{"paper_id":"2b1eb6ba-97ef-493d-badb-f7fce0ebdb47","body_text":"Manuscript type: Original Article\nPhotodegradation accelerates standing dead\nlitter decomposition in monsoonal mountain\ngrasslands of South America\nAgust´ ın Sarquis1,2, Ignacio A. Siebenhart 1,2, Marcela S. M´ endez1,2,3,4, and\nAmy T. Austin 1,2\n1Instituto de Investigaciones Fisiol´ ogicas y Ecol´ ogicas Vinculadas a la\nAgricultura (IFEVA, FAUBA-CONICET), Buenos Aires, Argentina\n2C´ atedra de Ecolog´ ıa, Departamento de Recursos Naturales y Ambiente,\nFacultad de Agronom´ ıa, University of Buenos Aires, Buenos Aires,\nArgentina\n3C´ atedra de Qu´ ımica de las Biomol´ eculas, Departamento de Biolog´ ıa\nAplicada y Alimentos, Facultad de Agronom´ ıa, University of Buenos Aires,\nBuenos Aires, Argentina\n4Instituto Nacional de Investigaci´ on Agropecuaria (INIA), La Estanzuela\nResearch Station, Colonia, Uruguay\nCorresponding author:Agust´ ın Sarquis (email address: agusarquis@agro.uba.ar)\nOpen Research Statement: Data are not yet provided as the study is currently under\npeer review. Upon acceptance, data will be archived in Zenodo. However, full datasets can\nbe made available to the editorial board if required for the evaluation of the manuscript.\nKey words: Carbon cycle; Grassland; Photodegradation; Photofacilitation; Physical litter\ntraits; Plant litter decomposition\n1\n.CC-BY 4.0 International licenseavailable under a \n(which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nAbstract1\nPlant litter decomposition is the process through which plant-derived organic matter is2\nrecycled in terrestrial ecosystems. One of the drivers of decomposition is photodegradation,3\nlight-induced reactions that result in litter mass loss and transformations that accelerate4\nthe decomposition process. Photodegradation has been mostly studied in arid ecosystems,5\nbut mesic grasslands with monsoonal climate have been almost absent in the literature. In6\nthese ecosystems, standing dead biomass might remain exposed to solar radiation during7\ndry winters while the effects of photodegradation accumulate. With the start of the warm8\nand humid seasons, biotic decomposition might increase as a consequence of the changes in9\nlitter quality caused by sunlight. We aimed to study the impact of different wavelengths of10\nsolar radiation on litter mass loss and litter quality changes in a montane grassland with a11\nmonsoonal climate. We incubated litter from two dominant grasses under filters that12\ngenerated treatments of full solar radiation, reduced UV radiation and reduced UV to13\nshort-wave visible radiation. We tracked changes in physical litter traits throughout the14\nexperiment under the three light treatment levels. We found an increase in litter mass loss15\ndue to sunlight exposure for both species, but each species reacted to a different range of16\nwavelengths. We found evidence of enhancement of biotic decomposition by solar radiation17\n(photofacilitation) in one of the two species, through an increase in β-glucosidase18\nenzymatic activity. Seasonality affected litter decomposition of one species only by19\nincreasing mass loss depending on whether it was placed in the field during the dry winter20\nor the humid spring. Finally, we found evidence of changes in physical litter traits caused21\nby solar radiation, mainly in leaf mass per area (LMA) and water adsorption capacity. Our22\nresults represent the first proof of photodegradation in a productive grassland in this23\n2\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nregion, and highlight the fact that photodegradation is not constrained only to arid24\nenvironments. Additionally, this study emphasizes the importance of litter physical traits25\nin regulating carbon cycling through plant litter decomposition in terrestrial ecosystems.26\nIntroduction27\nPlant litter decomposition is the process through which plant-derived organic matter is28\nbroken down into inorganic components. As a result, plant litter decomposition releases29\ncarbon (C) fixed by plants back to the atmosphere and intervenes in the formation of soil30\norganic matter (SOM) (Cotrufo et al., 2015). The main biotic driver of decomposition is31\nthe metabolic activity of fungi and bacteria (Bradford et al., 2017), but soil fauna can also32\nbe important (Zanne et al., 2022; Lejoly et al., 2026). The magnitude of biotic33\ndecomposition is also determined by climate (Gholz et al., 2000) and litter chemistry34\n(Cornwell et al., 2008; Wu et al., 2025; Zhang et al., 2008). Litter decomposition thus35\nresides at a main intersection between C losses and sequestration in terrestrial ecosystems.36\nFor instance, rising atmospheric temperatures can increase CO 2 emissions from litter37\ndecomposition (Hosseiniaghdam et al., 2023), reinforcing the greenhouse effect. Moreover,38\nthe formation of SOM from decomposing plant litter after decomposition is directly linked39\nto soil fertility and soil C sequestration (Cotrufo and Lavallee, 2022). Hence, it is of great40\ninterest to reach a better understanding of how decomposition affects the terrestrial C41\nbalance and how this process might be affected by global change.42\nA lesser-known driver of litter decomposition in terrestrial ecosystems is photodegradation,43\nthe photochemical mineralization of organic matter due to exposure to solar radiation44\n(Austin and Ballar´ e, 2024; King et al., 2012). It has been identified as important for its45\n3\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\ncontrol on C release in a number of ecosystems, particularly in arid and semiarid zones46\n(Austin and Vivanco, 2006; Berenstecher et al., 2020; Brandt et al., 2007; Day et al., 2007;47\nHuang et al., 2017). Specifically, sunlight wavelengths in the range of ultraviolet (UV-B,48\n280-315 nm; and UV-A, 315-400 nm) and blue-green (400-550 nm) are largely responsible49\nfor these reactions (Austin and Ballar´ e, 2010; Brandt et al., 2009; Day and Bliss, 2019).50\nThese photochemical reactions are possible due to the capacity of compounds in secondary51\ncell walls to absorb sunlight, principally lignin, and produce photo-oxidative reactions52\n(Austin and Ballar´ e, 2010; Kommedal et al., 2023; Moorhead and Callaghan, 1994).53\nMoreover, this process is independent from microbial activity and can release C directly to54\nthe atmosphere as CO 2, CH4 and CO (Brandt et al., 2010; Lee et al., 2012; Schade et al.,55\n2012).56\nPhotodegradation can also produce transformations in litter that make some carbohydrates57\nto be more accessible to microbial consumption (Austin et al., 2016). This, in turn,58\nincreases litter mass loss as a complementary effect of sunlight called photofacilitation59\n(Austin et al., 2016; Gallo et al., 2006; M´ endez et al., 2022). For instance, exposure to solar60\nradiation of litter can increaseβ -glucosidase enzymatic activity, an enzyme associated with61\nthe last step of cellulose degradation (Berenstecher et al., 2020, 2022; M´ endez et al., 2019,62\n2022), and phenol-oxidase, associated with the degradation of phenolic compounds of lignin63\n(Baker and Allison, 2015; Yao et al., 2022). It has been suggested that lignin degradation64\nby sunlight could be responsible for this increase in biotic decomposition, due to the65\ntransformations of the lignin-cellulose matrix that increases microbial access to66\ncarbohydrates (Austin and Ballar´ e, 2024; Austin et al., 2016; M´ endez et al., 2022).67\nThe importance of