, demonstrating a compensatory mechanism that stabilizes coastal carbon cycling under SCEs. Key words: blue carbon; coastal wetlands; decomposition; climate change; litter decomposition; microbial community Introduction Coastal ecosystems play an outsized role in global blue carbon storage, with the persistence of this carbon sink largely dependent on the suppression of litter decomposition by saline and anoxic conditions (Hao et al., 2024; Nahlik & Fennessy, 2016; Temmink et al., 2022; Xia et al., 2022). Recently, warmer winters and springs coupled with drier summers and autumns (IPCC, 2023; Piao et al., 2010; Xia et al., 2014) pose a major yet poorly quantified threat to coastal wetlands. A key unsolved question is whether, and by what mechanisms, such seasonal compound climate extremes (SCEs, Zscheischler et al., 2018) alter litter decomposition dynamics in these ecosystems. Litter decomposition is not a uniform process but follows a stage-structured functional succession, shifting from early leaching and copiotrophic bacteria to later reliance on fungi and oligotrophic microbes for the degradation of recalcitrant matter (Bani et al., 2018; Raza et al., 2023). While the temporal progression of microbial metabolic roles in terrestrial ecosystems is well-documented (Purahong et al., 2016; Zheng et al., 2021), its vulnerability to seasonally asynchronous climate stressors remains untested in coastal wetlands. This knowledge gap is critical because climate-driven disruption of these successional dynamics could fundamentally alter the timing of nutrient release and the stabilization of organic carbon, independent of changes in annual mean climate. Coastal wetlands are particularly vulnerable to disruptions from SCEs (Xi et al., 2021). Unlike upland ecosystems, where warming typically accelerates decomposition (Dawson-Glass et al., 2023; Hou et al., 2023; Liu et al., 2017), decomposition in coastal ecosystem is tightly constrained by strong hydrological and salinity feedbacks (Minick et al., 2019; Zhu et al., 2022; Stagg et al., 2018). For example, warming can enhance evaporation and increase porewater salinity and osmotic stress (Dai et al., 2018; Sun et al., 2021), while reduced rainfall can suppress microbial activity and constrain nutrient leaching (Ochoa-Hueso et al., 2018; Schimel, 2018; Zia et al., 2021). These abiotic shifts frequently trigger microbial community turnover, where stress-tolerant generalists may displace functionally specialized decomposers (Chen et al., 2024; Hicks et al., 2025; Veach & Zeglin, 2020). This vulnerability is amplified in systems with annual aboveground senescence, where seasonal litter pulses are tightly synchronized with microbial activity and plant phenology (Dai et al., 2022; Kim et al., 2022; Liao et al., 2023). A SCEs-induced decoupling of resource supply and microbial processing could therefore have cascading effects on nutrient cycling and persistence of organic carbon. To address this knowledge gap, we conducted a field manipulation experiment explicitly designed to simulate SCEs (i.e., winter–spring climate warming and summer–autumn rainfall reduction) in the Yellow River Delta of China, a temperate estuarine wetland of global conservation significance (Ni et al., 2025). We deliberately selected a non-tidal wetland system to isolate the effects of seasonally asymmetric climate change from the overpowering physical forcing of tides, thereby allowing a more straightforward interpretation of direct climate-microbe-decomposition interactions. By integrating measurements of mass loss, litter chemistry, and soil microbial community dynamics, we tested two hypotheses: (1) winter–spring warming accelerates early-stage decomposition via microbial activation but suppresses later decay through substrate depletion; and (2) summer–autumn drought increases salinity stress, inhibits microbial degradation, and promotes short-term carbon retention. More broadly, we assessed whether SCEs disrupt the natural trajectory of microbial functional succession, a shift with critical implications for the long-term carbon storage function in coastal wetlands. Material and methods Study site The study was conducted at the Yellow River Delta Coastal Wetland Ecological Research Station, Shandong, China (37°45′50″N, 118°59′24″E). This region is characterized by sandy clay loam soils and a warm-temperate semi-humid monsoon climate. The mean annual temperature is 12.5 °C, and annual precipitation ranges from 550 to 600 mm, with approximately 70% of the total occurring during July–September (Sun et al., 2021). The flat terrain maintains a shallow water table (mean depth: 1.1 m), leading to waterlogged, saline soils and periodic surface ponding following heavy