Responses Of Soil Enzyme Activities To Simulated Component Variation In Nitrogen Deposition In Songnen Grassland

With the development of the society andeconomy,the use of fossil fuels and fertilizers has led to an increase in atmospheric nitrogen deposition rates and made it an important part of globalclimate change.From 1980 s to 2000 s,the annual increase in nitrogen deposition in China is 0.41 Kg N ha-1,and developed areas are higher than remote areas.In addition to the great changes in nitrogen deposition rates,the changes in nitrogen deposition components have become the focus of scientific research and attracted a lot of public attention.The inorganic nitrogen components,such as NO3-,NH4+and its ecological effects are well known while the organic nitrogen components(amines,hydrocarbons,amino acids etc.)and its ecological effects are less understood.In this experiment,we studied the effect of simulated nitrogen deposition level and composition on soil enzyme activity using both field experiment and indoor culture experiment inSongnen salinized grassland.In this study,two nitrogen fertilizer gradients were set up: 10 g N m-2 yr-1 and 20 g N m-2 yr-1;and four different ratios of organic nitrogen(urea and glycine)and inorganic nitrogen(ammonium nitrate)were designed at each nitrogen level,TIN(100% inorganic nitrogen);LON(70% inorganic nitrogen,30% organic nitrogen);MON(30% inorganic nitrogen,70% organic nitrogen);HON(10% inorganic nitrogen,90% organic nitrogen).In addition,a nitrogen-free blank control treatmentwas set.The field experimental part was carried out at the Songnen Grassland Ecological Research Station of Northeast Normal University,usingthe in situ potting method,withsoil column sampledfrom the saline grassland.The indoor control experiment was carried out at the Northeast Normal University,and the saline soil was cultured using an incubator.The main results of the experiment are:(1)For fieldenzymatic activity,most of the enzymes(?-glucosidase,alkaline phosphatase,leucine aminopeptidase and acetylgucosaminidaseetc.)were significantly higher in the treatments with organic nitrogen than only adding inorganic nitrogen treatment,while most enzymes exhibit maximum activity under the MON treatment.(2)The nitrogen addition level had significant effect on soil total nitrogen and microbial carbon,but had no significant effect on soil organic carbon and soil pH.Different rations of organic nitrogen to inorganic nitrogen had significant effect on the measuredfour soil factors.There were interaction between the nitrogen addition level and ratio for the soil environmental factorsexcept soil organic carbon.This indicates that the nitrogen addition level and the proportion of different nitrogen fertilizer components are important factors for microbial and enzyme activities.(3)The results of correlation analysis showed that most of the enzymes had a significant correlation with soil environmental factors,while the results of fitting equations were mostly monovalent quadratic.(4)The plant density and aboveground biomass had a significant response to the nitrogen addition level.There was no interaction between the nitrogen application level and the nitrogen ratio in the four vegetation indices.There was no significant difference between the speciesnumber under the different nitrogen ratios.However,there was a significant difference between the control treatment and the TIN? LON? HON treatments at the nitrogen addition level of 10 g N.(5)The enzyme activity under controltreatmentin the culture experiment wasgenerallyincreasedfirstandthendecreased,thenincreasedanddecreasedagain,orshowedatend encytoincreasefirstandthendecrease.And the highest enzyme activities were detected at about 45 days for most enzymes after culture under nitrogen treatment.The results of this study showed thatvariation in the nitrogen application level and component(organic vs inorganic)significantly changed the microbial biomass and enzyme activity in Songnen grassland.The future nitrogen deposition studies should also pay attention to the changes of nitrogen deposition components and their related ecological effects,in addition to the effect of nitrogen deposition rate on the structure and function of ecosystems.The results of this study will help us to understand the response process and trend of biogeochemical cycles for the key resources of Songnen grassland,especially in the context of global change,when nitrogen deposition and compositionare predicted to change dramatically.