photodegradation was first appreciated in arid zones (Austin et al.,68\n2006; Day et al., 2007; Gallo et al., 2009) and has been shown to be important in a variety69\n4\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nof ecosystems, from deserts to arid grasslands and woodlands (Almagro et al., 2015, 2017;70\nBaker and Allison, 2015; Brandt et al., 2010; Day et al., 2018; Henry et al., 2008). These71\narid and semiarid ecosystems are characterized by low plant productivity and72\nheterogeneous vegetation, which contribute to the interception of solar radiation by plant73\nlitter. Photodegradation has been evaluated in both monsoonal (Brandt et al., 2010; Day74\net al., 2022; Pancotto et al., 2003) as well as Mediterranean climates (Austin et al., 2006;75\nRuhland and Fraley, 2023; Rutledge et al., 2010) of low mean annual precipitation (MAP).76\nThese studies have proven that sunlight affects decomposition in both types of climates,77\nwith divergent patterns at local scales. In Mediterranean climates, the peak of solar78\nradiation in summer does not coincide with the rainy season, whereas in monsoonal79\nclimates peaks in solar radiation and precipitation coincide. This suggests that whether80\npeaks in radiation and rains are synchronous or not can determine how abiotic and biotic81\nfactors influencing decomposition may interact with each other. More recent research on82\nphotodegradation has expanded to other types of ecosystems including temperate forests,83\ntropical ecosystems and agroecosystems (Keiser et al., 2021; Marinho et al., 2020; Wang84\net al., 2021; Cabrera et al., 2026), but the majority of mesic ecosystems have not yet been85\nevaluated for the potential importance of solar radiation on C turnover, particularly for the86\nrelative importance of direct photodegradation and photofacilitation effects (Austin and87\nBallar´ e, 2024).88\nConsidering grasslands are some of the most productive and widespread ecosystems on89\nEarth (Gibson, 2008), it would be important to advance our understanding of how solar90\nradiation affects this portion of the terrestrial C balance (Keiser and Nieland, 2025).91\nCurrently, most studies in grasslands have been set in low-rainfall sites, many of which92\nshowed marked positive effects of photodegradation (Yang et al., 2025; Butler et al., 2023;93\n5\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nWang et al., 2017), but some of them found either neutral or negative effects as well94\n(Erdenebileg et al., 2018; Yang et al., 2024; Almagro et al., 2017). However, fewer studies95\nhave been carried out in more productive grasslands with higher mean annual precipitation96\n(MAP) (Brandt et al., 2010; Butler et al., 2023; van Asperen et al., 2015). These97\ngrasslands are particularly relevant for the question of the importance on photodegradation98\ndue to the accumulation of very large quantities of standing dead material associated with99\ntussock grass formation, also known as marcescence (Mudr´ ak et al., 2023; Sarmiento, 1992)100\n. These large amounts of plant litter are exposed to solar radiation for long periods before101\ntouching the ground. Circling back to seasonality in these grasslands, even fewer studies102\nhave been done in grasslands with monsoonal climate (Yao et al., 2024). It is worth103\nexploring whether in monsoonal grasslands abiotic photodegradation acts during the dry104\nwinter and photofacilitation begins at the onset of the humid and warm seasons.105\nSeveral field studies have also specifically addressed the effect of litter traits on106\nphotodegradation and photofacilitation. This is important because part of how sunlight107\nimpacts decomposition is through modifying litter chemistry and structure and thus108\nchanging its decomposability (Austin et al., 2016; Foereid et al., 2010; Gallo et al., 2006).109\nMost studies have focused on chemical traits like C, N and C fractions like lignin,110\nhemicellulose and cellulose, (Ball et al., 2019; Day et al., 2022; Ma et al., 2017; Yang et al.,111\n2025). The focus on these traits, specifically on lignin, is because of its importance as a112\ncontrol on both biotic decomposition and photodegradation (Austin and Ballar´ e, 2010).113\nHowever, there is a need to expand studies to incorporate other litter traits that might114\npotentially be linked to photodegradation, including how those traits change over time. In115\nthat regard, few physical traits have been measured in photodegradation studies in the116\nfield, typically SLA/LMA (Wang et al., 2024; Li et al., 2024; Gaxiola and Armesto, 2015),117\n6\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nleaf toughness (Araujo et al., 2022; Masubelele and Bond, 2022), water and vapor holding118\ncapacity (Almagro et al., 2017; Logan et al., 2022), and light-interacting traits (i.e.:119\nreflectance, transmittance, and absorptance) (Day and Bliss, 2020; Day et al., 2015;120\nRuhland and Fraley, 2023). There is a clear opportunity for the advancement of121\nphotodegradation research in how physical litter traits change over time and how this feeds122\nback to C release at the ecosystem scale.123\nOur objectives in this study were to understand the importance of photodegradation for C124\nturnover and to evaluate the spectral dependence on C release from plant litter in a125\nmonsoonal mountain grassland in central Argentina. Previous studies for the region were126\ncarried out in grass-dominated ecosystems with similar MAP and mean annual temperature127\n(MAT), but with a different rainfall seasonality (i.e. Mediterranean climate; Berenstecher128\net al., 2020; M´ endez et al., 2019). We specifically designed our study to directly address129\nthe seasonal contributions and the role of dominant species identity in the relative130\nimportance of sunlight on litter decomposition of standing dead biomass. We hypothesized131\nthat exposure to sunlight accelerates C cycling throughout the year in this monsoonal132\ngrassland, and that this relationship is modulated by seasonality. During dry winters,133\nsunlight acts predominantly through the abiotic photo-oxidative route. Instead, during the134\nwarm and humid seasons, direct abiotic photodegradation and biotic decomposition occur135\nsimultaneously. Additionally, sunlight exposure during the dry season promotes biotic136\ndecomposition during the following humid season through changes in litter quality137\n(photofacilitation). Lastly, photodegradation of the litter produces changes in its physical138\nquality over time. We applied a temporally asynchronous design to disentangle the relative139\nimportance of contrasting seasons in order to test our hypothesis. Only a few studies have140\ntried to address directly how marked seasonal differences affect litter decomposition with141\n7\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nthis design (i.e.: Berenstecher et al., 2020; Li et al., 2024). We found that both seasonality142\nand species identity and their associated litter traits were key in defining the importance of143\nphotodegradation as a control on decomposition in this montane grassland.144\nMethods145\nStudy site146\nOur study site was located in a native montane grassland in a plateau known as Pampa de147\nAchala located in the C´ ordoba mountain ranges of central Argentina (2000 – 2300 m a.s.l.).148\nThe mean temperatures of the coldest and warmest months are 5.0 and 11.4 °C,149\nrespectively. Average mean annual precipitation is 900 mm, highly