rainfall events (Fan et al., 2012). The vegetation is composed of three plant functional groups: grasses (e.g., Phragmites australis ), forbs (e.g., Suaeda glauca ), and semi-shrubs (e.g., Apocynum venetum ) (Han et al., 2015). Aboveground biomass senesces annually, serving as the primary source of litter input each autumn. Climate manipulation experiment To simulate seasonal compound climate extremes (SCEs), a field manipulation experiment was established in May 2017 using a 4×4 Latin square design (with row and column positions included as random effects in subsequent analyses to control for spatial heterogeneity) with 2-m buffer zones between plots. Pre-treatment surveys confirmed no significant baseline differences in vegetation or soil properties (Tukey HSD, P > 0.05). Four treatments were applied: control (C), winter-spring warming (W, December–May), summer-autumn rainfall reduction (RR, June–November), and their combination (WR). Warming was achieved via infrared radiators (LPR2420/1500; 2000 W) suspended 2 m above ground, increasing the mean annual soil temperature at a 10-cm depth by 1.4 °C (2018–2024). Rainfall reduction was implemented using transparent V-channel polycarbonate panels (10 cm width; 10 cm spacing), which intercept 50% of precipitation. The control plots were equipped with inactive heaters and inverted V-shelters to account for any potential shading or structural artifacts associated with the experimental apparatus. Litter decomposition experiment Litter decomposition was monitored over two years (December 2021–December 2023). Senesced leaves and stems of P. australis , S. glauca , and A. venetum were collected outside the experimental plots, oven-dried at 65 °C, and placed in litterbags (10×15 cm; 2-mm mesh; 2.00 ± 0.01 g dry mass per bag). Bags were deployed in December 2021 and retrieved sequentially after 77, 128, 185, 282, 365, 580, and 730 days (n = 672; 7 replicates per species per treatment). The residual mass was gently rinsed, dried at 65 °C to a constant mass (with a precision of 0.0001g), and weighed to determine the mass loss. Litter mass loss (%) and decomposition constants ( k , yr -1 ) were calculated using the following equations (Olson, 1963): \(\text{Mass\ loss\ }\left(\%\right)=\frac{M_{0}-M_{t}}{M_{0}}\ \times 100\%\ \ \) (1) \(\frac{M_{t}}{M_{0}}=e^{-kt}\) (2) where M 0 is the initial dry mass and Mt is the residual mass at time t . For seasonal analyses, the first year was divided into winter–spring (0–185 days) and summer–autumn (186–365 days). Analyses focused on the first year to capture the most dynamic phase of decomposition and to avoid the diminished variability associated with late-phase stabilization before late-phase stabilization. Microclimate and biochemical monitoring Soil temperature (ST), volumetric moisture (SM), and electrical conductivity (EC) were recorded hourly at 10-cm depth using Hydra Probe II sensors (Campbell Sci, USA) connected to a CR1000 datalogger. Daily air temperature and precipitation data were obtained from an on-site eddy covariance tower. To characterize the soil environmental conditions for the winter-spring and summer-autumn stages, the mean values for each year, from January to June and from July to December, were used, respectively. Litter carbon (C) and nitrogen (N) contents were determined using an elemental analyzer (Vario EL cube; Germany), and phosphorus (P) content was analyzed with a flow analyzer (LACHAT QC8500; France) after H₂SO₄–H₂O₂ digestion. Lignin and cellulose contents were measured using spectrophotometric quantification (UV1800; Japan) after acid detergent fiber extraction. Topsoil (0–10 cm) was sampled twice a year (May and October). Microbial biomass C and N (MBC, MBN) were quantified by the chloroform fumigation–extraction method (24 h; Vance et al., 1987) using TOC analysis with a conversion factor ( K = 0.45). Bacterial and fungal communities were targeted by sequencing the 16S rRNA V4 region (primers 515F/806R) and the ITS2 region (primers ITS3/ITS4), respectively. Amplicon sequence variants (ASVs) were inferred using the DADA2 pipeline within QIIME 2. Following the removal of chimeric and non-target sequences (e.g., chloroplasts and mitochondria), taxonomic classification was performed against the SILVA 138.1 database for bacteria and the UNITE database for fungi. Alpha diversity indices (Chao1 and Simpson indices) were calculated for each sample. To characterize the relatively stable, long-term microbial community states shaped by the chronic climate manipulations rather than transient annual fluctuations, bacterial and fungal diversity indices (Chao1 richness and Simpson dominance) and the relative abundance of major taxa (defined as bacterial phyla and fungal