Photodegradation accelerates standing dead litter decomposition in monsoonal mountain grasslands of South America

represent the first proof of photodegradation in a productive grassland in this23 2 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint region, and highlight the fact that photodegradation is not constrained only to arid24 environments. Additionally, this study emphasizes the importance of litter physical traits25 in regulating carbon cycling through plant litter decomposition in terrestrial ecosystems.26 Introduction27 Plant litter decomposition is the process through which plant-derived organic matter is28 broken down into inorganic components. As a result, plant litter decomposition releases29 carbon (C) fixed by plants back to the atmosphere and intervenes in the formation of soil30 organic matter (SOM) (Cotrufo et al., 2015). The main biotic driver of decomposition is31 the metabolic activity of fungi and bacteria (Bradford et al., 2017), but soil fauna can also32 be important (Zanne et al., 2022; Lejoly et al., 2026). The magnitude of biotic33 decomposition is also determined by climate (Gholz et al., 2000) and litter chemistry34 (Cornwell et al., 2008; Wu et al., 2025; Zhang et al., 2008). Litter decomposition thus35 resides at a main intersection between C losses and sequestration in terrestrial ecosystems.36 For instance, rising atmospheric temperatures can increase CO 2 emissions from litter37 decomposition (Hosseiniaghdam et al., 2023), reinforcing the greenhouse effect. Moreover,38 the formation of SOM from decomposing plant litter after decomposition is directly linked39 to soil fertility and soil C sequestration (Cotrufo and Lavallee, 2022). Hence, it is of great40 interest to reach a better understanding of how decomposition affects the terrestrial C41 balance and how this process might be affected by global change.42 A lesser-known driver of litter decomposition in terrestrial ecosystems is photodegradation,43 the photochemical mineralization of organic matter due to exposure to solar radiation44 (Austin and Ballar´ e, 2024; King et al., 2012). It has been identified as important for its45 3 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint control on C release in a number of ecosystems, particularly in arid and semiarid zones46 (Austin and Vivanco, 2006; Berenstecher et al., 2020; Brandt et al., 2007; Day et al., 2007;47 Huang et al., 2017). Specifically, sunlight wavelengths in the range of ultraviolet (UV-B,48 280-315 nm; and UV-A, 315-400 nm) and blue-green (400-550 nm) are largely responsible49 for these reactions (Austin and Ballar´ e, 2010; Brandt et al., 2009; Day and Bliss, 2019).50 These photochemical reactions are possible due to the capacity of compounds in secondary51 cell walls to absorb sunlight, principally lignin, and produce photo-oxidative reactions52 (Austin and Ballar´ e, 2010; Kommedal et al., 2023; Moorhead and Callaghan, 1994).53 Moreover, this process is independent from microbial activity and can release C directly to54 the atmosphere as CO 2, CH4 and CO (Brandt et al., 2010; Lee et al., 2012; Schade et al.,55 2012).56 Photodegradation can also produce transformations in litter that make some carbohydrates57 to be more accessible to microbial consumption (Austin et al., 2016). This, in turn,58 increases litter mass loss as a complementary effect of sunlight called photofacilitation59 (Austin et al., 2016; Gallo et al., 2006; M´ endez et al., 2022). For instance, exposure to solar60 radiation of litter can increaseβ -glucosidase enzymatic activity, an enzyme associated with61 the last step of cellulose degradation (Berenstecher et al., 2020, 2022; M´ endez et al., 2019,62 2022), and phenol-oxidase, associated with the degradation of phenolic compounds of lignin63 (Baker and Allison, 2015; Yao et al., 2022). It has been suggested that lignin degradation64 by sunlight could be responsible for this increase in biotic decomposition, due to the65 transformations of the lignin-cellulose matrix that increases microbial access to66 carbohydrates (Austin and Ballar´ e, 2024; Austin et al., 2016; M´ endez et al., 2022).67 The importance of photodegradation was first appreciated in arid zones (Austin et al.,68 2006; Day et al., 2007; Gallo et al., 2009) and has been shown to be important in a variety69 4 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint of ecosystems, from deserts to arid grasslands and woodlands (Almagro et al., 2015, 2017;70 Baker and Allison, 2015; Brandt et al., 2010; Day et al., 2018; Henry et al., 2008). These71 arid and semiarid ecosystems are characterized by low plant productivity and72 heterogeneous vegetation, which contribute to the interception of solar radiation by plant73 litter. Photodegradation has been evaluated in both monsoonal (Brandt et al., 2010; Day74 et al., 2022; Pancotto et al., 2003) as well as Mediterranean climates (Austin et al., 2006;75 Ruhland and Fraley, 2023; Rutledge et al., 2010) of low mean annual precipitation (MAP).76 These studies have proven that sunlight affects decomposition in both types of climates,77 with divergent patterns at local scales. In Mediterranean climates, the peak of solar78 radiation in summer does not coincide with the rainy season, whereas in monsoonal79 climates peaks in solar radiation and precipitation coincide. This suggests that whether80 peaks in radiation and rains are synchronous or not can determine how abiotic and biotic81 factors influencing decomposition may interact with each other. More recent research on82 photodegradation has expanded to other types of ecosystems including temperate forests,83 tropical ecosystems and agroecosystems (Keiser et al., 2021; Marinho et al., 2020; Wang84 et al., 2021; Cabrera et al., 2026), but the majority of mesic ecosystems have not yet been85 evaluated for the potential importance of solar radiation on C turnover, particularly for the86 relative importance of direct photodegradation and photofacilitation effects (Austin and87 Ballar´ e, 2024).88 Considering grasslands are some of the most productive and widespread ecosystems on89 Earth (Gibson, 2008), it would be important to advance our understanding of how solar90 radiation affects this portion of the terrestrial C balance (Keiser and Nieland, 2025).91 Currently, most studies in grasslands have been set in low-rainfall sites, many of which92 showed marked positive effects of photodegradation (Yang et al., 2025; Butler et al., 2023;93 5 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Wang et al., 2017), but some of them found either neutral or negative effects as well94 (Erdenebileg et al., 2018; Yang et al., 2024; Almagro et al., 2017). However, fewer studies95 have been carried out in more productive grasslands with higher mean