Response Of Soil Nitrogen Cycle To Simulated Precipitation Reduction In A Southern Subtropical Castanopsis Hystrixplantation

The global precipitation pattern has changed in time and space and increasing seasonal drought is one of the important characteristics of global climate change.Soil nitrogen cycle is an important component of the nutrient cycle of forest ecosystems,directly affecting the ecological service functions such as carbon cycle,food supply,as well as soil and water conservation.The change in precipitation pattern will affect soil nitrogen cycle of forest ecosystems by altering soil nitrogen input,nitrogen transformation,and nitrogen output,which in turn affects the productivity and biodiversity of terrestrial ecosystems.Therefore,a typical plantation in the southern subtropical region——Castanopsis hystrix plantation was selected as the research subject.A continuous five-year field precipitation controlled experiment including 50%reduction of throughfall treatment(TRT)and natural rainfall(control)was conducted in C.hystrix plantation.Using litter basket and decomposition bag methods,buried plastic pipe with caps,and static box method,we analyzed the dynamic changes of litterfall production and decomposition rates,soil nitrogen mineralization processes and its mechanism,the relationship between soil N2O and CO2 emissions under the throughfall reduction condition.This study is aimed at revealing the responses of soil nitrogen cycle to simulated precipitation reduction in the southern subtropical C.hystrix plantation ecosystem,and provide a theoretical basis for developing forest management strategies to adapt to global climate change.The main results were as follows:(1)Compared with the control,the TRT caused significant increases in the branch litterfall in 2015(30%)and the total litterfall in 2017(36%),suggesting that TRT had significant effects on the litterfall production in C.hystrix plantation,which was yearly different,and differed among litterfall components.With the increase of decomposition time,the carbon concentration of leaf litter showed a trend of "rapid declined and then leveled out",while the nitrogen concentration of leaf litter exhibited "rapid increased,leveled off and then re-increased" in the trend,indicating that the release patterns of leaf litter carbon and nitrogen were distinct.After two years of leaf litter decomposition,the TRT did not significantly influence the decomposition coefficient,remaining mass,carbon concentration,and carbon/nitrogen ratio of leaf litter,although the TRT resulted in a 25%reduction in the decomposition coefficient of leaf litter but 32%and 33%increases in the time of 50%and 95%losses of litter quality compared to the control without statistical differences between treatments.(2)TRT had no significant effects on soil ammonia nitrogen,total nitrogen,dissolved organic nitrogen,net ammonification rate(NAR)and net nitrogen mineralization rate(NMR).However,in the wet season,TRT significantly increased the NNR at 0-10 cm soil depth by 136%compared to the control;in the dry season,TRT significantly decreased the NNR at 10-20 cm soil depth by 125%relative to the control.The changes in the soil moisture in dry and wet seasons influenced the soil microbial community structure,especially arbuscular mycorrhizal fungi,and soil net nitrification rate,further altered the soil nitrogen cycle of the C.hystrix plantation.In addition,the soil microbial biomass and microbial community structure were decoupled under the TRT in C.hystrix plantation.The contents of soil nitrate nitrogen,dissolved organic carbon and dissolved organic nitrogen were the main factors affecting soil microbial community structure at 0-10 cm soil depth in the wet season of C.hystrix plantation.(3)TRT resulted in a significant increase of 39%in the soil CO2 emission flux in the wet season compared with the control,while did not significantly affect soil N2O emission flux in C.hystrix.Moreover,there was a significant positive correlation between soil CO2 and N2O fluxes.According to the Pearson's correlation analysis,soil CO2 emission flux showed a binary linear correlation with soil temperature and moisture at soil depth of 5 cm.However,soil N2O emission flux was not significantly correlated with soil temperature and moisture at soil depth of 5 cm.