orders with a relative abundance > 0.1%) were averaged across the 2018–2024 period to provide robust microbial trait baselines for each plot. Statistical analysis Linear mixed-effects models ( lme4 package) (Bates et al., 2015) were used to test the main and interactive effects of warming (W) and rainfall reduction (RR) on soil parameters (treating time and blocks as random effects) and litter decomposition (treating blocks as random effects). Latin square row and column positions and time were included as random effects to account for spatial heterogeneity and temporal dependence. To identify the mechanistic drivers of litter mass loss linear mixed-effects models were fitted, with litter species and organ included as random effects. Fixed predictors included soil properties (ST, SM, EC), litter traits (C:N:P ratios), and microbial attributes (diversity indices, MBC, MBN). A Principal Component Analysis (PCA) was used to select the single best indicator representing litter quality from the initial quality parameters (C:N ratio, C:P ratio, cellulose, and lignin). Pearson correlations were used to assess pairwise relationships. Predictor importance was quantified by standardizing coefficients and partitioning their relative contributions to variance explained (Canessa et al., 2022). Marginal R² (R² m ) represented variance explained by fixed factors, while conditional R² (R² c ) reflected variance explained by both fixed and random factors combined (Nakagawa & Schielzeth, 2013). All statistical analyses were performed using the R statistical environment, version 4.5.0 (R Core Team, 2025). Seasonal patterns of litter decomposition Litter decomposition displayed a clear three-phase pattern: a substantial initial rapid mass loss (26.3 ± 5.6% during 0–185 days), followed by a period of continued high mass loss (accumulating an additional 57.6 ± 8.4% by day 365), and eventual stabilization with a cumulative loss of 61.1 ± 8.3% by day 730 (Fig. 1A-F). Mass loss during the late stage was minimal, with only 4.0% occurring between 365–529 days and 3.0% between 529–730 days. Accordingly, subsequent analyses focused on the first year (0-365 days), when decomposition was most pronounced and treatment effects were most evident. Leaves decomposed significantly faster than stems across all species ( P. australis, S. glauca, A. venetum ; organ effect: P < 0.001; Table S1). Decomposition rates also varied significantly among species ( P < 0.001; Table S1), ranked as S. glauca having the highest rates (leaf : 1.88 yr -1 ; stem : 0.59 yr -1 ), followed by P. australis (leaf : 1.09 yr -1 ; stem : 0.49 yr -1 ) and A. venetum (leaf: 0.90 yr -1 ; stem : 0.48 yr -1 ) (Fig. 1G). Importantly, winter–spring warming (W) and summer-autumn rainfall reduction (RR) exerted consistent main effects across species and organs, with no significant treatment × species or treatment × organ interactions ( P > 0.05; Table S1). Contrary to expectations, both W and RR suppressed mass loss during the first winter–spring period (0-185 days) by 7.13% (range: -10.28% to +0.36%; P < 0.01) and 3.58% (range: -6.35% to +0.80%; P < 0.05), respectively (Fig. 2). In contrast, both treatments enhanced decomposition in the subsequent summer–autumn period (W: +7.54%, range: -1.35% to +19.39%, P < 0.01; RR: +5.77%, range: -3.89% to +17.03%, P 0.05; Table S1), indicating a clear seasonal compensation. Drivers of decomposition across seasons Climatic treatments significantly altered soil conditions (Fig. 3). In winter–spring, winter–spring warming (W) increased soil temperature (ST; +0.95 ± 0.13 °C, P < 0.001), whereas summer-autumn rainfall reduction (RR) elevated soil moisture (SM; +0.95 ± 0.37 vol.%, P < 0.01). In summer–autumn, RR increased both SM (+3.00 ± 0.63 vol.%, P < 0.001) and soil electrical conductivity (EC, +0.08 ± 0.00 S m⁻¹; P < 0.01), while also reducing litter C:P ratios (-139.78 ± 93.14, P < 0.05). Biologically, W during winter–spring increased microbial biomass carbon and nitrogen content (MBC: +82.55 ± 42.9 mg kg -1 , P < 0.05; MBN: +82.55 ± 42.9 mg kg -1 , P < 0.05) but reduced bacterial Chao1 richness (Chao b : -139.28 ± 34.51 OTUs, P < 0.05). In contrast, RR during summer–autumn reduced MBC (-26.65 ± 15.46 mg kg -1 ( P < 0.05). Fungal Simpson diversity (Simpson f ) increased under both W (+0.05 ± 0.01, P < 0.05) and RR (+0.09 ± 0.01, P < 0.001) in summer-autumn. Mixed-effects models revealed a clear seasonal shift in decomposition drivers (Fig.4). The litter C:N ratio was selected through PCA analysis to represent litter quality, as detailed in Fig. S1. In winter–spring, litter type (included as a random effect) explained most of the variance (conditional R 2 [R 2 c ] ≈0.89–0.90). Among the fixed factors, ST (marginal R 2 [R 2 m ] = 0.014), EC (R 2 m =0.007), MBC (R 2 m =0.007), and Chao b ( R 2 m =0.013) were modest predictors (all P < 0.05, Fig. 4A). Variance partitioning attributed 50.1% of explained variance to litter properties, 38.2% to microbial traits, and 11.6% to climate factors with only Chao b being statistically significant ( P < 0.05, Fig. 4B). In summer–autumn, the explanatory power of litter identity diminished (R 2 c ≈0.65–0.73). Key fixed predictors were ST (R 2 m =0.017), Simpson f (R 2 m =0.027), and litter quality (C:N ratio: R 2 m =0.158; C:P ratio: R 2 m =0.144) (Fig. 4C). Microbial traits (43.1%) explained a similar proportion of variance as litter traits (42.7%), with Simpson f and litter C:N ratio being the dominant significant factors ( P < 0.05; Fig 4D). Microbial community responses Microbial composition shifted markedly with season and treatment (Fig. 5). In winter–spring, winter–spring warming (W) enriched the relative abundance of α-proteobacteria (+1.27 ± 0.53% absolute, P < 0.001), Bacteroidota (+0.74 ± 0.66%, P < 0.05), and Firmicutes (+0.20 ± 0.16%, P < 0.05), but suppressed Acidobacteria (-2.05 ± 0.62%, P < 0.001) and Planctomycetota (-0.45 ± 0.14%, P < 0.01). W also increased Eurotiales (+0.50 ± 0.32%, P < 0.05) but decreased Hypocreales (-1.93 ± 0.86, P < 0.01). summer-autumn rainfall reduction (RR) mainly reduced γ-Proteobacteria (-1.19 ± 0.43%, P < 0.001). In summer–autumn, α-proteobacteria (W: +1.91 ± 0.42%, P < 0.001; RR: +1.05 ± 0.49, P < 0.05), Chloroflexi (RR: +1.03 ± 0.33%, P < 0.05), and Bacteroidota (W: +1.41 ± 1.04%, P < 0.05; RR: +1.61 ± 1.11%, P < 0.01) increased, while Acidobacteria (W: -0.72 ± 0.46%, P < 0.05; RR: -1.04 ± 0.41%, P < 0.01) and γ-Proteobacteria (RR: -2.84 ± 1.52%, P < 0.001) declined (Table S7). For fungal, Thelebolales (-5.92 ± 3.06%, P < 0.01) reduced under W, while Hypocreales (+2.35 ± 1.06%, P < 0.01), Pleosporales (+1.13 ± 0.71%, P < 0.01), and Lobulomycetales (+0.61 ± 0.44%, P < 0.05) increased under RR. Discussion 1. Seasonal reversal of decomposition under SCEs Our study reveals a seasonal reversal in decomposition dynamics, in which winter–spring warming (W) and summer–autumn rainfall reduction (RR) suppress early-stage decay but enhance late-stage decomposition. This counterintuitive outcome challenges the prevailing paradigm of uniformly accelerated decomposition under warming and suppression under drought (Chen et al., 2024; Schimel, 2018; Zhao et al., 2025) and establishes that SCEs redistribute rather than accelerates litter decay in coastal wetlands. Contrary to patterns widely observed in upland systems (Dawson-Glass et al., 2023; Liu et al., 2017; Santonja et al., 2015), both W and RR reduced litter mass loss by 7.13% and 3.58%, respectively, in winter–spring, yet stimulated it by 7.54% and 5.77%, respectively, in the subsequent summer–autumn. This seasonal compensation resulted in no net changes in annual decomposition (Table S1), a phenomenon that would be obscured in conventional annual-scale studies. Mechanistically, this reversal aligns with stage-structured decomposition theory (Berg, 2014), where early decay is primarily constrained by abiotic stress that impairs copiotrophic communities, whereas late-phase stimulation suggests a release from these constraints or the recruitment of oligotrophic specialists (Bani et al., 2018; Purahong et al., 2016). Our seasonally targeted manipulations reveal that the classic trajectory of microbial functional succession can be decoupled from its typical seasonal progression by asymmetric climate forcing, likely mediated through environmental filtering. A fundamental finding is that SCEs drive a seasonal shift in microbial regulation. While litter quality consistently explained the largest share of variance (49.1% in winter–spring; 42.7% in summer–autumn), microbial drivers shifted from bacterial dominance in winter–spring to fungal mediation in summer–autumn. This demonstrates that the relative importance of microbial control is not static but somewhat seasonally partitioned, necessitating a seasonally resolved perspective to predict coastal carbon cycles under such climate regimes. 