concentrated between150\nOctober and April, and frosts occur year round (Cingolani et al., 2015). Additionally, fog151\nevents are common year round and represent an additional water input into the system152\n(Poca et al., 2018). Soils are Mollisols derived from granitic rocks and fine texture particles153\noriginated from wind erosion (Cabido et al., 1987). The landscape is an undulating plain154\nwith patches of rocky outcrops, Polylepis australis Bitter woodlands, and short-statured155\ngrazing lawns, all immersed in a tussock grassland matrix with varying degrees of openness156\ndominated by Poa stuckertii (Hack.) Parodi and Deyeuxia hieronymi (Hack.) T¨ urpe157\n(Cingolani et al., 2014; von M¨ uller et al., 2017; Zeballos et al., 2024). These tussock158\ngrassland physiognomies are highly productive and generate each year large quantities of159\nsenescent material during the dry season (fall-winter) that stay attached to the plants for160\nextended periods (Pucheta et al., 1998). This implies that a considerable amount of dry161\nbiomass is exposed to sunlight during an extended period of time, which makes this system162\nan ideal laboratory to study the effects of photodegradation. In this system, the relatively163\n8\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nhigh stocking rates and wildfires help maintain the grazing lawns (Cingolani et al., 2013),164\nwhile in zones with lower stocking rates, tall tussock grasses dominate (i.e. P. stuckertii165\nand D. hieronymi ). We worked within the limits of Quebrada del Condorito National Park,166\nwhere grazing is used as a management tool with conservation aims and to stop the167\naccumulation of excessive amounts of flammable standing dead biomass (Cingolani et al.,168\n2014; von M¨ uller et al., 2017). The rangeland where we carried out this study had an169\neffective stocking rate for the period 2020-2021 of 0.13 AU ha−1 (Animal Units per hectare)170\n(Administraci´ on de Parques Nacionales, 2021).171\nExperimental design172\nThe general experimental design consisted of the decomposition in the field of two173\ndominant native grass species, P. stuckertii and D. hieronymi. Leaf litter was decomposed174\nin plots covered in the respective species, hanging as if it were standing dead biomass. Each175\nspecies was subjected to three levels of a sunlight treatment to evaluate the importance of176\nphotodegradation as a driver of litter decomposition in this site. We deployed samples in177\nthe field at three different dates to study the impact of seasonality on decomposition, and178\nwe collected the samples from the field on four dates along a 2-year period.179\nIn more detail, we established 5 pairs of plots on the same hillside with grassland cover180\n(Figure 1a) for the evaluation of photodegradation and seasonality on native plant litter181\ndecomposition (31º 37’ S, 64º 48’ W; 2150 m a.s.l.). Out of each pair of plots one had P.182\nstuckertii cover and another one, D. hieronymi cover. Samples of each of the two species183\nwere later incubated in plots covered in their respective species. Plots were located south184\nof rocky pavements to avoid shading from vegetation. We chose this exposure for the plots185\nto maximize sunlight exposure in the Southern Hemisphere.186\n9\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nIn July 2019, we collected standing dead material (hereafter ‘litter’) of P. stuckertii and D.187\nhieronymi in the same rangeland where the experiment was done, and in a neighboring188\nrangeland. We took the litter to Buenos Aires where we stored it in boxes in a dry and189\ncool space in the Instituto de Investigaciones Fisiol´ ogicas y Ecol´ ogicas Vinculadas a la190\nAgricultura (IFEVA). We made sure to flip the litter in the boxes periodically to keep it191\naired until processing. We selected senescent blades with the least signs of decomposition.192\nWe cut blades in 10-16 cm segments and weighed samples of 1.5 g of air-dried litter. We193\npreserved 10 samples for measurements of initial litter quality, and the rest were randomly194\nassigned to a treatment.195\nTo evaluate mass loss, we designed envelopes made of hexagonal galvanized wire mesh with196\nopening of 13 mm, 20 cm in height and 10 cm in width (Figure 1b). We oriented the197\nenvelopes on the hangers in a north-south direction. The north side of the envelopes had a198\nplastic filter that blocked a specific range of the wavelength in order to create three distinct199\nsunlight treatment levels: full radiation (R+), blocked ultraviolet (UV-), and all200\nphotochemically active radiation blocked (R-). The R+ filter was a 100 µm thickness201\npolyethylene with a transmittance from UV-B to the visible spectrum (Appendix S1:202\nFigure S1; 280-800 nm; Ever Wear S. A.). The UV- filter blocked radiation in the UV203\nrange (280-400 nm; Costech, 226 UV). The R- filter blocked radiation from UV to204\nblue-green light (280-550 nm; Rosco® Nº135 Deep Golden Amber). This type of filter has205\nproven to be efficient in reducing radiation with photo-oxidative effects in several other206\nstudies (Austin and Ballar´ e, 2010; Brandt et al., 2009; Day and Bliss, 2019). It is worth207\nnoting that these filters cannot separate the abiotic effects of photodegradation (direct208\nphotomineralization) from the biotic effects (photofacilitation), nor the deleterious effects209\nof sunlight on microbes. Thus, these filters allow us to demonstrate an overall sunlight210\n10\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\neffect that is the balance of these three processes. We punched holes in the filters and211\ncovered the southern side of the envelopes with fiberglass mesh of 2 mm openings to allow212\nthe passage of air and humidity. We added a polyester gauze pocket to the bottom of the213\nenvelopes to avoid fragmentation and loss from the bottom.214\nBack in the field, within each plot we constructed a horizontal iron hanger 1 m high and215\n1.6 m long in front of the vegetation. We hung the envelopes with litter samples in two216\nrows at 60-80 and 80-100 cm high. These heights are within range for the vegetation in the217\narea (Vaieretti et al., 2010), thus we simulated decomposition of standing dead material.218\nWe measured temperature of the samples in the field on 5 occasions using an infrared219\nthermometer (model 63, Fluke), and we did not find a significant effect of filters on litter220\ntemperature (Appendix S1: Table S1).221\nSamples were placed and collected along a two year trajectory in order to evaluate the222\neffect of different seasons on photodegradation (Figure 1c). The first group of samples223\nbegan in June 2021, at the onset of the dry and cold season and it lasted for 2 years on the224\nfield (hereafter Winter Group 1). The second group started on October 2021, at the onset225\nof the warm and humid season, and it lasted for 1.7 years (hereafter Spring Group). The226\nlast group started on June 2022, and it lasted for 1 year (hereafter Winter Group 2). We227\nretrieved samples on 4 dates at 0.3, 1, 1.3 and 2 years. Each time we collected one litter228\nenvelope per group, species and light treatment per plot. In total we had 270 samples: 2229\nspecies x 5 plots x 3 sunlight levels x (4 harvests for Winter Group 1 + 3 harvests for230\nSpring Group + 2 harvests for Winter Group 2). On each collection date, we put samples231\nin paper envelopes and sealed them in plastic zipper bags individually for their232\ntransportation