Effects Of Increased Temperature And UV-B Radiation On Soil Nitrogen Supply And Apparent Balance In Rice And Azolla Rotation Paddy Field

Warming and UV-B radiation enhancement are two major global climate change issues today and in the future,both of which have significant impacts on farmland ecosystems and their material cycles.However,the combined effects of the two have limited understanding of the effects of nitrogen supply and balance in paddy soils.The Yuanyang terraced field is located in the southern Yunnan Plateau.The paddy field does not apply chemical fertilizers,and relies on rice straw returning and Azolla nitrogen fixation to maintain the nitrogen balance and supply of the paddy field for thousands of years.However,its response to warming and UV-B radiation enhancement is unclear.A field experiment was carried out in a paddy field with an altitude of 1600 m in Yuanyang terraced field.The local rice varieties“Baijiao Laojing”and Azolla were used as the research objects.Simulated warming(+2℃)and UV-B radiation(5 kJ·m-2)treatments were used to collect samples of rice and Azolla,paddy soil and atmosphere during rice growth period(tillering stage,jointing stage,booting stage and maturity stage)and winter fallow period(November,December 2021 and January,April 2022).The effects of temperature and UV-B radiation on plant biomass,nitrogen content,photosynthetic physiology of rice and Azolla and soil nitrogen form and content,nitrogen transformation related functional enzyme activities were studied.The results showed that:(1)Warming promoted the growth of rice and Azolla and significantly increased the number of tillers,biomass of rice and biomass of Azolla by 40.93%,50.89%and 19.97%,respectively.The growth of Azolla was inhibited by UV-B radiation,and the tillering number,biomass and biomass of Azolla decreased by 8.27%,27.29%and 6.27%,respectively.However,warming and UV-B radiation increased the number of rice tillers by 3%,and the total biomass of rice and Azolla decreased by 5.89%and 16.77%,respectively.Warming alleviated the inhibitory effect of UV-B radiation on rice biomass and tiller number.(2)Warming significantly increased the nitrogen content and nitrogen accumulation of rice weed plants,and the nitrogen accumulation of rice increased by 81.47%;UV-B radiation significantly increased the nitrogen content of rice and Azolla plants,but decreased the nitrogen accumulation by 10.10%.The nitrogen content of rice was lower than that of single factor,but the nitrogen accumulation of rice increased by 2.77%.Warming alleviated the inhibition effect of UV-B radiation on nitrogen accumulation in rice.(3)Warming significantly reduced soil alkali-hydrolyzed nitrogen content by 14.13%-31.83%,and increased soil protease and urease activities;UV-B radiation significantly increased NO3--N content in soil by 9.19%-101.61%,but significantly reduced soil microbial biomass nitrogen content by 16.83%-36.56%.Warming and UV-B radiation resulted in a significant decrease of 30.54%in soil inorganic nitrogen content at booting stage and 22.28%-24.12%in soil alkali-hydrolyzed nitrogen content at winter idle stage.Warming,warming and UV-B radiation decreased soil nitrogen supply capacity,while UV-B radiation increased soil nitrogen supply capacity.(4)CK,warming,UV-B radiation,warming and UV-B radiation caused soil nitrogen input of paddy fields to be 12.05,16.34,12.89,13.11 g·m-2,and nitrogen output to be 10.50,15.15,9.65,10.88 g·m-2,respectively.The decrease of soil inorganic nitrogen was 0.81,1.63,-2.32 and 3.97 g·m-2,respectively,and the apparent loss of soil nitrogen was 2.36,2.82,0.92 and 6.20 g·m-2,respectively.Compared with CK,warming,warming and UV-B radiation increased soil nitrogen apparent loss,and UV-B radiation decreased soil nitrogen apparent loss.(5)Correlation analysis showed that NH4+-N content was positively correlated with alkali-hydrolyzed nitrogen content and protease activity.Soil urease activity was positively correlated with alkali-hydrolyzed nitrogen and microbial biomass nitrogen content,and negatively correlated with NO3--N content.In conclusion,warming,warming and UV-B radiation both decreased soil nitrogen supply and increased apparent nitrogen loss.UV-B radiation increased soil nitrogen supply capacity and reduced apparent nitrogen loss.Warming can promote soil mineralization by increasing the activities of soil protease and urease.It also alleviated the inhibition effect of UV-B radiation on rice nitrogen accumulation,promoted rice to absorb soil inorganic nitrogen,increased soil nitrogen output,and reduced soil inorganic nitrogen residue,thus reducing soil nitrogen supply capacity,and ultimately led to increased soil nitrogen apparent loss.