Ultraviolet radiation effects on biodegradation depend on precipitation amount, litter quality, and time in an arid land

Although the effects of ultraviolet (UV) radiation on litter decomposition rates are well-documented, the influence of UV radiation on biodegradation and its relative importance to litter decomposition under elevated precipitation in arid regions remain unclear. Here, we conducted a 12-month experiment to examine the effects of UV radiation and precipitation on litter decomposition and microbial community properties, including mass loss and element dynamics, enzyme activities, and microbial biomass, diversity, and community composition. Results showed that UV radiation and precipitation accelerated litter decomposition, with their effects on biodegradation dependent on season and litter type. In the summer, UV radiation exerted photo-inhibitory effects on biodegradation, reducing bacterial biomass (particularly the relative abundance of UV-sensitive taxa), diversity, and enzyme activities. In contrast, precipitation enhanced biodegradation by increasing these factors. However, in the winter, UV radiation exerted photo-facilitatory effects on biodegradation by promoting bacterial and fungal biomass (particularly the relative abundance of UV-resistant taxa) and enhancing lignocellulolytic enzyme activities. Precipitation continued to enhance biodegradation by facilitating the production of fungal biomass and phenol oxidase enzyme activity. Moreover, litter mass loss and most microbial community properties were higher in the high-quality litter. Subsequently, effect size analysis revealed that, while precipitation did not alter the relative importance of UV radiation to litter mass loss, it decreased photo-inhibitory effects in the summer and photo-facilitatory effects of UV radiation on biodegradation in the winter. Interestingly, interactions between UV radiation and litter type amplified the photo-facilitatory effect of UV radiation on decomposition and microbial activity in high-quality litter during the winter. Overall, our results show that the relative abundance of specific bacterial taxa primarily drives litter biodegradation, and that increased precipitation and litter quality significantly modify photodegradation-biodegradation interactions by shifting these key microbial groups. Future studies should encompass a broader diversity of ecosystems to enhance the generalizability of the findings.