annual precipitation96 (MAP) (Brandt et al., 2010; Butler et al., 2023; van Asperen et al., 2015). These97 grasslands are particularly relevant for the question of the importance on photodegradation98 due to the accumulation of very large quantities of standing dead material associated with99 tussock grass formation, also known as marcescence (Mudr´ ak et al., 2023; Sarmiento, 1992)100 . These large amounts of plant litter are exposed to solar radiation for long periods before101 touching the ground. Circling back to seasonality in these grasslands, even fewer studies102 have been done in grasslands with monsoonal climate (Yao et al., 2024). It is worth103 exploring whether in monsoonal grasslands abiotic photodegradation acts during the dry104 winter and photofacilitation begins at the onset of the humid and warm seasons.105 Several field studies have also specifically addressed the effect of litter traits on106 photodegradation and photofacilitation. This is important because part of how sunlight107 impacts decomposition is through modifying litter chemistry and structure and thus108 changing its decomposability (Austin et al., 2016; Foereid et al., 2010; Gallo et al., 2006).109 Most studies have focused on chemical traits like C, N and C fractions like lignin,110 hemicellulose and cellulose, (Ball et al., 2019; Day et al., 2022; Ma et al., 2017; Yang et al.,111 2025). The focus on these traits, specifically on lignin, is because of its importance as a112 control on both biotic decomposition and photodegradation (Austin and Ballar´ e, 2010).113 However, there is a need to expand studies to incorporate other litter traits that might114 potentially be linked to photodegradation, including how those traits change over time. In115 that regard, few physical traits have been measured in photodegradation studies in the116 field, typically SLA/LMA (Wang et al., 2024; Li et al., 2024; Gaxiola and Armesto, 2015),117 6 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint leaf toughness (Araujo et al., 2022; Masubelele and Bond, 2022), water and vapor holding118 capacity (Almagro et al., 2017; Logan et al., 2022), and light-interacting traits (i.e.:119 reflectance, transmittance, and absorptance) (Day and Bliss, 2020; Day et al., 2015;120 Ruhland and Fraley, 2023). There is a clear opportunity for the advancement of121 photodegradation research in how physical litter traits change over time and how this feeds122 back to C release at the ecosystem scale.123 Our objectives in this study were to understand the importance of photodegradation for C124 turnover and to evaluate the spectral dependence on C release from plant litter in a125 monsoonal mountain grassland in central Argentina. Previous studies for the region were126 carried out in grass-dominated ecosystems with similar MAP and mean annual temperature127 (MAT), but with a different rainfall seasonality (i.e. Mediterranean climate; Berenstecher128 et al., 2020; M´ endez et al., 2019). We specifically designed our study to directly address129 the seasonal contributions and the role of dominant species identity in the relative130 importance of sunlight on litter decomposition of standing dead biomass. We hypothesized131 that exposure to sunlight accelerates C cycling throughout the year in this monsoonal132 grassland, and that this relationship is modulated by seasonality. During dry winters,133 sunlight acts predominantly through the abiotic photo-oxidative route. Instead, during the134 warm and humid seasons, direct abiotic photodegradation and biotic decomposition occur135 simultaneously. Additionally, sunlight exposure during the dry season promotes biotic136 decomposition during the following humid season through changes in litter quality137 (photofacilitation). Lastly, photodegradation of the litter produces changes in its physical138 quality over time. We applied a temporally asynchronous design to disentangle the relative139 importance of contrasting seasons in order to test our hypothesis. Only a few studies have140 tried to address directly how marked seasonal differences affect litter decomposition with141 7 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint this design (i.e.: Berenstecher et al., 2020; Li et al., 2024). We found that both seasonality142 and species identity and their associated litter traits were key in defining the importance of143 photodegradation as a control on decomposition in this montane grassland.144 Methods145 Study site146 Our study site was located in a native montane grassland in a plateau known as Pampa de147 Achala located in the C´ ordoba mountain ranges of central Argentina (2000 – 2300 m a.s.l.).148 The mean temperatures of the coldest and warmest months are 5.0 and 11.4 °C,149 respectively. Average mean annual precipitation is 900 mm, highly concentrated between150 October and April, and frosts occur year round (Cingolani et al., 2015). Additionally, fog151 events are common year round and represent an additional water input into the system152 (Poca et al., 2018). Soils are Mollisols derived from granitic rocks and fine texture particles153 originated from wind erosion (Cabido et al., 1987). The landscape is an undulating plain154 with patches of rocky outcrops, Polylepis australis Bitter woodlands, and short-statured155 grazing lawns, all immersed in a tussock grassland matrix with varying degrees of openness156 dominated by Poa stuckertii (Hack.) Parodi and Deyeuxia hieronymi (Hack.) T¨ urpe157 (Cingolani et al., 2014; von M¨ uller et al., 2017; Zeballos et al., 2024). These tussock158 grassland physiognomies are highly productive and generate each year large quantities of159 senescent material during the dry season (fall-winter) that stay attached to the plants for160 extended periods (Pucheta et al., 1998). This implies that a considerable amount of dry161 biomass is exposed to sunlight during an extended period of time, which makes this system162 an ideal laboratory to study the effects of photodegradation. In this system, the relatively163 8 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint high stocking rates and wildfires help maintain the grazing lawns (Cingolani et al., 2013),164 while in zones with lower stocking rates, tall tussock grasses dominate (i.e. P. stuckertii165 and D. hieronymi ). We worked within the limits of Quebrada del Condorito National Park,166 where grazing is used as a management tool with conservation aims and to stop the167 accumulation of excessive amounts of flammable standing dead biomass (Cingolani et al.,168 2014; von M¨ uller et al., 2017). The rangeland where we carried out this study had an169 effective stocking rate for the period 2020-2021 of 0.13 AU ha−1 (Animal Units per hectare)170 (Administraci´ on de Parques Nacionales, 