Soil Microbial Responses To Precipitation And Its Regulation Of The Nitrogen Cycle In Songnen Meadow Steppe

Human activities lead to continuous global warming,resulting in drastic changes in precipitation pattern.The change of precipitation will significantly affect the structure and function of temperate grassland ecosystem with water and nitrogen as the limiting factors.Grasslands account for more than 40% of the land area of China,and they play a vital role in the nitrogen cycle process of terrestrial ecosystems.Nitrogen cycle is prominent in biogeochemical cycle and is closely related to human life.The nitrogen transformation processes were driven by microorganisms.The effect of altered precipitation on nitrogen cycle was mainly regulated by soil microorganism,but its response and regulation mechanisms are far from being clear.Therefore,study on the soil microbial responses to precipitation and regulation of the nitrogen cycle is helpful to understand the interaction between the aboveground and underground biomes and the nutrient cycle process of grassland ecosystem.This study was conducted at the Songnen meadow steppe,which is dominated by Leymus chinensis(Trin.)Tzvel.,a C3 perennial rhizomatous grass.We used three manipulative experiments to simulating precipitation changes,including a 50% reduction to 50% increases in precipitation,different levels of drought and seasonal severe drought.To investigate the adaptation and response of soil microorganisms and nitrogen cycle to precipitation change,we aimed to reveal the response of soil microorganism to precipitation change and its regulation mechanism to nitrogen cycle.We report the main results and conclusions as follows:(1)Using the in situ precipitation increase and decrease experiment,soil bacteria and fungi were selected as experimental microorganisms,we conducted the rainout shelters over two consecutive growing seasons(2016 and 2017),so as to study the response mechanism of soil microbial community diversity and composition to precipitation changes.Altered precipitation(from-50% to +50%)had significant impacts on bacterial diversity,the ambient precipitation plots had the greatest bacterial diversity.However,the effects of altered precipitation on fungal diversity were not significant.Interannual variation in annual precipitation had greater impacts on fungal diversity than bacterial diversity.The precipitation gradient significantly altered the structure of the bacterial community in both years,but not on the structure of the fungal community.Extracellular enzyme activities of carbon cycling enzymes and nitrogen cycling enzymes gradually increased;whereas phosphorus cycling enzyme gradually declined along the increasing precipitation gradient.This means that increased precipitation promoted microbial activity associated with the carbon and nitrogen cycle,and phosphorus cycle decoupled.Soil water content was the unique driver for bacterial diversity;aboveground biomass and soil properties were identified as significant drivers for fungal diversity.Microbial activity was mainly related to changes of soil water content,aboveground biomass and soil properties(soil organic carbon,total nitrogen,total phosphorus and p H),which were altered by changes in precipitation.(2)Using the in situ reduction precipitation experiment,we conducted the rainout shelters over three consecutive growing seasons(from 2016 to 2018),and we focused on the nitrogen transformation rates and microbial functional genes,so as to study the response mechanism of soil nitrogen transformation rates by microbial functional genes regulated to different drought levels.With the severity and duration of the drought,soil nitrate content increased significantly,while ammonium content decreased significantly,which indicating that drought promoted the nitrification.Drought had no significant effect on soil nitrification rate and mineralization rate,however,the results of the structural equation model showed that the change of precipitation was positively correlated with the nitrification and mineralization rate.That is,with the increase of precipitation,the trend of nitrification and mineralization rates were increasing.Drought significantly reduced the growing season mean nitrous oxide flux.Severe drought resulted in significant decreases in the abundance of nif H gene,AOB gene and nir K gene,while the abundance of denitrifying nar G gene was significantly increased.Under the condition of precipitation change,the abundance of microbial functional genes directly regulates the soil nitrogen transformation rates,especially the nitrification functional genes.AOB gene mainly regulated soil net nitrification rate and mineralization rate,and AOA gene mainly regulated nitrous oxide emission,that indicating precipitation changes mainly affect the flux of nitrous oxide by the nitrification process.In addition,soil water content and inorganic nitrogen content were also important factors affecting soil nitrogen transformation rates.(3)Using the in situ reduction rainfall in summer and snow in winter experiment,we conducted the rainout shelters and simulated snow removal over one year(from 2015 to 2016),and we focused on the soil nitrogen availability and microbial biomass,so as to study the response mechanism of soil nitrogen availability by microbial biomass regulated to seasonal severe drought.Shoot biomass was significantly decreased by both drought treatments,and there was significant effect of the winter drought treatment on plant cover.There was a significant decline in bacterial,fungal and total microbial biomass under the drought treatments.As the drought continued,the dominant treatment explaining variation in soil microbial community structure shifted from the winter drought treatment to the summer drought treatment.Throughout the drought period,each nitrogen variable increased compared with the control.Summer drought significantly increased the concentrations of soil inorganic nitrogen.Drought promoted accumulation of the soil inorganic nitrogen,and rewetting can decrease inorganic nitrogen concentrations after drought,which potentially resulted in the decreased resistance of nitrogen availability.Nitrogen availability in response to winter drought was associated with biotic factors.Nitrogen availability in response to summer drought was linked to abiotic and biotic factors.In summary,altered precipitation significantly changed the diversity and structure of the bacterial community in both years,but not on the diversity and structure of the fungal community.The moderate range of growing season precipitation(200-280 mm)resulted in higher microbial diversity in Songnen meadow steppe.Compared with bacterial diversity,fungal diversity was sensitive to interannual differences in precipitation.Precipitation changes soil nitrogen transformation rates were regulated by microbial functional gene abundance.Under drought conditions,nitrous oxide emission was mainly controlled by ammonia oxidizing archaea through nitrification in the study area.Soil net nitrogen mineralization and nitrification rates were mainly controlled by ammonia oxidizing bacteria,soil water content and inorganic nitrogen content.Both changes in abiotic factors and biotic factors explained the drought effect on soil nitrogen availability over summer,only changes in biotic factors explained the legacy effect of winter drought.Our results highlight that both the effects of summer and winter drought are important to consider in predicting these nitrogen variables responses.In this study,we analyzed the response of soil microorganisms to precipitation change and their regulation mechanism to nitrogen cycle.The results provide important experimental data support for predicting the adaptability of soil microorganisms and the functional response strategy of ecosystem nitrogen cycle under the future changed precipitation regime.