Effects Of UV-B Radiation And Moisture On Straw Decomposition And Soil Organic Carbon Turnover

Straw returning into soil is one of the most common and effective practices for crop field management. In the background of global warming, the straw decomposition and its effect on soil carbon cycle after its returning have become a hotspot and attracting for many studies. There are many factors influencing rice straw decomposition in soil and the global carbon cycle, such as temperature, moisture and chemical properties of the straws. In recent years, the incidence of UV-B radiation on the earth enhanced as the decrease of ozone layer and the response of terrestrial ecosystem carbon cycle on UV-B radiation got more and more attention from many researchers. However, there are not enough researching about the decomposition of straws in response of climate change and UV-B radiation on soil carbon cycle in agroecosystems and the interaction between them. Therefore, in this study, we designed several experiments to explore the effects of straw addition on soil organic carbon turnover at dry and slightly flooded conditions, as well as the response of rice straw decomposition and organic carbon turnover in soil and soil+straw mixture under UV-B radiation through a few laboratory experiments and a long-time exposure experiment. The main results are as follows:1) After straw addition, CO2emission rate, DOC and alkali extractable phenols (AEP) increased quickly no matter at dry or slightly flooded condition, with a average rise of278.2%,42.2%and47.2%, respectively, while NH4+-N declined to a relative low level in one week; at slightly flooded condition, soil phenol oxidase activity and Fe2+concentration were higher than that at dry condition, with average increase of4.9and42.4times, respectively, and straw addition enhanced DOC content and cumulative C emission at slightly flooded condition.2) In laboratory incubation experiment, the effects of UV-B radiation on straw decomposition and subsequent microbial availability differed significantly at dry and wet conditions. At dry condition, UV-B radiation had no significant effect on straw chemical properties, while at wet condition, UV-B radiation decreased DOC, water extractable phenols (WEP), C, N and aromatic compounds, but increased DON content only (average rise by15.3%). Moisture also affected straw decomposition. The DOC, DON and aromatic compounds in wet straw were higher than that in dry straw, with average increase of13.7%,45.7%and26.1%, respectively, while the C content and microbial availability in wet straw were lower than that in dry straw.3) In laboratory, UV-B radiation had no significant effect on dry soil organic carbon turnover, but decrease flooded soil DOC and Fe2+concentration by25.6%and50.5%respectively in the end of the experiment. After straw addition, UV-B did not influence dry soil significantly, but brought down (most seriously during day5to15) the DOC and phenol oxidase in the flooded soil and water layer and raised soil WEP by an average rate of42.0%.4) In field exposure condition, UV-B radiation had no significant influence on mass loss and C, N content in dry straw, but boosted DOC and WEP content of dry straw by20.6%and10.7%respectively in the end of the field experiment, as well as subsequent microbial availability. Artificial precipitation decreased the straw mass loss by about30%and straw DOC and WEP by rates greater than50%, as well as straw microbial availability. UV-B radiation had no significant influence on the DOC, WEP and microbial availability of straw with artificial precipitation, but raised its C content in the late period by12.1%5) In field condition, UV-B increased the DOC and WEP of dry soil and soil+straw mixture slightly, as well as microbial availability in soil with artificial precipitation. Artificial precipitation decreased DOC (11.7%,25.3%) and subsequent microbial availability of soil and soil+straw mixture. Straw mulching raised soil DOC and subsequent cumulative C emission by77.7%and96.2%, respectively, and UV-B radiation decreased DOC content of soil with straw mulching.6) To sum up, straw addition will improve soil organic carbon turnover and the moisture condition will influence straw decomposition. In laboratory condition, UV-B radiation has no significant effect on dry straw and dry soil, but in the field condition, UV-B radiation will promote straw decomposition and soil organic carbon turnover. The moisture condition of straw and soil and the climate factors, such as precipitation would also promote or inhibit the effect of UV-B radiation. UV-B radiation can affect straw decomposition and soil organic carbon turnover indirectly through changing the subsequent microbial availability of straw and soil.