to Buenos Aires. We kept samples cold with ice packs during transportation233\nand stored them in a freezer until processing.234\n11\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nMass loss235\nFirst, we brushed samples and cleared extraneous material with tweezers. We measured236\nfresh mass and extracted 0.200 g for enzymatic activity assays (see below). We dried the237\nrest of the sample at 50 °C for 48 h and measured dry mass. We calculated water content238\nand used it to estimate the dry mass of the sub-sample used for enzymatic activity assays239\nand finally calculated total sample dry mass. We ground samples and corrected for ash240\ncontent using combustion at 450 °C in a muffle furnace (Harmon et al., 1999). We fit a241\nnegative exponential model for each plot using lineal regression following equation242\nln(Mt/M0) = b − kt\nwhere M0 is ash-free initial dry mass, Mt is ash-free dry mass at time t, b is the intercept,243\nand k is the slope, also called decomposition rate (Olson, 1963). We only used k constants244\nat the plot level if the regression was significant (p < 0.05). We did not use data from245\nWinter Group 2 because it was not possible to fit the model with confidence for only 3246\ntime points. Because of that, for samples that stayed in the field for a year we calculated247\ndaily organic mass loss as an alternative. In this way, we compared daily mass loss up to 1248\nyear and k until 2 years.249\nPhysical litter traits250\nWe measured leaf area of initial and field samples, except for the October 2021 date. We251\nused a scanner (model V370, Epson, Japan) and analyzed images with ImageJ software252\n(Schneider et al., 2012). We calculated leaf mass per area (LMA, mg mm −2) dividing dry253\nmass by area (P´ erez-Harguindeguy et al., 2013). We measured leaf toughness using the254\n12\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\npunch test (P´ erez-Harguindeguy et al., 2013), for initial samples, and samples from Winter255\nGroup 2 at the October 2022 date, and for all groups at the July 2023 sampling date. We256\nused a digital dynamometer (DFX2-010, Chatillon) coupled with a flat needle 1.91 mm in257\ndiameter. To calculate force to punch, we divided the force by the perimeter of the needle258\n(N mm−1). When blades were thinner than the needle, we approximated the perimeter to a259\nrectangle long as the diameter of the needle, and wide as the sample. Blades of P.260\nstuckertii and D. hieronymi fold at senescence, thus it was necessary to unfold them before261\nmeasuring. However, this was not possible for D. hieronymi because it is too fine and262\nfragile when dry. So, D. hieronymi values correspond to the force to punch two tissue263\nlayers. We recognize this is not comparable to other measurements, but it is more realistic264\ngiven that D. hieronymi litter persists in this form during decomposition.265\nWe measured water adsorption capacity with a technique modified from Day et al. (2022).266\nThis is a measure of hydrophilicity of litter. We did this for initial samples, for Winter267\nGroup 2 at the October 2022 date, and for all July 2023 samples. We dried samples of268\n0.250 g at 60 °C for 48 h and registered initial dry mass (W i). We put samples in nylon269\nbags (40 µm pores) and submerged them in distilled water for 30 min. After draining the270\nbags, we dried excess water adhered to samples with paper and measured wet mass (W w).271\nWe dried samples again at 60 °C for 48 h and measured final dry mass (W f). We calculated272\nwater adsorption capacity as (W w − Wf)/Wf, as percentage. We also calculated potential273\nleaching mass loss as (W i − Wf)/Wi, as percentage.274\nChemical litter traits275\nWe measured initial soluble carbohydrates, hemicellulose, cellulose and lignin with the acid276\ndetergent sequential digestion method (Van Soest et al., 1991). We put 0.5000 g (w 0) of277\n13\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\ndry ground samples in Ankom F57 filter bags and put them in a sequential automatized278\nanalyzer (Ankom 220, Ankom Technology, NY, USA). We first washed samples in water at279\n25°C for 1 h, dried them for 24 h at 100 ° C and measured dry mass (w 1). Then, we280\nperformed an acid detergent digestion at 100 °C for 1 h, after which we washed samples 3281\ntimes in water at 90 °C and twice in acetone, for 15 min each time. We dried samples and282\nmeasured dry mass (w 2). After this, we put samples in sulfuric acid at a 72% concentration283\nfor 3 h, washed 3 times with hot water and twice with acetone. We dried samples and284\nmeasured dry mass (w 3). We finally combusted samples in a muffle furnace at 450 °C for 4285\nh and calculated ash content (w 4) to correct for inorganic mass. We calculated the286\nproportion of soluble carbohydrates as ( w0 − w1)/(w0 − w4), hemicellulose as287\n(w1 − w2)/(w0 − w4), cellulose as (w 2 − w3)/(w0 − w4) and lignin as (w 3 − w4)/(w0 − w4).288\nWe measured total initial sugars following de DuBois et al. (1956). First, we digested 0.035289\ng of dry grinded samples in 7 ml of HCl 2.5 N at 100 °C for 3 h. We neutralized it with290\nNaCO3 and added 93 ml of distilled water. We took an aliquot of 1 ml and added 1 ml of291\n5% phenol and 5 ml of 96% sulfuric acid. We measured absorptance at 490 nm with a292\nUV-VIS spectrophotometer (Shimadzu Scientific Instruments, Japan). We calculated total293\nsugar concentrations with a calibration curve made using a solution of dextrose in distilled294\nwater.295\nWe measured initial saccharification (accessibility to cell wall polysaccharides by microbial296\nenzymes) (Breuil and Saddler, 1985; Chen and Dixon, 2007; Ghose, 1987; M´ endez et al.,297\n2022). We prepared a 50 U/ml solution of the Trichoderma viride cellulase enzyme (C9422,298\nSigma Aldrich) in acetate buffer 50 mM at 5.5 pH. We added 1 ml of enzymatic solution, 4299\nml of buffer and 0.2 ml of toluene to tubes with 0.0250 g of dry grinded samples. We300\nincubated the tubes for 72 h at 50 °C under constant shaking. We took 1 ml aliquots and301\n14\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nadded 2 ml of distilled water and 3 ml of a solution 100 ml solution made from 1% NaOH302\nwith 1 g of dinitrosalicylic acid, 0.2 g of sodium sulfite and 0.05 g of phenol. We incubated303\nthe samples for 5 min at 100°C, after which we added 1 ml of Rochelle salt solution at 40%304\n(40 g of sodium and potassium tartrate in 100 ml of distilled water). Once the samples305\nwere chilled, we measured absorptance at 575 nm. We quantified sugar concentrations306\nusing a calibration curve.307\nWe extracted total polyphenols from 40 mg of dried ground sample in 20 mL of 50%308\nmethanol at 80°C for 1 h. We determined polyphenols concentration with the309\nFolin-Ciocalteu method (Cadisch and Giller, 1997), measuring absorptance at 760 nm. For310\nthe quantification we used a gallic acid calibration curve. We also measured sunscreens311\n(hereafter A305), which are phenolic compounds that absorb light at 305 nm (Mazza et al.,312\n2000). We extracted A305 with methanol:HCl 99:1 for 48 h at -20 °C and measured313\nabsorptance at 305 nm.314\nEnzymatic activity315\nWe measured potential β-glucosidase and phenol-oxidase enzymatic activities of litter316\nincubated in the field at each date (Sinsabaugh et al., 1999). We put 0.200 g of fresh litter317\nin 7 ml of distilled water and shook it. For β-glucosidase activity we prepared a 10 mM318\nsolution of