Litter Mixing Effects Modulate UV-Induced Photodegradation Across Grasslands

UV radiation can strongly affect litter decomposition across broad climatic gradients, but the magnitude of its contribution differs among ecosystems. Current understanding of this process remains largely based on single-species litter experiments, which overlook the compositional complexity of natural litter inputs and the potential role of litter mixing in modulating UV-driven photodegradation. To address this gap, we conducted a mixed-litter decomposition experiment within a UV-manipulation network across a 25° latitudinal gradient spanning four grassland types in China. Using biomass-weighted mixtures of locally dominant species to represent natural litter inputs at each experimental site, we examined how variations in litter diversity and trait dispersion shaped the regional pattern of UV effects on mixed-litter decomposition. After four-year field decomposition, UV attenuation prolonged litter mean residence time by 3%-35% across sites. Litter mixtures with higher taxonomic and phylogenetic diversity showed faster early-stage mass loss, a pattern explained by greater availability of dissolved organic carbon for microbial decomposition. In contrast, greater dispersion in species-level C/N ratios within the litter mixtures imposed increasingly stronger stoichiometric constraint on biotic degradation as decomposition progressed. The magnitude of UV-induced photodegradation increased with UV intensity. However, faster decomposition in phylogenetically diverse mixtures shortened the duration of radiation exposure, thereby constraining the strength of UV-induced litter photodegradation. This mixing-effect-mediated constraint presented a stronger influence than UV exposure intensity on explaining regional variation in UV effects on litter decomposition. Overall, these findings underscore a previously underappreciated role of local litter mixture complexity in modulating the spatial pattern of UV-induced photodegradation across grassland ecosystems. By linking litter mixing effects with UV-driven decomposition, this study extends photodegradation research beyond the common focus on single-species litter and highlights the importance of community-level litter complexity for predicting grassland carbon turnover concurrent with climate change and biodiversity loss.