2021).171 Experimental design172 The general experimental design consisted of the decomposition in the field of two173 dominant native grass species, P. stuckertii and D. hieronymi. Leaf litter was decomposed174 in plots covered in the respective species, hanging as if it were standing dead biomass. Each175 species was subjected to three levels of a sunlight treatment to evaluate the importance of176 photodegradation as a driver of litter decomposition in this site. We deployed samples in177 the field at three different dates to study the impact of seasonality on decomposition, and178 we collected the samples from the field on four dates along a 2-year period.179 In more detail, we established 5 pairs of plots on the same hillside with grassland cover180 (Figure 1a) for the evaluation of photodegradation and seasonality on native plant litter181 decomposition (31º 37’ S, 64º 48’ W; 2150 m a.s.l.). Out of each pair of plots one had P.182 stuckertii cover and another one, D. hieronymi cover. Samples of each of the two species183 were later incubated in plots covered in their respective species. Plots were located south184 of rocky pavements to avoid shading from vegetation. We chose this exposure for the plots185 to maximize sunlight exposure in the Southern Hemisphere.186 9 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint In July 2019, we collected standing dead material (hereafter ‘litter’) of P. stuckertii and D.187 hieronymi in the same rangeland where the experiment was done, and in a neighboring188 rangeland. We took the litter to Buenos Aires where we stored it in boxes in a dry and189 cool space in the Instituto de Investigaciones Fisiol´ ogicas y Ecol´ ogicas Vinculadas a la190 Agricultura (IFEVA). We made sure to flip the litter in the boxes periodically to keep it191 aired until processing. We selected senescent blades with the least signs of decomposition.192 We cut blades in 10-16 cm segments and weighed samples of 1.5 g of air-dried litter. We193 preserved 10 samples for measurements of initial litter quality, and the rest were randomly194 assigned to a treatment.195 To evaluate mass loss, we designed envelopes made of hexagonal galvanized wire mesh with196 opening of 13 mm, 20 cm in height and 10 cm in width (Figure 1b). We oriented the197 envelopes on the hangers in a north-south direction. The north side of the envelopes had a198 plastic filter that blocked a specific range of the wavelength in order to create three distinct199 sunlight treatment levels: full radiation (R+), blocked ultraviolet (UV-), and all200 photochemically active radiation blocked (R-). The R+ filter was a 100 µm thickness201 polyethylene with a transmittance from UV-B to the visible spectrum (Appendix S1:202 Figure S1; 280-800 nm; Ever Wear S. A.). The UV- filter blocked radiation in the UV203 range (280-400 nm; Costech, 226 UV). The R- filter blocked radiation from UV to204 blue-green light (280-550 nm; Rosco® Nº135 Deep Golden Amber). This type of filter has205 proven to be efficient in reducing radiation with photo-oxidative effects in several other206 studies (Austin and Ballar´ e, 2010; Brandt et al., 2009; Day and Bliss, 2019). It is worth207 noting that these filters cannot separate the abiotic effects of photodegradation (direct208 photomineralization) from the biotic effects (photofacilitation), nor the deleterious effects209 of sunlight on microbes. Thus, these filters allow us to demonstrate an overall sunlight210 10 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint effect that is the balance of these three processes. We punched holes in the filters and211 covered the southern side of the envelopes with fiberglass mesh of 2 mm openings to allow212 the passage of air and humidity. We added a polyester gauze pocket to the bottom of the213 envelopes to avoid fragmentation and loss from the bottom.214 Back in the field, within each plot we constructed a horizontal iron hanger 1 m high and215 1.6 m long in front of the vegetation. We hung the envelopes with litter samples in two216 rows at 60-80 and 80-100 cm high. These heights are within range for the vegetation in the217 area (Vaieretti et al., 2010), thus we simulated decomposition of standing dead material.218 We measured temperature of the samples in the field on 5 occasions using an infrared219 thermometer (model 63, Fluke), and we did not find a significant effect of filters on litter220 temperature (Appendix S1: Table S1).221 Samples were placed and collected along a two year trajectory in order to evaluate the222 effect of different seasons on photodegradation (Figure 1c). The first group of samples223 began in June 2021, at the onset of the dry and cold season and it lasted for 2 years on the224 field (hereafter Winter Group 1). The second group started on October 2021, at the onset225 of the warm and humid season, and it lasted for 1.7 years (hereafter Spring Group). The226 last group started on June 2022, and it lasted for 1 year (hereafter Winter Group 2). We227 retrieved samples on 4 dates at 0.3, 1, 1.3 and 2 years. Each time we collected one litter228 envelope per group, species and light treatment per plot. In total we had 270 samples: 2229 species x 5 plots x 3 sunlight levels x (4 harvests for Winter Group 1 + 3 harvests for230 Spring Group + 2 harvests for Winter Group 2). On each collection date, we put samples231 in paper envelopes and sealed them in plastic zipper bags individually for their232 transportation to Buenos Aires. We kept samples cold with ice packs during transportation233 and stored them in a freezer until processing.234 11 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Mass loss235 First, we brushed samples and cleared extraneous material with tweezers. We measured236 fresh mass and extracted 0.200 g for enzymatic activity assays (see below). We dried the237 rest of the sample at 50 °C for 48 h and measured dry mass. We calculated water content238 and used it to estimate the dry mass of the sub-sample used for enzymatic activity assays239 and finally calculated total sample dry mass. We ground samples and corrected for ash240 content using combustion at 450 °C in a muffle furnace (Harmon et al., 1999). We fit a241 negative exponential model for each plot using lineal regression following equation242 ln(Mt/M0) = b − kt where M0 is ash-free initial dry mass, Mt is ash-free dry mass at time t, b is the intercept,243 and k is the slope, also called decomposition rate (Olson, 1963). We only used k constants244 at the plot level if the regression was significant (p < 0.05). We did not use data from245 Winter Group 2 because it was not possible to fit the model with confidence for only 3246 time points. Because of that, for samples that stayed in the field for a year we calculated247 