p-nitrophenyl-β-D-glucopyranoside substrate in 50 mM acetate buffer at 5.5 pH.319\nFor each sample we prepared a tube with a 1 ml of litter solution plus 1 ml of substrate,320\nand a tube with 1 ml of litter solution plus 1 ml of buffer. We also prepared tubes with 1321\nml of substrate and 1 ml of distilled water as blank tubes. We incubated the tubes for 2 h322\nat 24 °C and centrifuged them. We stopped the reaction with 0.2 ml of 1M NaOH and323\nmeasured absorptance at 410 nm in a spectrophotometer.324\n15\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nTo measure phenol-oxidase enzymatic activity, we prepared a 12.5 mM solution of the325\n3,4-dihydroxy-L-phenylalanine substrate with the same acetate buffer as before. For each326\nsample we prepared a tube with 2 ml of litter solution plus 1 ml of substrate, and a tube327\nwith 2 ml of litter solution plus 1 ml of buffer. We also prepared blank tubes with 1 ml of328\nsubstrate and 2 ml of distilled water. We incubated the test tubes for 3 h at 24 °C and329\ncentrifuged them. We measured absorptance at 460 nm with a spectrophotometer. For330\nboth enzymes we calculated their activity (A) as331\nA = a\n1.6·b · h (1)\nwhere a is net absorptance, 1.6 is the extinction coefficient in µM, b is the litter sample in g332\nper ml of aliquot, and h is incubation time in hours.333\nStatistical analysis334\nWe used ANOVA to analyze remaining organic matter and accumulated enzymatic activity335\ncomparing light filters per each date, species and starting group separately. We also336\nanalyzed remaining organic matter between June and October 2022 dates per filter and337\nspecies. We only compared these dates because we were interested in knowing what338\nhappened during the dry cold season. We compared daily mass loss and k constants339\nbetween filters per species and starting group separately using ANOVA. We also compared340\nthese variables between starting groups per species only for filter level R+ as a way to341\ncompare the impact of seasonality on decomposition under near-ambient conditions. We342\nanalysed differences in initial traits between the two species with ANOVA. We removed one343\nvalue leaf toughness and A305 that was defined as an outlier based on Cook’s distance.344\n16\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nWe tested normality of errors using Shapiro-Wilk’s test and homoscedasticity with345\nLevene’s test. To fulfil ANOVA assumptions, we transformed using natural logarithm346\naccumulatedβ -glucosidase activity results from P. stuckertii of Winter Group 1 at 1 year,347\nand from Winter Group 2 in all its dates. We transformed using ln +1 results from348\naccumulated phenol-oxidase activity of Winter Group 1 at 1.3 and 2 years, and of Spring349\nGroup at 0.7 years. We did not include anomalous data from plot 6 of phenol-oxidase350\nactivity of P. stuckertii of Spring Group at 1 year. We could not fulfil normality351\nassumption of phenol-oxidase activity in P. stuckertii of Winter Group 1 at 1 year because352\nof excess of zeros, and using mixed models for zero-inflation did not improve model fit.353\nFinally, to evaluate changes in litter traits over time in comparison with initial values, we354\nperformed t-tests and calculated 95% confidence intervals. We performed all statistical355\nanalyses in R (R Core Team, 2020).356\nResults357\nEffects of sunlight and season on litter decomposition and358\nenzymatic activity359\nResults for organic mass loss of litter and accumulated β-glucosidase activity during the360\ntwo years of the experiment are shown in Figure 2 only for Winter Group 1 (ANOVA361\nresults in Appendix S1: Table S2). After 1.3 years of field exposure, P. stuckertii showed a362\n15% significant increase in mass loss due to visible light (Figure 2a). Interestingly,363\ndecomposition seemed to halt during the dry (winter) season, since there were no364\ndetectable differences between June and October 2022 for any attenuation treatment365\n17\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\n(Figure 3;p: 0.2,R 2: 0.3).D. hieronymi, in turn, showed a relative increase of 46% in mass366\nloss due to UV light at 1.3 years (Figure 2b). Here, there was also a halt in decomposition367\nduring the dry season for UV- (p: 0.3, R2: 0.2) and R- (p: 0.7, R2: 0.02) filters, but not for368\nfull sunlight filters (R+) which lost 5.0 ± 2.1 % more mass during that period (Figure 3; p:369\n0.047, R2: 0.5). After 2 years in the field, UV light increased mass loss by 30% (Figure 2b).370\nAccumulated β-glucosidase activity in P. stuckertii increased over time but showed no371\nclear response to solar radiation attenuation (Figure 2c). In contrast, D. hieronymi372\npresented a significant 50% increase in enzyme activity after 2 years in the field due to UV373\nexposure (Figure 2d). Phenol-oxidase activity did not have a clear response to sunlight in374\nthis experiment (Appendix S1: Table S3). Full results for Spring Group and Winter Group375\n2 can be found in the Appendix S1 (Figure S2, Table S4). Generally, results from the other376\ntwo groups followed similar seasonal patterns in response to water availability compared to377\nWinter Group 1.378\nLitter decomposition across seasons and dominant grassland379\nspecies380\nDecomposition for litter starting in winter (Winter group 1) and in spring are shown in381\nFigure 4 (ANOVA results in Appendix S1: Table S5). For results after 1 year in the field,382\nwe show daily mass losses. P. stuckertii did not show a response to light filters in daily383\nmass loss at 1 year for any of the seasonal groups (Figure 4a). There was however a384\nsignificant effect of starting season, since R+ samples decomposed 18% faster when they385\nwere placed to decompose in winter, when compared to spring (p: 0.01, R2: 0.6). For386\nresults at the end of the experiment, we show k values (yr−1). Radiation attenuation had a387\n18\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nsignificant effect on integrated decomposition, with a 19% increase at the spring start.388\nLight treatments did not differ significantly between filters for the winter group for P.389\nstuckertii (Figure 4c). Additionally, the effect of starting season at the end of the390\nexperiment switched compared to 1 year results. R+ samples that started decomposing in391\nthe spring decomposed 23% faster compared to those that started in the winter (p: 0.02,392\nR2: 0.5).393\nIn the case of D. hieronymi, daily mass loss at 1 year increased due to UV exposure by 25%394\nand 20% for the spring and winter groups, respectively (Figure 4b). In contrast, the season395\nof start did not influence daily mass loss for this species (p: 0.08, R2: 0.4). At the end of396\nthe experiment, k values of the winter group showed a 56% increase due to UV light397\n(Figure 4d). Instead, the Spring Group reacted to both UV and visible light with a 24%398\nand 35% increase, respectively. Again, this species did not react to season of start (p: 0.7,399\nR2: 0.02). Overall, decomposition rates suggest that P. stuckertii responds more to400\nseasonality, while D. hieronymi responds more to sunlight exposure.401\nInitial litter quality402\nWe observed differences in physical and chemical initial litter traits between both species403\n(Figure 5, Appendix S1: Table S6). Out of 7 chemical traits, only 3 significantly differed404\nbetween species. P. stuckertii litter had 22% less absorptance at 305 nm, 15% more405\ncellulose, and 8% less hemicellulose than D. hieronymi. Additionally, all 4 physical traits406\ndiffered between species. P. stuckertii litter was almost three times as tougher, twice as407\ndenser (higher LMA), adsorbed a third more water, and lost almost 7 times more mass408\nthrough leaching than D. hieronymi.409\n19\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nChanges in litter traits410\nThere were substantial changes in physical litter traits during the field incubation with411\nrespect to initial values (Figure 6). Starting with P. stuckertii, its LMA decreased around412\n9% as a response to visible light at 0.7 years (p: 0.009). This pattern continued until 1.7413\nyears when litter under all filters had lower LMA than initially. Water adsorption capacity414\nincreased by 33-41% at the 1.7 year sampling date due to exposure to visible light with415\nrespect to initial values (p: 0.0008). At the end of the experiment, however, all samples416\nbecame more hydrophilic (R+ samples too, even when the effect was not significant, the417\nmean relative effect had the same magnitude as the other treatment levels). Leaf toughness418\nin P. stuckertii showed an apparent hardening at 0.3 years (only significant under R-; p:419\n0.04), but overall, leaf toughness decreased for all samples over time independently of light420\ntreatment. Finally, there was initially an increase in potential leaching (mass lost after421\nsoaking in water), followed by a decrease until the end of the experiment (Appendix S1:422\nFigure S3).423\nIn contrast, D. hieronymi did not show a clear response in LMA to sunlight exposure. In424\nterms of water adsorption capacity, this species responded to UV light at the beginning of425\nthe experiment with a 42% increase at 0.3 years (p: 0.005), and this pattern continued426\nuntil 1 year (+46%; p: 0.02). All samples became more hydrophilic at the end of the427\nexperiment. Leaf toughness showed no clear response to sunlight exposure, although leaf428\ntoughness for this species also decreased over time. Finally, D. hieronymi had continuously429\nhigher leaching throughout the experiment compared to initial conditions but with no clear430\nresponse to sunlight exposure (Appendix S1: Figure S3).431\n20\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nDiscussion432\nIn this study, we set out to understand how important photodegradation was for C433\nturnover of plant litter in a monsoonal mountain grassland. We first hypothesized that434\nsunlight accelerates C cycling at this site and that this dynamic is modulated by435\nseasonality. Our results confirm this hypothesis, although we found diverging patterns436\nbetween the two species of dominant grasses. Moreover, we hypothesized that437\nphotofacilitation would occur during humid seasons due to sunlight exposure in the438\nprevious dry season. This was partially confirmed by results in D. hieronymi, but not in P.439\nstuckertii, reinforcing how differently both species react to sunlight exposure. Our last440\nhypothesis proposed that photodegradation produces changes in litter physical traits over441\ntime. We found partial support for this hypothesis, most clearly with changes in P.442\nstuckertii’s LMA and D. hieronymi ’s water adsorption capacity.443\nGenerally, highly productive grasslands have been almost absent from photodegradation444\nresearch (Austin and Ballar´ e, 2024). Overall, we demonstrate that photodegradation can445\nconsiderably enhance plant litter decomposition rates in productive mountain grasslands.446\nHowever, we did not expect to find such contrasting patterns for spectral dependence of447\nphotodegradation and response to sunlight exposure between the two species under study,448\ngiven that most aspects of initial litter quality were similar (Figure 5). Most notably, D.449\nhieronymi showed a 46% increase in mass loss after 1.3 years of UV-light exposure, while450\nP. stuckertii had only a 15% increase in mass loss due to visible-light exposure at the same451\ndate (Figure 2). Other field experiments in grasslands with similar precipitation to our452\nstudy site found either high (Butler et al., 2023) or intermediate (Brandt et al., 2010)453\neffects of photodegradation. Butler et al. (2023) found an increase of up to 50% in mass454\n21\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nloss with full sunlight exposure. Meanwhile, Brandt et al. (2010) found 17% increase in455\ndecomposition rates in one species but not in the other one. This could have been456\nexplained by the higher N concentration of the former species due to a confounded effect of457\nbiotic decomposition or even photofacilitation. We did not measure N concentration of our458\nsamples, butD. hieronymihas a slightly higher N concentration than P. stuckertii which459\nmight explain the bigger mass losses (Poca et al., 2014). These studies are not directly460\ncomparable, however, due to the fact that they did not include all the attenuation461\ntreatments (Brandt et al., 2010), or could not distinguish UV and visible light effects462\n(Butler et al., 2023). Moreover, our study is the only one to assess photodegradation of463\nstanding dead litter in this type of grassland. Taken together, this makes comparisons464\ndifficult and highlights the need to include both sunlight spectra in future experiments.465\nPhotofacilitation can be defined as the increase in microbially mediated decomposition as a466\nconsequence of litter alterations due to the effect of photodegradation (Austin and Ballar´ e,467\n2024). We were able to infer accelerated biotic decomposition at our grassland site by468\nmeasuring extracellular enzymatic activities, and again we found striking differences469\nbetween the two species. The accumulated activity of the hydrolytic enzyme β-glucosidase470\ndid not significantly respond to light in P. stuckertii, but it did increase 50% under471\nUV-light exposure in D. hieronymi after 2 years (Figure 2). This supports the idea that472\nidentity of dominant species is important in determining both magnitude and direction in473\nbiogeochemical processes (Fan et al., 2024). Connecting back to studies in similar474\ngrasslands, Brandt et al. (2010) did not find any photodegradation effect on β-glucosidase475\nover the course of their study. Both experiments lasted for about 2 years, but there were476\nseasonality differences between the monsoonal regime (this study) and the Mediterranean477\nclimate in Brandt et al. (2010). We were able to measure photofacilitation towards the end478\n22\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nof the experiment, probably because this coincided with the end of the humid season.479\nMeanwhile, their study ended during the dry season, when biotic decomposition is typically480\nreduced. In this regard, a study with a similar design to ours in a Mediterranean open481\nwoodland did find photofacilitation effects of β-glucosidase activity (Berenstecher et al.,482\n2020). Their study ended after the wet season, just like ours, which possibly allowed for the483\neffects of photodegradation to accumulate over time, finally boosting biotic decomposition484\nonce humidity became available. This suggests that seasonal variation of rainfall and485\ntemperature can determine how abiotic and biotic decomposition mechanisms unfold.486\nIt is clear that precipitation seasonality plays a big role in determining the directions and487\nmagnitude of photodegradative effects in grassland ecosystems. Aside from the evidence