Effects Of Nitrogen Addition On Soil Gross Nitrogen Mineralization In Typical Grassland Of Inner Mongolia

Nitrogen is usually the most limited nutrient in terrestrial ecosystems, especially in grassland ecosystems. Recently, with the increasing global nitrogen deposition and severely degraded grassland vegetation, the nitrogen cycle has become one of the main issues from various countries’scientists. Nitrogen cycle is the biochemical processes droved by soil microorganisms. Ammonification and nitrification are critical steps of microbial nitrogen transformation. Our study was carried out in two experimental points, including typical grassland modify N deposition since2008in Xilinhot, Inner Mongolia, and abandoned grassland of farming-pastoral zone after continuous N addition and mowing since2006in Duolun, Inner Mongolia. Using15N pool dilution techniques, we studied soil gross N mineralization rate in grassland ecosystems. In addition, through comparing differences between different treatments, we analyzed the response mechanisms of Inner Mongolia grassland ecosystem to the increasing global nitrogen deposition, and simultaneously provided scientific foundation for restoration and reconstruction of grassland vegetation in Inner Mongolia.(1) The main conclusions of this study in typical grasslands in Xilinhot, Inner Mongolia were as follows:①Nitrogen was the limited nutrient in Inner Mongolia grassland ecosystem. Nitrogen addition significantly promoted aboveground net primary productivity (ANPP), and simultaneously increased soil ammonium (NH4+-N), nitrate (NO3--N) and inorganic N pools (NH4+-N+NO3--N) concentrations. In addition, these characteristics were increasing with N addition quantity in grassland ecosystem.②Compared with control treatment, low N addition, including adding2gN·m-2·a-1、10gN·m-2·a-120gN·m-2·a-1NH4NO3, had no significant effects on soil microbial biomass carbon (MBC), microbial biomass N (MBN) and microbial biomass C to N ratio (MBC/MBN). However, high N addition (50g N·m-2·a-1NH4NO3) significantly increased soil MBN in Inner Mongolia grassland ecosystem.③Compared with control treatment, low N addition had no significant effects on soil gorss ammonification rate, while high N addition significantly promoted gross ammonification rate. Soil gross nitrification rate significantly increased after low N addition (except2gN·m-2·a-1NH4NO3). Moreover, we also found that soil gross nitrificantion rate through appling N twice a year was higher than applying every month equally, and differences among different treatments reached a significant level after adding50g20gN·m-2·a-1NH4NO3④With the increasing N addition quantity, soil microbial N transformation tended to increase in typical grasslands of Inner Mongolia. However, there were no significant difference between low N addition frequency (twice a year) and high N addition frequency (every month equally). (2) The main conclusions of the study in abandoned grassland of farming-pastoral zone in Duolun, Inner Mongolia were as follows:①Nitrogen addition(10gN·m-2·a-1NH4NO3) increased gross nitrification rate, while it had no effects on gross ammonification rate, soil MBC, MBN, MBC/MBN, microbial respiration (MR), and microbial metabolic quotient (qCO2). Moreover, N addition significantly enhanced ANPP, and simultaneously increased soil inorganic N pool by115%and196%, respectively.②Mowing had no significant effects on gross nitrification and ammonification rates, while it significantly reduced soil MR.③After N addition+mowing for five to seven years, soil gross ammonification and nitrification rates had no significant change. N addition+mowing had significant positive impact on ANPP, NH4+-N+NO3---N and qCO2, while it had significant negative effects on MBC and MBC/MBN.④N addition was an effective mean to increase productivity in abandoned grassland management, but we need deeply think about if long-term N addition could promote soil microbial N transformation or not in abandoned grassland.