The role of UV-B radiation and precipitation on straw decomposition and topsoil C turnover

In arid and semi-arid area, ultraviolet-B (UV-B) radiation plays a mainly positive role in litter decomposition. In subtropical area, however, the role of UV-B radiation remains uncertain due to the interference of precipitation. To evaluate the potential contribution of precipitation to the role of UV-B radiation, we exposed 84 bags of rice straw to ambient and reduced UV-B radiation, with and without water addition after each precipitation event, on the roof of a building at Huazhong Agricultural University (Wuhan, China). Additionally, the indirect effects of UV-B radiation on soil dissolved organic carbon (DOC) were investigated by placing 70-g soil samples beneath 5-g straw bags with precipitation treatments. Mass loss, DOC and water extractable phenols (WEP) were monitored over 228 days. Subsequently, microbial facilitation was studied, by incubating straw and soil samples exposed for the longest period (228 days) in the laboratory at 25 °C, and by afterwards analyzing their CO2-C emission. Our results demonstrated that UV-B radiation did not significantly affect straw mass loss, but induced an increase in straw DOC, WEP and CO2-C emission by 20.6%, 10.7% and 20.4%, respectively, under dry conditions. Whereas, combined with precipitation, the effects of UV-B radiation on straw DOC, WEP and CO2-C emission were negligible. Only a small decrease in soil DOC (9.9%) and CO2-C (4.0%) was observed. Furthermore, UV-B radiation interacted significantly with precipitation during straw decomposition. These results indicate that for dry conditions UV-B radiation accelerates straw decomposition by increasing DOC content. For wet conditions, however, the effects of UV-B radiation on straw decomposition are balanced out and even negative on topsoil C turnover.