of488\nphotofacilitation mentioned above, we were able to detect other effects of seasonality on489\ndecomposition at this grassland. This was possible due to our explicit experiment design490\nthat was customized to closely follow wet and dry seasons, following Berenstecher et al.491\n(2020). First, we detected a halt in decomposition during the second dry season (between 1492\nand 1.3 years), except for D. hieronymi under full sun treatment (Figure 3). This suggests493\nthat biotic activity was negligible during this period of low humidity, but that UV494\nradiation exposure maintained C losses for that species. Moreover, this direct495\nphotodegradation effect probably caused the apparent photofacilitation found at the496\nsubsequent humid season, as previously discussed. Next, we found that the order of wet497\nand dry seasons affected decomposition rates of P. stuckertii but not of D. hieronymi498\n(Figure 4). Cumulatively, our results suggest that the former responds more strongly to499\nrainfall seasonality, probably due to its higher water adsorption capacity (Figure 5), while500\nthe latter is more responsive to solar radiation.501\nOnly two photodegradation studies used a similar temporal design to evaluate seasonality502\n23\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\neffects (Berenstecher et al., 2020; Li et al., 2024). Berenstecher et al. (2020) found that503\nsamples under full sunlight after a year decomposed faster when they started in the dry504\nseason compared to starting in the humid season, similar to P. stuckertii in this study505\n(Figure 4). Li et al. (2024), additionally, studied photodegradation of forest litter with506\nseasonal snow cover. They found that litter that was exposed to sunlight at the start507\nduring the snow-free autumn decomposed faster than samples that started during the508\nsnow-covered winter. Evidently, although with differences between ecosystem types, the509\nalternation of precipitation and temperature seasons modulates the effects of510\nphotodegradation, and more studies should apply such experimental design to expand our511\nunderstanding of this interaction.512\nDecomposition not only entails organic mass loss, but also chemical and physical changes513\nas a response to biotic and abiotic drivers (Prescott and Vesterdal, 2021). It is important514\nto study these changes through time because they in turn affect the trajectory of the515\ndecomposition process and determine the chemical quality of the remaining organic matter516\nentering the soil (Cotrufo et al., 2015). However, most studies focus on changes in chemical517\ntraits over time (Brandt et al., 2010; Uselman et al., 2011; M´ endez et al., 2019), and nearly518\nno photodegradation studies have focused on physical traits. We detected a clear decrease519\nin LMA as an effect of blue and green light on P. stuckertii (Figure 6). Only one previous520\nstudy reported a similar decrease in LMA with exposure to UV light after 5 months521\n(Gaxiola and Armesto, 2015). A likely explanation for this result is that leaf mass was lost522\nwith little changes in leaf area, resulting in thinner litter and thus decreased LMA. We also523\ndetected a clear increase in water adsorption capacity in both species (Figure 6), a proxy of524\nlitter hydrophilia (Talhelm and Smith, 2018). This response to solar radiation could be a525\nconsequence of the degradation of hydrophobic leaf cuticles (Logan et al., 2022), and other526\n24\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nhydrophobic structural leaf compounds like lignin that respond strongly to527\nphotodegradation (Austin and Ballar´ e, 2010; Wang et al., 2024). Interestingly, the changes528\nin these two litter traits might be connected to lignin losses through photodegradation,529\nwhich would explain why litter became lighter and more hydrophilic over time (Austin and530\nBallar´ e, 2024). Considering physical litter traits affect decomposition dynamics and in turn531\nthose traits change over time with decomposition, it becomes evident that the relationship532\nbetween them adjusts dynamically over time and determines the fate of litter C (Sun et al.,533\n2022).534\nConclusions535\nOur study shows clear evidence that the effect of photodegradation is not limited to arid536\nenvironments, but it also affects mesic and highly productive ecosystems like grasslands.537\nWe show proof that in these ecosystems a considerable amount of mass loss happens in the538\nair before litter even touches the ground, both through direct abiotic and biotic539\ncontributions of solar radiation. Grasslands are estimated to cover 22.8% of the global land540\nsurface, representing a substantial proportion of terrestrial area (MacDougall et al., 2026).541\nIn these grass-dominated ecosystems, standing dead biomass constitutes a large portion of542\ntotal biomass (Sarmiento, 1992; Yang et al., 2024). Hence, we can project that wherever543\ngrasslands with marked seasonality are found and standing dead biomass persists for long544\nperiods, photodegradation will be an important driver of C cycling (Keiser and Nieland,545\n2025; Yang et al., 2025). Further, focusing on grasslands with precipitations ruled by a546\nmonsoon, the impact of photodegradation could potentially be even larger. Recently,547\nresearch has shown that in global drylands seasonality is an important driver of548\n25\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\ndecomposition, with faster decomposition in sites under monsoonal compared to549\nMediterranean climates (Siebenhart et al., 2025). The coincidence of peaks in temperature550\nand precipitation can boost microbial processes, including interactive effects with abiotic551\nmechanisms like photofacilitation. Photodegradation has many implications in the552\nterrestrial C cycle (Austin and Ballar´ e, 2024), from direct C emissions through553\nphotomineralization to changes in litter quality that affect the afterlife of plant-derived554\norganic matter in soils. Taking all into consideration, it becomes clear that the study of555\nphotodegradation in productive grasslands with monsoonal climates should be a priority556\nfor a better understanding of the terrestrial C cycle.557\nAcknowledgements558\nWe would like to thank Quebrada del Condorito National Park, especially Fernanda559\nFabbio, for facilitating our field experiment; Cecilia Palmieri for granting us access to her560\nproperty during field trips; Jos´ e Luis Lois, Franco Fern´ andez and Lucio Biancari for their561\nassistance during field work; Laura Ventura for her assistance in the laboratory; Gonzalo562\nArias and Lucas Enrico from IMBiV for their help with leaf toughness measurements.563\nAuthor Contributions564\nAgust´ ın Sarquis: Conceptualization, Methodology, Formal Analysis, Investigation, Writing565\n- Original Draft, Visualization, Funding Acquisition, Project Administration. Ignacio A.566\nSiebenhart: Investigation, Writing - Review and Editing. Marcela S. M´ endez:567\nInvestigation, Writing - Review and Editing. Amy T. Austin: Conceptualization,568\nMethodology, Funding Acquisition, Resources, Supervision, Writing - Review and Editing.569\n26\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nConflict of Interest Statement570\nWe declare that none of the authors have any conflict of interest.571\nReferences572\nAdministraci´ on de Parques Nacionales (2021). Disposici´ on aprobatoria de las Cargas573\nGanaderas del Programa de Herbivor´ ıa Dom´ estica del Parque Nacional Quebrada del574\nCondorito: DI-2021-44637752-APN-DRC#APNAC. Technical report.575\nAlmagro, M., F. T. Maestre, J. Mart´ ınez-L´ opez, E. Valencia, and A. Rey (2015). 