2. Functional bottlenecks underlie early-stage suppression We found that winter-spring suppression was not simply a reduction in microbial activity but reflected a collapse in functional complementarity, mediated through distinct pathways under W and RR. This loss of metabolic partnership formed the fundamental bottleneck constraining early decay. Under W, elevated temperature and advanced vegetation phenology (-6.41 days) (Jiang, 2023) likely stimulated root activity and increased labile carbon inputs into the rhizosphere (Ren et al., 2024; Wang et al., 2021; Xiong et al., 2019). This resource pulse, combined with direct thermal effects, favored taxa proficient in utilizing labile organic carbon (e.g., Bacteroidota: +0.74%, Rhodobacterales within Alphaproteobacteria: +0.67%). This shift coincided with a reduced abundance of taxa often associated with oligotrophic lifestyles and the degradation of more recalcitrant carbon degradation (e.g., Acidobacteria: -2.05%, Planctomycetota: -0.45%) (Fu et al., 2022; Yang et al., 2023; Zhao et al., 2024), likely driven by competitive exclusion and their documented higher thermal sensitivity (Oliverio et al., 2017; Wang & Kuzyakov, 2024; Zhao et al., 2024), narrowing the spectrum of functional roles available for decomposition (Wang et al., 2025). Under RR, suppression arose primarily from abiotic stress. Although soil moisture increased, concurrent rises in salinity and oxygen depletion (Chu et al., 2018; Perri et al., 2022) selectively inhibited aerobic labile organic carbon degraders, most notably γ-Proteobacteria (−1.19%, P < 0.01). By contrast, Acidobacteria, which were strongly suppressed under W, were less affected under RR, consistent with their tolerance to suboxic and mildly saline conditions (Eichorst et al., 2018). However, their persistence alone was insufficient to sustain decomposition, since their complementary partners were simultaneously impaired. Critically, despite their differing in the initial triggers, both W and RR converged on the exact outcome of disrupting the synergistic relationship between labile organic and recalcitrant carbon decomposers. Our results demonstrate that decomposition efficiency depends less on total microbial biomass than on the preservation of functional complementarity. The loss of this complementarity under asymmetric climate stressors creates a bottleneck that limits the initiation of decomposition. 3. Fungal recruitment drives late-stage recovery The recovery of decomposition in summer-autumn, despite persistent salinity stress, was primarily mediated by a restructuring of the fungal community, which compensated for the persistent bacterial bottleneck. The increased fungal diversity under both W (+0.05, P correlation with salinity, suggest that salinity functioned as an environmental filter (R² = 0.30, P < 0.05; Fig. S4) (Liao et al., 2023), promoting stress-tolerant decomposers and broadening the community’s functional potential. Bacterial communities largely maintained winter-spring trends, with the continued enrichment of LOC utilizers (e.g., Bacteroidota: +1.41 to +2.66%, Rhodobacterales: +1.91% to +2.05%) and the decline of RC specialists (e.g. Acidobacteria: -0.72 to -2.22%) (all P could not reverse the early-phase functional imbalance, thereby highlighting the essential role of fungi in driving recovery. Fungal responses, differed across treatments. Under RR, responses were broad, with enrichment of Ascomycetes orders often harboring taxa with strong lignocellulolytic potential (e.g., Hypocreales: +2.35%; Pleosporales: +1.13 %) (Herzog et al., 2019; Qiu et al., 2023). Under W, changes were subtler but still functionally relevant, characterized by a decline in the less-specialized Thelebolales (−5.92%, P previously low-abundance Agaricales (absolute: +0.30 ± 0.12%; relative: +221%, P < 0.05) (Voříšková & Baldrian, 2013), a Basidiomycete lineage that included many efficient lignin decomposers. Together, these shifts support the interpretation that the salinity-filtered fungal community provided the functional compensation that bacteria lacked. By metabolizing the litter retained from the suppressed winter-spring phase, fungi drove the observed recovery of decomposition rate in summer-autumn. This functional succession, from bacterial bottlenecks to fungal compensation, illustrates how microbial succession stabilizes ecosystem processes under SCEs. Our results demonstrate that SCEs reshape the balance of microbial functional roles, with fungi acting as a critical biochemical buffer that maintains carbon turnover under such stress. 4. Ecosystem implications of seasonal compensation The seasonal reversal of decomposition we observed risks decoupling the carbon and nutrient cycles, potentially threatening the nutrient-use efficiency of coastal wetlands. While suppressed winter-spring decay temporarily retained nutrients, accelerated summer-autumn decomposition and mineralized them during a period of peak microbial respiration, likely elevating CO 2 emissions (Possinger et al., 2025). Although we detected no immediate plant nutrient limitation (unchanged δ¹⁵N; Table S4, Fig. S3), the temporal mismatch between microbial nutrient release and plant uptake suggests a potential for progressively weakened ecosystem-scale nutrient retention over time (Broadbent et al., 2024). Critically, the observed seasonal compensation that buffered net carbon loss is likely precarious. Projected intensification of climate extremes could disrupt this balance. If fungal-driven summer-autumn decomposition continues to intensify, it may eventually override winter-spring suppression, potentially weakening the carbon sink function. This risk is compounded by observed shifts in vegetation toward more recalcitrant, high C:N species (e.g., increased P. australis dominance under RR, Fig. S3) (Berg, 2014), which could further challenge microbial decomposition capacity and alter ecosystem stoichiometry. 