daily organic mass loss as an alternative. In this way, we compared daily mass loss up to 1248 year and k until 2 years.249 Physical litter traits250 We measured leaf area of initial and field samples, except for the October 2021 date. We251 used a scanner (model V370, Epson, Japan) and analyzed images with ImageJ software252 (Schneider et al., 2012). We calculated leaf mass per area (LMA, mg mm −2) dividing dry253 mass by area (P´ erez-Harguindeguy et al., 2013). We measured leaf toughness using the254 12 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint punch test (P´ erez-Harguindeguy et al., 2013), for initial samples, and samples from Winter255 Group 2 at the October 2022 date, and for all groups at the July 2023 sampling date. We256 used a digital dynamometer (DFX2-010, Chatillon) coupled with a flat needle 1.91 mm in257 diameter. To calculate force to punch, we divided the force by the perimeter of the needle258 (N mm−1). When blades were thinner than the needle, we approximated the perimeter to a259 rectangle long as the diameter of the needle, and wide as the sample. Blades of P.260 stuckertii and D. hieronymi fold at senescence, thus it was necessary to unfold them before261 measuring. However, this was not possible for D. hieronymi because it is too fine and262 fragile when dry. So, D. hieronymi values correspond to the force to punch two tissue263 layers. We recognize this is not comparable to other measurements, but it is more realistic264 given that D. hieronymi litter persists in this form during decomposition.265 We measured water adsorption capacity with a technique modified from Day et al. (2022).266 This is a measure of hydrophilicity of litter. We did this for initial samples, for Winter267 Group 2 at the October 2022 date, and for all July 2023 samples. We dried samples of268 0.250 g at 60 °C for 48 h and registered initial dry mass (W i). We put samples in nylon269 bags (40 µm pores) and submerged them in distilled water for 30 min. After draining the270 bags, we dried excess water adhered to samples with paper and measured wet mass (W w).271 We dried samples again at 60 °C for 48 h and measured final dry mass (W f). We calculated272 water adsorption capacity as (W w − Wf)/Wf, as percentage. We also calculated potential273 leaching mass loss as (W i − Wf)/Wi, as percentage.274 Chemical litter traits275 We measured initial soluble carbohydrates, hemicellulose, cellulose and lignin with the acid276 detergent sequential digestion method (Van Soest et al., 1991). We put 0.5000 g (w 0) of277 13 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint dry ground samples in Ankom F57 filter bags and put them in a sequential automatized278 analyzer (Ankom 220, Ankom Technology, NY, USA). We first washed samples in water at279 25°C for 1 h, dried them for 24 h at 100 ° C and measured dry mass (w 1). Then, we280 performed an acid detergent digestion at 100 °C for 1 h, after which we washed samples 3281 times in water at 90 °C and twice in acetone, for 15 min each time. We dried samples and282 measured dry mass (w 2). After this, we put samples in sulfuric acid at a 72% concentration283 for 3 h, washed 3 times with hot water and twice with acetone. We dried samples and284 measured dry mass (w 3). We finally combusted samples in a muffle furnace at 450 °C for 4285 h and calculated ash content (w 4) to correct for inorganic mass. We calculated the286 proportion of soluble carbohydrates as ( w0 − w1)/(w0 − w4), hemicellulose as287 (w1 − w2)/(w0 − w4), cellulose as (w 2 − w3)/(w0 − w4) and lignin as (w 3 − w4)/(w0 − w4).288 We measured total initial sugars following de DuBois et al. (1956). First, we digested 0.035289 g of dry grinded samples in 7 ml of HCl 2.5 N at 100 °C for 3 h. We neutralized it with290 NaCO3 and added 93 ml of distilled water. We took an aliquot of 1 ml and added 1 ml of291 5% phenol and 5 ml of 96% sulfuric acid. We measured absorptance at 490 nm with a292 UV-VIS spectrophotometer (Shimadzu Scientific Instruments, Japan). We calculated total293 sugar concentrations with a calibration curve made using a solution of dextrose in distilled294 water.295 We measured initial saccharification (accessibility to cell wall polysaccharides by microbial296 enzymes) (Breuil and Saddler, 1985; Chen and Dixon, 2007; Ghose, 1987; M´ endez et al.,297 2022). We prepared a 50 U/ml solution of the Trichoderma viride cellulase enzyme (C9422,298 Sigma Aldrich) in acetate buffer 50 mM at 5.5 pH. We added 1 ml of enzymatic solution, 4299 ml of buffer and 0.2 ml of toluene to tubes with 0.0250 g of dry grinded samples. We300 incubated the tubes for 72 h at 50 °C under constant shaking. We took 1 ml aliquots and301 14 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint added 2 ml of distilled water and 3 ml of a solution 100 ml solution made from 1% NaOH302 with 1 g of dinitrosalicylic acid, 0.2 g of sodium sulfite and 0.05 g of phenol. We incubated303 the samples for 5 min at 100°C, after which we added 1 ml of Rochelle salt solution at 40%304 (40 g of sodium and potassium tartrate in 100 ml of distilled water). Once the samples305 were chilled, we measured absorptance at 575 nm. We quantified sugar concentrations306 using a calibration curve.307 We extracted total polyphenols from 40 mg of dried ground sample in 20 mL of 50%308 methanol at 80°C for 1 h. We determined polyphenols concentration with the309 Folin-Ciocalteu method (Cadisch and Giller, 1997), measuring absorptance at 760 nm. For310 the quantification we used a gallic acid calibration curve. We also measured sunscreens311 (hereafter A305), which are phenolic compounds that absorb light at 305 nm (Mazza et al.,312 2000). We extracted A305 with methanol:HCl 99:1 for 48 h at -20 °C and measured313 absorptance at 305 nm.314 Enzymatic activity315 We measured potential β-glucosidase and phenol-oxidase enzymatic activities of litter316 incubated in the field at each date (Sinsabaugh et al., 1999). We put 0.200 g of fresh litter317 in 7 ml of distilled water and shook it. For β-glucosidase activity we prepared a 10 mM318 solution of p-nitrophenyl-β-D-glucopyranoside substrate in 50 mM acetate buffer at 5.5 pH.319 For each sample we prepared a tube with a 1 ml of litter solution plus 1 ml of substrate,320 and a tube with 1 ml of litter solution plus 1 ml of buffer. We also prepared tubes with 1321 ml of substrate and 1 ml of distilled water as blank tubes. We incubated the tubes for 2 h322 at 24 °C and centrifuged them. We stopped the reaction with 0.2 ml of 1M NaOH and323 measured absorptance at 410 nm in a spectrophotometer.324 15 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint To measure phenol-oxidase enzymatic activity, we prepared a 12.5 mM solution of the325 3,4-dihydroxy-L-phenylalanine