Effects Of Different Nitrogen Addition On Soil Net Nitrogen Mineralization In Grassland Ecosystem Of Northern Agro-pastoral Transitional Zone

Productivity of terrestrial ecosystems are generally limited by nitrogen.However,the excessive use of nitrogen fertilizer leads to a large amount of industrial nitrogen fixation entering the terrestrial ecosystem nitrogen cycle in the process of modern agricultural development,which leads to increase of atmospheric nitrogen deposition.It is likely to have a profound impact on soil nitrogen pool and available nitrogen.Therefore,soil nitrogen retention,mineralization,turnover and other dynamic characteristics have become one of the hot spots of ecological research.Grassland ecosystem accounts for 40% of the land area in China,and its vegetation change affect not only the development of animal husbandry in China,but also the national ecological security.Soil available nitrogen plays a key role in grassland ecosystem vegetation change.Therefore,it is very important to study the soil net nitrogen mineralization in grassland ecosystem.Based on the grassland ecosystem experiment platform in Youyu County,Shanxi Province,eight different nitrogen additions（0,1,2,4,8,16,24,32 g/Nm2）were used for in-situ cultivation in the field.Soil samples were collected regularly and the contents of ammonium nitrogen and nitrate nitrogen in soil were measured by flow analyzer.The nitrogen mineralization rate was calculated in order to analyzing the influence of different nitrogen levels on soil nitrogen mineralization and its influencing factors.The main results are as follows:（1）The results showed that ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）and inorganic nitrogen（In N）content in soil were increased by adding nitrogen in the two-year nitrogen application experiment.The addition of nitrogen could increase the content of ammonium nitrogen（NO₃^--N）and inorganic nitrogen in soil.High nitrogen treatment(N₁₆,N₂₄,N₃₂)significantly increased the content of soil NO₃^--N and In N.The content of NO₃^--N and In N increased with the increase of nitrogen addition quantity.Soil ammonium,nitrate and inorganic nitrogen increased significantly in 2018 compared with those of2017（P < 0.05）.It indicates that short-term continuous nitrogen addition increases the soil inorganic pool.（2）Effect of different nitrogen addition on soil mineralization of semi-arid grassland did not enhance with the increase of nitrogen addition,and the ammonium rate of soil in growing season was inhibited by the monthly nitrogen application in growing season instead of increase.Compared with two years nitrogen application treatment were lower than control treatment.According to the ammonium rate,nitrification rate and net nitrogen mineralization rate for two years,we predicted that long-term nitrogen application might decrease the soil nitrogen supply capacity.（3）The main factors of affecting mineralization rate are soil temperature,moisture,MBC and MBN.Soil temperature and moisture content were positively correlated with ammonium rate,nitrification rate and net nitrogen mineralization rate.Soil temperature and moisture had significant effects on ammonium rate（P< 0.05）.Soil microbial biomass C and N were positively correlated with ammonium rate,nitrification rate and net nitrogen mineralization rate.Soil microbial biomass C had significant effects on nitrification rate and net nitrogen mineralization rate（P < 0.01）.Soil microbial biomass N had significant effects on nitrification rate and net nitrogen mineralization rate（P < 0.05）.