5. Limitation and concluding synthesis The two-year duration and seasonal resolution of our study, set within a long-term platform, capture key decomposition dynamics processes. This robust design provides confidence in the observed seasonal dynamics, although our mechanistic interpretation primarily relies on microbial composition and diversity data. While we also assessed extracellular enzyme activities (Table S5), their lack of significant correlation with decomposition in this saline wetland system indicates that a longer-term investment in functional potential is a more responsive indicator of progress changes than short-term enzyme expression levels. Resolving direct functional pathways will require further studies employing metatranscriptomic or metaproteomic approaches. Furthermore, the present findings are derived from a non-tidal wetland. Extrapolation to tidal wetlands should be made with caution, as tidal inundation can interact with or modify the mechanisms identified in this study (Peng et al., 2022). However, the consistent responses across multiple plant functional groups and organs, within a system characterized by annual aboveground senescence, support the broader relevance of the seasonally asymmetric decomposition phenomenon and the underlying mechanism in other non-tidal coastal wetlands. Our ecosystem-scale inferences highlight potential risks rather than specific outcomes. The scenarios of long-term C-N decoupling and reduced sink stability should be viewed as plausible under intensified climate extremes and vegetation shifts, rather than inevitable trajectories. In conclusion, our results demonstrate that SCEs reorganize the temporal dynamics of decomposition, with winter–spring suppression linked to bacterial bottlenecks compensated by fungal recovery in summer–autumn. This restructured sequence of microbial drivers underscores the importance of community reassembly in sustaining wetland functions under climate extremes. More broadly, our findings call for Earth system models to incorporate seasonally resolved microbial–vegetation interactions and to account for the dynamic assembly of soil communities under shifting hydroclimatic regimes. Integrating such mechanisms into model frameworks will be critical for improving projections of wetland carbon cycling and the long-term stability of coastal blue carbon under future climate change. Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2022YFF0802100) and the National Natural Science Foundation of China (32325033). References Bani, A., Pioli, S., Ventura, M., Panzacchi, P., Borruso, L., Tognetti, R., et al. (2018). The role of microbial community in the decomposition of leaf litter and deadwood. Applied Soil Ecology , 126 , 75-84. 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J., Pitman, A., et al. (2018). Future climate risk from compound events. Nature Climate Change , 8 (6), 469-477. Supplementary Material File (images.docx) Download 713.95 KB Information & Authors Information Version history V1 Version 1 16 October 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords blue carbon climate change coastal wetlands decomposition litter decomposition microbial community Authors Affiliations Huizhu Li East China Normal University View all articles by this author Zhenyu Wang East China Normal University View all articles by this author Xueke Wang East China Normal University View all articles by this author Wei Liu East China Normal University View all articles by this author Jiamin Shi East China Normal University View all articles by this author Ming Jiang View all articles by this author Guangxuan Han Chinese Academy of Sciences View all articles by this author Liming Yan 0009-0003-7712-1863 [email protected] East China Normal University View all articles by this author Jianyang Xia 0000-0001-5923-6665 East China Normal University View all articles by this author Metrics & Citations Metrics Article Usage 201 views 121 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Huizhu Li, Zhenyu Wang, Xueke Wang, et al. 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