The Response Of Soil Extracellular Enzyme Activities To Nitrogen Deposition And Precipitation Changes In The Temperate Grassland

Human activity has globally increased atmospheric nitrogen?N?deposition as well as altered precipitation regime.How these global change factors influence soil carbon turnover has been recognized as the key process of carbon cycling in terrestrial ecosystems.During the decomposition of soil organic matter,soil microorganism secret extracellular enzymes to breakdown the complex compounds as soluble low-molecular-weight compounds for assimilation as well as respiration.Thus,soil extracellular enzymes are often the rate-limiting step in the decomposition of soil organic matter.However,there is still a lack of studies on how enzyme activities respond to global changes and how these enzyme activities regulate the responses of soil organic matter decomposition to global change.Here,one long-term multi-level N addition experiment in typical steppe and one precipitation manipulation experiment across three grassland ecosystems were conducted in Duolun County,Inner Mongolia,respectively.We examined soil carbon?C?,N,phosphorus?P?-acquisition hydrolases,oxidase activities,microbial biomass,physical and chemical properties and heterotrophic respiration?Rh?to explore the responses of extracellular enzyme activities to N deposition and changing precipitation,as well asthe regulatory mechanism of microbial decomposition by enzyme activities.The results of the N addition study showed that with the increasing N addition level,soil p H value,Rh,microbial biomass,C-and N-acquisition hydrolytic enzyme activities all exhibited declining trends.The P-acquisition hydrolase activity enhanced by 106.9% and 67.2% at high nitrogen addition levels(32 g N m^-2 y^-1 and 64 g N m^-2 y^-1).The activity of oxidase showed a non-linear response to the increasing N addition level with increasing under low level of N addition but decreasing under high level of N addition.In addition,ratios of both C to N and C to P of specific acquisition enzymes declined with increasing N addition level.The structural equation model further showed that the activity of ?-glucosidase?BG?and soil p H together explained 63% of variation in soil heterotrophic respiration,revealing that the N-induced inhabitation of microbial respiration was due to the suppression of BG activity and soil acidification,rather than the reduced microbial biomass.Overall,our findings provide enzymatic pathway and regulatory mechanism of the reduced microbial respiration under N deposition.The results of precipitation manipulative experiment showed that precipitation interacted with grassland ecosystem type to affect soil extracellular enzyme activities.The decreased precipitation reduced soil C-,N-,and P-acquisition hydrolase activities by 34.9%,41.6%,38.2% and 54.6% in the typical steppe,respectively.Soil P-acquisition hydrolase and oxidase activities were reduced by the decreased precipitation by 23.3% and 44.8% in the desert steppe,respectively.However,the decreased precipitation did not influence soil enzyme activities in meadow steppe.Different from the effects of the decreased precipitation treatment,the increased precipitation stimulated soil C-,N-,and P-acquisition hydrolase activities by 40.7%,30.0%,41.6% and 70.6% in the meadow steppe,respectively.Soil oxidase activity was stimulated by the increased precipitation in the desert steppe only.However,there were no significant effects of the increased precipitation on soil enzyme activities.In addition,the changed precipitation had no impact on the stoichiometry of soil extracellular enzymes.The structural equation model demonstrated that 91% of the variations of soil Rh were ascribed to changes in soil BG activity and soil temperature,suggesting that soil BG activity and temperature jointly mediated soil Rh response to shift in precipitation regime.Our findings facilitate the understanding how soil extracellular enzyme activities respond to N deposition and changed precipitation and the underlying mechanisms.The results disentangle pathways by which soil extracellular enzymes regulate microbial respiration under global changes,quantify the contribution of soil enzymes in mediating soil organic matter decomposition.It is needed to combin e soil microbial communities and functions to further explore microbial regulatory process under global changes in the future studies.