substrate with the same acetate buffer as before. For each326 sample we prepared a tube with 2 ml of litter solution plus 1 ml of substrate, and a tube327 with 2 ml of litter solution plus 1 ml of buffer. We also prepared blank tubes with 1 ml of328 substrate and 2 ml of distilled water. We incubated the test tubes for 3 h at 24 °C and329 centrifuged them. We measured absorptance at 460 nm with a spectrophotometer. For330 both enzymes we calculated their activity (A) as331 A = a 1.6·b · h (1) where a is net absorptance, 1.6 is the extinction coefficient in µM, b is the litter sample in g332 per ml of aliquot, and h is incubation time in hours.333 Statistical analysis334 We used ANOVA to analyze remaining organic matter and accumulated enzymatic activity335 comparing light filters per each date, species and starting group separately. We also336 analyzed remaining organic matter between June and October 2022 dates per filter and337 species. We only compared these dates because we were interested in knowing what338 happened during the dry cold season. We compared daily mass loss and k constants339 between filters per species and starting group separately using ANOVA. We also compared340 these variables between starting groups per species only for filter level R+ as a way to341 compare the impact of seasonality on decomposition under near-ambient conditions. We342 analysed differences in initial traits between the two species with ANOVA. We removed one343 value leaf toughness and A305 that was defined as an outlier based on Cook’s distance.344 16 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint We tested normality of errors using Shapiro-Wilk’s test and homoscedasticity with345 Levene’s test. To fulfil ANOVA assumptions, we transformed using natural logarithm346 accumulatedβ -glucosidase activity results from P. stuckertii of Winter Group 1 at 1 year,347 and from Winter Group 2 in all its dates. We transformed using ln +1 results from348 accumulated phenol-oxidase activity of Winter Group 1 at 1.3 and 2 years, and of Spring349 Group at 0.7 years. We did not include anomalous data from plot 6 of phenol-oxidase350 activity of P. stuckertii of Spring Group at 1 year. We could not fulfil normality351 assumption of phenol-oxidase activity in P. stuckertii of Winter Group 1 at 1 year because352 of excess of zeros, and using mixed models for zero-inflation did not improve model fit.353 Finally, to evaluate changes in litter traits over time in comparison with initial values, we354 performed t-tests and calculated 95% confidence intervals. We performed all statistical355 analyses in R (R Core Team, 2020).356 Results357 Effects of sunlight and season on litter decomposition and358 enzymatic activity359

for organic mass loss of litter and accumulated β-glucosidase activity during the360 two years of the experiment are shown in Figure 2 only for Winter Group 1 (ANOVA361

in Appendix S1: Table S2). After 1.3 years of field exposure, P. stuckertii showed a362 15% significant increase in mass loss due to visible light (Figure 2a). Interestingly,363 decomposition seemed to halt during the dry (winter) season, since there were no364 detectable differences between June and October 2022 for any attenuation treatment365 17 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint (Figure 3;p: 0.2,R 2: 0.3).D. hieronymi, in turn, showed a relative increase of 46% in mass366 loss due to UV light at 1.3 years (Figure 2b). Here, there was also a halt in decomposition367 during the dry season for UV- (p: 0.3, R2: 0.2) and R- (p: 0.7, R2: 0.02) filters, but not for368 full sunlight filters (R+) which lost 5.0 ± 2.1 % more mass during that period (Figure 3; p:369 0.047, R2: 0.5). After 2 years in the field, UV light increased mass loss by 30% (Figure 2b).370 Accumulated β-glucosidase activity in P. stuckertii increased over time but showed no371 clear response to solar radiation attenuation (Figure 2c). In contrast, D. hieronymi372 presented a significant 50% increase in enzyme activity after 2 years in the field due to UV373 exposure (Figure 2d). Phenol-oxidase activity did not have a clear response to sunlight in374 this experiment (Appendix S1: Table S3). Full results for Spring Group and Winter Group375 2 can be found in the Appendix S1 (Figure S2, Table S4). Generally, results from the other376 two groups followed similar seasonal patterns in response to water availability compared to377 Winter Group 1.378 Litter decomposition across seasons and dominant grassland379 species380 Decomposition for litter starting in winter (Winter group 1) and in spring are shown in381 Figure 4 (ANOVA results in Appendix S1: Table S5). For results after 1 year in the field,382 we show daily mass losses. P. stuckertii did not show a response to light filters in daily383 mass loss at 1 year for any of the seasonal groups (Figure 4a). There was however a384 significant effect of starting season, since R+ samples decomposed 18% faster when they385 were placed to decompose in winter, when compared to spring (p: 0.01, R2: 0.6). For386

at the end of the experiment, we show k values (yr−1). Radiation attenuation had a387 18 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint significant effect on integrated decomposition, with a 19% increase at the spring start.388 Light treatments did not differ significantly between filters for the winter group for P.389 stuckertii (Figure 4c). Additionally, the effect of starting season at the end of the390 experiment switched compared to 1 year results. R+ samples that started decomposing in391 the spring decomposed 23% faster compared to those that started in the winter (p: 0.02,392 R2: 0.5).393 In the case of D. hieronymi, daily mass loss at 1 year increased due to UV exposure by 25%394 and 20% for the spring and winter groups, respectively (Figure 4b). In contrast, the season395 of start did not influence daily mass loss for this species (p: 0.08, R2: 0.4). At the end of396 the experiment, k values of the winter group showed a 56% increase due to UV light397 (Figure 4d). Instead, the Spring Group reacted to both UV and visible light with a 24%398 and 35% increase, respectively. Again, this species did not react to season of start (p: 0.7,399 R2: 0.02). Overall, decomposition rates suggest that P. stuckertii responds more to400 seasonality, while D. hieronymi responds more to sunlight exposure.401 Initial litter quality402 We observed differences in physical and chemical initial litter traits between both species403 (Figure 5, Appendix S1: Table S6). Out of 7 chemical traits, only 3 significantly differed404 between species. P. stuckertii litter had 22% less absorptance at 305 nm, 15% more405 cellulose, and 8% less hemicellulose than D. hieronymi. Additionally, all 