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Grace,886\nE. Granda, H. M. Griffiths, M. Guerra Lara, M. Hasegawa, M. M. Hefting,887\nN. Hinko-Najera, L. B. Hutley, J. Jones, A. Kahl, M. Karan, J. A. Keuskamp,888\nT. Lardner, M. Liddell, C. Macfarlane, C. Macinnis-Ng, R. F. Mariano, M. S. M´ endez,889\nW. S. Meyer, A. S. Mori, A. S. Moura, M. Northwood, R. Ogaya, R. S. Oliveira,890\nA. Orgiazzi, J. Pardo, G. Peguero, J. Penuelas, L. I. Perez, J. M. Posada, C. M. Prada,891\n41\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nT. Pˇ r´ ıvˇ etiv´ y, S. M. Prober, J. Prunier, G. W. Quansah, V. Resco de Dios, R. Richter,892\nM. P. Robertson, L. F. Rocha, M. A. R´ ua, C. Sarmiento, R. P. Silberstein, M. C. Silva,893\nF. F. Siqueira, M. G. Stillwagon, J. Stol, M. K. Taylor, F. P. Teste, D. Y. P. Tng,894\nD. Tucker, M. T¨ urke, M. D. Ulyshen, O. J. Valverde-Barrantes, E. van den Berg, R. S. P.895\nvan Logtestijn, G. F. C. Veen, J. G. Vogel, T. J. Wardlaw, G. Wiehl, C. Wirth, M. J.896\nWoods, and P.-C. Zalamea (2022, sep). Termite sensitivity to temperature affects global897\nwood decay rates.Science (80-. ). 377(6613), 1440–1444.898\nZeballos, S. R., J. J. Cantero, M. A. Giorgis, A. T. R. Acosta, C. O. N´ u˜ nez, M. V.899\nPalchetti, D. S. Argibay, and M. R. Cabido (2024, oct). Classification of montane900\ngrasslands in central Argentina.Appl. Veg. Sci. 27(4).901\nZhang, D., D. Hui, Y. Luo, and G. Zhou (2008, jun). Rates of litter decomposition in902\nterrestrial ecosystems: global patterns and controlling factors. J. Plant Ecol. 1 (2), 85–93.903\n42\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nFigure Captions904\nFigure 1: Field plot with D. hieronymi cover (a); close-up of litter envelopes of P.905\nstuckertii (b); time frame for the litterbag placement and overall experimental design (c).906\nDark green triangles mark sample collection dates. Photographs by A. Sarquis.907\nFigure 2: Organic mass (OM) loss (%; a, b) and accumulated β-glucosidase activity (µmol908\ng−1; c, d) over time for both species of Winter Group 1. Asterisks denote significant909\ndifferences between filters per date: ** p < 0.01, * p < 0.05. Upper-case letters denote910\ndifferences between filters following Tukey HSD test. Bars represent mean values and911\nstandard errors. Results in this figure show OM mass loss, but statistical analyses were912\nperformed on remaining OM. R+: full radiation. UV-: UV blocked. R-: UV, blue and913\ngreen light blocked.914\nFigure 3: Organic mass (OM) remaining (%) pre- and post-dry season of 2022. Shapes are915\nmean values and bars are standard errors. Asterisk denotes significant differences between916\ndates per filter: * p < 0.05. Only Winter Group 1 values are included in this figure. R+:917\nfull radiation. UV-: UV blocked. R-: UV, blue and green light blocked.918\nFigure 4: Decomposition rates for each filter by species and starting group. Daily organic919\nmass (OM) loss (% day −1; a, b) was calculated for samples after 1 year and k constants920\nwere calculated at the end of the experiment. Asterisks denote significant differences921\nbetween filters of the same starting group: *** p < 0.001, ** p < 0.01, * p < 0.05.922\nUpper-case letters denote differences from Tukey test between filters of the same starting923\ngroup. Plus signs denote significant differences between starting groups (for filter level R+924\nonly): ++ p < 0.01, + p < 0.05. Bars are mean values with standard errors. R+: full925\n43\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nradiation. UV-: UV blocked. R-: UV, blue and green light blocked.926\nFigure 5: Initial physical and chemical initial traits of the two species. Asterisks denote927\nsignificant differences between species: *** p < 0.001, ** p < 0.01, * p < 0.05. A305:928\nsunscreens with absorptance at 305 nm. LMA: leaf mass per area.929\nFigure 6: Relative effect of field decomposition on physical traits compared to initial values930\nfor both species under each filter. Colored squares denote significant differences for a t-test931\n(p < 0.05). Colors represent direction of change and intensity: blue shades represent932\nincreases in trait values and pink shades represent decreases, while lighter shades represent933\nlow-moderate changes (0 – ±0.5) and darker shades represent bigger changes (±0.5 –934\ninfinite). LMA: leaf mass per area.935\n44\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nFigures936\na\nb\nDry season Humid season Dry season Humid season\n2021\nJ J A S O N D J F MA M J J J JA S O N D J F MA M\n2022 2023\nWinter group 1\nSpring group\nWinter group 2\nc\nFigure 1: Experimental design\n45\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nOM loss (%)\nYears\nR+ UV- R-\n0\n10\n20\n30\n40\n50 a\n**\nA A B\n0.3 1 1.3 2\n**A\nB B\n**A\nB B\nb\nβ-glucosidase activity\n(umol . g-1)\nc\n0\n20000\n40000\n60000\n0.3 1 1.3 2\nP. stuckertii D. hieronymi\nd *\nBB\nA\nFigure 2: Mass loss and enzymatic activity\n46\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\n70\n75\n80\n85\n70 75 80 85\nOM remaining pre−dry season (%)\nOM remaining post−dry season (%)\nFilter\n● R+\n● UV-\n● R-\nSpecies\nDeyeuxia\nPoa\n*\n●\n●\n●\nFigure 3: Dry season mass loss\n47\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nOM loss  (% day-1)\nStart season\n***A\nB\nB\n**\nA\nBB\nb\n D. hieronymi\n***A\nB\nB\n***A\nB\nC\nWinter Spring SpringWinter\nd\n0.00\n0.02\n0.04\n0.06 ++\na\nk (yr-1)\nP. stuckertii\n0.0\n0.1\n0.2\n0.3\n*A\nBAB\n+\nc\nR + UV - R - Winter Spring\nFigure 4: Decomposition rates\n48\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nSoluble \ncarbohydrates (%)\nD. hieronymi\nP. stuckertii\nSpecies\nPhysical\nChemical\nTraits\n200\n173\n9\n8\n38\n43\n53\n49\n5 5\n1 4\n0.1 0.3\n65\n82\n0.14\n0.12 0.07\n0.08\n0.7\n0.50.4\n0.09\nHemicellulose\n(%) **\nCellulose\n(%) ***\nLignin\n(%)\nSaccharification\n(mg/g)\nTotal\nsugars (mg/g)\nWater adsorption\ncapacity (%) *\nLMA\n(mg mm-2) ***\nToughness\n (N mm-1) ***\nPotential\nleaching (%) *\nTotal polyphenols\n (mg/g)\nA305 **\nFigure 5: Initial litter traits\n49\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint \n\nRelative effect\n0.5 - + Inf  \n0 - 0.5 \nn.s.\n-0.5 - 0 \n-Inf - -0.5 \nLMA\nR+\nUV-\nR-\nP. stuckertii\n-0.03 -0.1 -0.18 -0.16 -0.3 -0.27\n-0.25\n-0.19 -0.17\n-0.18 -0.21\n-0.09 -0.06\n-0.12 -0.08\n-0.1 0.04\n-0.03\n0.3 0.7 1.0 1.3 1.7 2.0\nR+\nUV-\nR-\nD. hieronymi\n0.35 0.03 0.19 0.29 0.22 0.29\n0.2 0.150.19 0.260.05 0.18\n0.35 0.07 0.24 0.16 0.17 0.35\nYears\nWater adsorption capacity\n0.45\n0.75\n0.72\n0.56\n1\n0.74\n0.12\n0.46\n0.18\n0.19\n0.42\n0.21\nD. hieronymi\n0.07\n0.41\n0.33\n0.25\n0.37\n0.45\n−0.14\n−0.02\n−0.02\n−0.01\n0.16\n0.12\nP. stuckertii\nR+\nUV-\nR-\n−0.14\n−0.25\n−0.19\n−0.31\n−0.38\n−0.32\n−0.02\n−0.12\n0.08\n0.36\n0.33\n0.37\nToughness\nP. stuckertii\nR+\nUV-\nR-\n0.3 1.0 1.7 2.0\n−0.28\n−0.41\n−0.54\n−0.38\n−0.47\n−0.43\n−0.27\n−0.41\n−0.24\n−0.25\n−0.12\n−0.25\nD. hieronymi\n0.3 1.0 1.7 2.0\nYears\nFigure 6: Traits changes\n50\n.CC-BY 4.0 International licenseavailable under a \n(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 \nThe copyright holder for this preprintthis version posted March 10, 2026. ; https://doi.org/10.64898/2026.03.06.709891doi: bioRxiv preprint","source_license":"CC-BY-4.0","license_restricted":false}