Photodegradation, especially by ultraviolet (UV) radiation, has been found to greatly accelerate litter decomposition in dryland ecosystems, even where microbial activity is severely inhibited by water limitation. However, our knowledge of photodegradation is mainly derived from laboratory or local-scale experiments. How UV radiation interacts with environmental factors to govern the pattern of regional or continental-scale litter decomposition is still unclear, which constrains our ability to quantify the contribution of photodegradation on terrestrial C cycling under the changing climate. In this study, we conducted a coordinated distributed photodegradation experiment covering alpine steppe, alpine meadow, typical steppe and desert steppe across a 5000 km transect in China in 2016. At each site, UV-transparent and UV-absorbing sheets were used to create different levels of UV exposure. The standard litter and litter from local dominant species were placed at each site, and the field decomposition lasted for 3.5 years. The mean residence time (MRT) of litter was calculated using Weibull litter decomposition model. We found that the UV-induced decrease in MRT was observed at all experimental sites, which was mainly contributed by the acceleration on litter decomposition at the late stage. We further partitioned the relative contribution of climate and litter quality to UV-induced reduction in MRT (ΔMRT) using a photodegradation model. We found that a progressive increase in ΔMRT was observed following the increase in UV intensity under the cold and dry environment. With increasing temperature and precipitation, the positive effect of UV radiation on abiotic photodegradation was gradually offset by its inhibitory effect on microbial activity. The mean litter layer MRT of grasslands in China is 1.7 years, and UV radiation averagely accelerates litter layer turnover by 0.3 years. Our findings elucidate the geographical pattern of photodegradation in grasslands in China. We emphasized the importance of photo-degradation in regulating the C cycling of dryland ecosystems, especially in a warmer and drier future.