4 physical traits406 differed between species. P. stuckertii litter was almost three times as tougher, twice as407 denser (higher LMA), adsorbed a third more water, and lost almost 7 times more mass408 through leaching than D. hieronymi.409 19 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Changes in litter traits410 There were substantial changes in physical litter traits during the field incubation with411 respect to initial values (Figure 6). Starting with P. stuckertii, its LMA decreased around412 9% as a response to visible light at 0.7 years (p: 0.009). This pattern continued until 1.7413 years when litter under all filters had lower LMA than initially. Water adsorption capacity414 increased by 33-41% at the 1.7 year sampling date due to exposure to visible light with415 respect to initial values (p: 0.0008). At the end of the experiment, however, all samples416 became more hydrophilic (R+ samples too, even when the effect was not significant, the417 mean relative effect had the same magnitude as the other treatment levels). Leaf toughness418 in P. stuckertii showed an apparent hardening at 0.3 years (only significant under R-; p:419 0.04), but overall, leaf toughness decreased for all samples over time independently of light420 treatment. Finally, there was initially an increase in potential leaching (mass lost after421 soaking in water), followed by a decrease until the end of the experiment (Appendix S1:422 Figure S3).423 In contrast, D. hieronymi did not show a clear response in LMA to sunlight exposure. In424 terms of water adsorption capacity, this species responded to UV light at the beginning of425 the experiment with a 42% increase at 0.3 years (p: 0.005), and this pattern continued426 until 1 year (+46%; p: 0.02). All samples became more hydrophilic at the end of the427 experiment. Leaf toughness showed no clear response to sunlight exposure, although leaf428 toughness for this species also decreased over time. Finally, D. hieronymi had continuously429 higher leaching throughout the experiment compared to initial conditions but with no clear430 response to sunlight exposure (Appendix S1: Figure S3).431 20 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Discussion432 In this study, we set out to understand how important photodegradation was for C433 turnover of plant litter in a monsoonal mountain grassland. We first hypothesized that434 sunlight accelerates C cycling at this site and that this dynamic is modulated by435 seasonality. Our results confirm this hypothesis, although we found diverging patterns436 between the two species of dominant grasses. Moreover, we hypothesized that437 photofacilitation would occur during humid seasons due to sunlight exposure in the438 previous dry season. This was partially confirmed by results in D. hieronymi, but not in P.439 stuckertii, reinforcing how differently both species react to sunlight exposure. Our last440 hypothesis proposed that photodegradation produces changes in litter physical traits over441 time. We found partial support for this hypothesis, most clearly with changes in P.442 stuckertii’s LMA and D. hieronymi ’s water adsorption capacity.443 Generally, highly productive grasslands have been almost absent from photodegradation444 research (Austin and Ballar´ e, 2024). Overall, we demonstrate that photodegradation can445 considerably enhance plant litter decomposition rates in productive mountain grasslands.446 However, we did not expect to find such contrasting patterns for spectral dependence of447 photodegradation and response to sunlight exposure between the two species under study,448 given that most aspects of initial litter quality were similar (Figure 5). Most notably, D.449 hieronymi showed a 46% increase in mass loss after 1.3 years of UV-light exposure, while450 P. stuckertii had only a 15% increase in mass loss due to visible-light exposure at the same451 date (Figure 2). Other field experiments in grasslands with similar precipitation to our452 study site found either high (Butler et al., 2023) or intermediate (Brandt et al., 2010)453 effects of photodegradation. Butler et al. (2023) found an increase of up to 50% in mass454 21 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint loss with full sunlight exposure. Meanwhile, Brandt et al. (2010) found 17% increase in455 decomposition rates in one species but not in the other one. This could have been456 explained by the higher N concentration of the former species due to a confounded effect of457 biotic decomposition or even photofacilitation. We did not measure N concentration of our458 samples, butD. hieronymihas a slightly higher N concentration than P. stuckertii which459 might explain the bigger mass losses (Poca et al., 2014). These studies are not directly460 comparable, however, due to the fact that they did not include all the attenuation461 treatments (Brandt et al., 2010), or could not distinguish UV and visible light effects462 (Butler et al., 2023). Moreover, our study is the only one to assess photodegradation of463 standing dead litter in this type of grassland. Taken together, this makes comparisons464 difficult and highlights the need to include both sunlight spectra in future experiments.465 Photofacilitation can be defined as the increase in microbially mediated decomposition as a466 consequence of litter alterations due to the effect of photodegradation (Austin and Ballar´ e,467 2024). We were able to infer accelerated biotic decomposition at our grassland site by468 measuring extracellular enzymatic activities, and again we found striking differences469 between the two species. The accumulated activity of the hydrolytic enzyme β-glucosidase470 did not significantly respond to light in P. stuckertii, but it did increase 50% under471 UV-light exposure in D. hieronymi after 2 years (Figure 2). This supports the idea that472 identity of dominant species is important in determining both magnitude and direction in473 biogeochemical processes (Fan et al., 2024). Connecting back to studies in similar474 grasslands, Brandt et al. (2010) did not find any photodegradation effect on β-glucosidase475 over the course of their study. Both experiments lasted for about 2 years, but there were476 seasonality differences between the monsoonal regime (this study) and the Mediterranean477 climate in Brandt et al. (2010). We were able to measure photofacilitation towards the end478 22 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint of the experiment, probably because this coincided with the end of the humid season.479 Meanwhile, their study ended during the dry season, when biotic decomposition is typically480 reduced. In this regard, a study with a similar design to ours in a Mediterranean open481 woodland did find