Legacy Effects Of Historical Nitrogen And Water Addition On Soil Microbiological Characteristics In A Typical Steppe

It was predicted that nitrogen deposition and precipitation will increase in eastern Inner Mongolia in the next 50 years.Nevertheless,it has also been reported that under the new pattern of global change,atmospheric nitrogen deposition and precipitation increment are not sustainable,and the directional change of both may stop or weaken at regional scale.The environmental effects of nitrogen deposition and precipitation increment have been extensively studied,however the legacy effects of nitrogen and water on ecosystem are still unclear.This study took advantage of a long-term(13-year)nitrogen and water addition field experiment in eastern Inner Mongolia.In 2018,the treatments were stopped in order to explore the legacy effects of historical nitrogen and water addition on soil physical and chemical properties,microbial biomass,and the soil enzymatic activities in a typical steppe.The main results are as follows:(1)Historical nitrogen addition had negative legacy effects on soil p H after two years,and positive legacy effects on ammonium nitrogen,nitrate nitrogen,soluble organic carbon and nitrogen.Historical water addition has positive legacy effect on soil p H.It indicated that most of the soil chemical properties were still influenced by historical nitrogen and water addition after two-years of cessation of N and water.(2)There were negative legacy effects of historical nitrogen on microbial biomass carbon(MBC),nitrogen(MBN),respiration rate,?-glucosidase(BG)and alkaline phosphomonoesterase(Alka PME),and positive residual effects on carbon availability index(CAI).Historical water addition had positive legacy effects on MBC,MBN,respiration rate,BG,NAG,and Alka PME activities.It suggested that the historical nitrogen deposition and water addition still affect the current soil microbiological characteristics in this typical grassland ecosystem.(3)The negative legacy effects of nitrogen on the MBC,basal respiration(BR),and the activities of BG,NAG and Alka PME were weakened by the background of historical increasing precipitation.The historical precipitation increment also weakens the positive legacy effect of nitrogen addition on dissolved organic nitrogen(DON)and CAI,which means that,in the context of historical water addition,the impacts of nitrogen on the ecosystem will recover more quickly than the regular precipitation treatments.(4)In the second year after the cessation of nitrogen and water addition,the positive legacy effect of nitrogen addition on Olsen-P and the negative legacy effect of nitrogen addition on microbial biomass,and the negative legacy effect of water addition on NO₃^--N and CAI disappeared.It indicates that a number of soil biotic and abiotic properties can be gradually recovered in a short period of time after 13 years of nitrogen and water addition treatment.These indicators may serve as early markers of ecosystem recovery.The current study proves that under the new global change pattern,with nitrogen deposition and precipitation increment might being weakened or stopped,historical nitrogen and water addition would still have legacy effects on most of the soil biotic and abiotic properties in a typical grassland within short-term duration.The increased nitrogen and water input not only impact the current ecosystem characteristics,but also affect the ecosystem in future time after cessation.Anyway,the legacy effects on specific soil chemical properties and microbiological characteristics were weakened or disappeared,suggesting that some ecosystem characteristics can be gradually restored.The present study provides supporting evidence to further understand the ecological impacts of atmospheric nitrogen deposition and global precipitation pattern changes on typical steppe ecosystem.