photofacilitation effects of β-glucosidase activity (Berenstecher et al.,482 2020). Their study ended after the wet season, just like ours, which possibly allowed for the483 effects of photodegradation to accumulate over time, finally boosting biotic decomposition484 once humidity became available. This suggests that seasonal variation of rainfall and485 temperature can determine how abiotic and biotic decomposition mechanisms unfold.486 It is clear that precipitation seasonality plays a big role in determining the directions and487 magnitude of photodegradative effects in grassland ecosystems. Aside from the evidence of488 photofacilitation mentioned above, we were able to detect other effects of seasonality on489 decomposition at this grassland. This was possible due to our explicit experiment design490 that was customized to closely follow wet and dry seasons, following Berenstecher et al.491 (2020). First, we detected a halt in decomposition during the second dry season (between 1492 and 1.3 years), except for D. hieronymi under full sun treatment (Figure 3). This suggests493 that biotic activity was negligible during this period of low humidity, but that UV494 radiation exposure maintained C losses for that species. Moreover, this direct495 photodegradation effect probably caused the apparent photofacilitation found at the496 subsequent humid season, as previously discussed. Next, we found that the order of wet497 and dry seasons affected decomposition rates of P. stuckertii but not of D. hieronymi498 (Figure 4). Cumulatively, our results suggest that the former responds more strongly to499 rainfall seasonality, probably due to its higher water adsorption capacity (Figure 5), while500 the latter is more responsive to solar radiation.501 Only two photodegradation studies used a similar temporal design to evaluate seasonality502 23 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint effects (Berenstecher et al., 2020; Li et al., 2024). Berenstecher et al. (2020) found that503 samples under full sunlight after a year decomposed faster when they started in the dry504 season compared to starting in the humid season, similar to P. stuckertii in this study505 (Figure 4). Li et al. (2024), additionally, studied photodegradation of forest litter with506 seasonal snow cover. They found that litter that was exposed to sunlight at the start507 during the snow-free autumn decomposed faster than samples that started during the508 snow-covered winter. Evidently, although with differences between ecosystem types, the509 alternation of precipitation and temperature seasons modulates the effects of510 photodegradation, and more studies should apply such experimental design to expand our511 understanding of this interaction.512 Decomposition not only entails organic mass loss, but also chemical and physical changes513 as a response to biotic and abiotic drivers (Prescott and Vesterdal, 2021). It is important514 to study these changes through time because they in turn affect the trajectory of the515 decomposition process and determine the chemical quality of the remaining organic matter516 entering the soil (Cotrufo et al., 2015). However, most studies focus on changes in chemical517 traits over time (Brandt et al., 2010; Uselman et al., 2011; M´ endez et al., 2019), and nearly518 no photodegradation studies have focused on physical traits. We detected a clear decrease519 in LMA as an effect of blue and green light on P. stuckertii (Figure 6). Only one previous520 study reported a similar decrease in LMA with exposure to UV light after 5 months521 (Gaxiola and Armesto, 2015). A likely explanation for this result is that leaf mass was lost522 with little changes in leaf area, resulting in thinner litter and thus decreased LMA. We also523 detected a clear increase in water adsorption capacity in both species (Figure 6), a proxy of524 litter hydrophilia (Talhelm and Smith, 2018). This response to solar radiation could be a525 consequence of the degradation of hydrophobic leaf cuticles (Logan et al., 2022), and other526 24 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint hydrophobic structural leaf compounds like lignin that respond strongly to527 photodegradation (Austin and Ballar´ e, 2010; Wang et al., 2024). Interestingly, the changes528 in these two litter traits might be connected to lignin losses through photodegradation,529 which would explain why litter became lighter and more hydrophilic over time (Austin and530 Ballar´ e, 2024). Considering physical litter traits affect decomposition dynamics and in turn531 those traits change over time with decomposition, it becomes evident that the relationship532 between them adjusts dynamically over time and determines the fate of litter C (Sun et al.,533 2022).534 Conclusions535 Our study shows clear evidence that the effect of photodegradation is not limited to arid536 environments, but it also affects mesic and highly productive ecosystems like grasslands.537 We show proof that in these ecosystems a considerable amount of mass loss happens in the538 air before litter even touches the ground, both through direct abiotic and biotic539 contributions of solar radiation. Grasslands are estimated to cover 22.8% of the global land540 surface, representing a substantial proportion of terrestrial area (MacDougall et al., 2026).541 In these grass-dominated ecosystems, standing dead biomass constitutes a large portion of542 total biomass (Sarmiento, 1992; Yang et al., 2024). Hence, we can project that wherever543 grasslands with marked seasonality are found and standing dead biomass persists for long544 periods, photodegradation will be an important driver of C cycling (Keiser and Nieland,545 2025; Yang et al., 2025). Further, focusing on grasslands with precipitations ruled by a546 monsoon, the impact of photodegradation could potentially be even larger. Recently,547 research has shown that in global drylands seasonality is an important driver of548 25 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint decomposition, with faster decomposition in sites under monsoonal compared to549 Mediterranean climates (Siebenhart et al., 2025). The coincidence of peaks in temperature550 and precipitation can boost microbial processes, including interactive effects with abiotic551 mechanisms like photofacilitation. Photodegradation has many implications in the552 terrestrial C cycle (Austin and Ballar´ e, 2024), from direct C emissions through553 photomineralization to changes in litter quality that affect the afterlife of plant-derived554 organic matter in soils. Taking all into consideration, it becomes clear that the study of555 photodegradation in productive grasslands with monsoonal climates should be a priority556 for a better understanding of the terrestrial C cycle.557 Acknowledgements558 We would like to thank Quebrada del Condorito National Park, especially Fernanda559 Fabbio, for facilitating our field experiment; Cecilia Palmieri for granting us access to her560 property during field trips; Jos´ e Luis Lois, Franco Fern´ andez and Lucio Biancari for their561 assistance during field work; Laura Ventura for her assistance in the laboratory; Gonzalo562 Arias and Lucas Enrico from IMBiV for their help with leaf toughness measurements.563 Author Contributions564 Agust´ ın Sarquis: Conceptualization, Methodology, Formal Analysis, Investigation, Writing565 - Original Draft, Visualization, Funding Acquisition, Project Administration. Ignacio A.566 Siebenhart: Investigation, Writing - Review and Editing. Marcela S. M´ endez:567 Investigation, Writing - Review and Editing. Amy T. Austin: Conceptualization,568 Methodology, Funding Acquisition, Resources, Supervision, Writing - Review and Editing.569 26 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Conflict of Interest Statement570 We declare that none of the authors have any conflict of interest.571 References572 Administraci´ on de Parques Nacionales (2021). Disposici´ on aprobatoria de las Cargas573 Ganaderas del Programa de Herbivor´ ıa Dom´ estica del Parque Nacional Quebrada del574 Condorito: DI-2021-44637752-APN-DRC#APNAC. Technical report.575 Almagro, M., F. T. Maestre, J. Mart´ ınez-L´ opez, E. Valencia, and A. Rey (2015). 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It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Figure Captions904 Figure 1: Field plot with D. hieronymi cover (a); close-up of litter envelopes of P.905 stuckertii (b); time frame for the litterbag placement and overall experimental design (c).906 Dark green triangles mark sample collection dates. Photographs by A. Sarquis.907 Figure 2: Organic mass (OM) loss (%; a, b) and accumulated β-glucosidase activity (µmol908 g−1; c, d) over time for both species of Winter Group 1. Asterisks denote significant909 differences between filters per date: ** p < 0.01, * p < 0.05. Upper-case letters denote910 differences between filters following Tukey HSD test. Bars represent mean values and911 standard errors. Results in this figure show OM mass loss, but statistical analyses were912 performed on remaining OM. R+: full radiation. UV-: UV blocked. R-: UV, blue and913 green light blocked.914 Figure 3: Organic mass (OM) remaining (%) pre- and post-dry season of 2022. Shapes are915 mean values and bars are standard errors. Asterisk denotes significant differences between916 dates per filter: * p < 0.05. Only Winter Group 1 values are included in this figure. R+:917 full radiation. UV-: UV blocked. R-: UV, blue and green light blocked.918 Figure 4: Decomposition rates for each filter by species and starting group. Daily organic919 mass (OM) loss (% day −1; a, b) was calculated for samples after 1 year and k constants920 were calculated at the end of the experiment. Asterisks denote significant differences921 between filters of the same starting group: *** p < 0.001, ** p < 0.01, * p < 0.05.922 Upper-case letters denote differences from Tukey test between filters of the same starting923 group. Plus signs denote significant differences between starting groups (for filter level R+924 only): ++ p < 0.01, + p < 0.05. Bars are mean values with standard errors. R+: full925 43 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint radiation. UV-: UV blocked. R-: UV, blue and green light blocked.926 Figure 5: Initial physical and chemical initial traits of the two species. Asterisks denote927 significant differences between species: *** p < 0.001, ** p < 0.01, * p < 0.05. A305:928 sunscreens with absorptance at 305 nm. LMA: leaf mass per area.929 Figure 6: Relative effect of field decomposition on physical traits compared to initial values930 for both species under each filter. Colored squares denote significant differences for a t-test931 (p < 0.05). Colors represent direction of change and intensity: blue shades represent932 increases in trait values and pink shades represent decreases, while lighter shades represent933 low-moderate changes (0 – ±0.5) and darker shades represent bigger changes (±0.5 –934 infinite). LMA: leaf mass per area.935 44 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Figures936 a b Dry season Humid season Dry season Humid season 2021 J J A S O N D J F MA M J J J JA S O N D J F MA M 2022 2023 Winter group 1 Spring group Winter group 2 c Figure 1: Experimental design 45 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint OM loss (%) Years R+ UV- R- 0 10 20 30 40 50 a ** A A B 0.3 1 1.3 2 **A B B **A B B b β-glucosidase activity (umol . g-1) c 0 20000 40000 60000 0.3 1 1.3 2 P. stuckertii D. hieronymi d * BB A Figure 2: Mass loss and enzymatic activity 46 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint 70 75 80 85 70 75 80 85 OM remaining pre−dry season (%) OM remaining post−dry season (%) Filter ● R+ ● UV- ● R- Species Deyeuxia Poa * ● ● ● Figure 3: Dry season mass loss 47 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint OM loss (% day-1) Start season ***A B B ** A BB b D. hieronymi ***A B B ***A B C Winter Spring SpringWinter d 0.00 0.02 0.04 0.06 ++ a k (yr-1) P. stuckertii 0.0 0.1 0.2 0.3 *A BAB + c R + UV - R - Winter Spring Figure 4: Decomposition rates 48 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Soluble carbohydrates (%) D. hieronymi P. stuckertii Species Physical Chemical Traits 200 173 9 8 38 43 53 49 5 5 1 4 0.1 0.3 65 82 0.14 0.12 0.07 0.08 0.7 0.50.4 0.09 Hemicellulose (%) ** Cellulose (%) *** Lignin (%) Saccharification (mg/g) Total sugars (mg/g) Water adsorption capacity (%) * LMA (mg mm-2) *** Toughness (N mm-1) *** Potential leaching (%) * Total polyphenols (mg/g) A305 ** Figure 5: Initial litter traits 49 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint Relative effect 0.5 - + Inf 0 - 0.5 n.s. -0.5 - 0 -Inf - -0.5 LMA R+ UV- R- P. stuckertii -0.03 -0.1 -0.18 -0.16 -0.3 -0.27 -0.25 -0.19 -0.17 -0.18 -0.21 -0.09 -0.06 -0.12 -0.08 -0.1 0.04 -0.03 0.3 0.7 1.0 1.3 1.7 2.0 R+ UV- R- D. hieronymi 0.35 0.03 0.19 0.29 0.22 0.29 0.2 0.150.19 0.260.05 0.18 0.35 0.07 0.24 0.16 0.17 0.35 Years Water adsorption capacity 0.45 0.75 0.72 0.56 1 0.74 0.12 0.46 0.18 0.19 0.42 0.21 D. hieronymi 0.07 0.41 0.33 0.25 0.37 0.45 −0.14 −0.02 −0.02 −0.01 0.16 0.12 P. stuckertii R+ UV- R- −0.14 −0.25 −0.19 −0.31 −0.38 −0.32 −0.02 −0.12 0.08 0.36 0.33 0.37 Toughness P. stuckertii R+ UV- R- 0.3 1.0 1.7 2.0 −0.28 −0.41 −0.54 −0.38 −0.47 −0.43 −0.27 −0.41 −0.24 −0.25 −0.12 −0.25 D. hieronymi 0.3 1.0 1.7 2.0 Years Figure 6: Traits changes 50 .CC-BY 4.0 International licenseavailable under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint