Impact Of Simulated Nitrogen Deposition On Production Function,Community Structure And Soil Respiration Of Grassland Ecosystem

With the global consumption of fossil fuels and the extensive use of chemical fertilizers,nitrogen deposition has been increasing over the past few decades.China has become the third largest concentration area of nitrogen deposition in the world,among which nitrogen deposition are much higher in North China,East China and the Midwest Inner Mongolia grasslands.Serious nitrogen saturation caused by nitrogen deposition greatly affected ecosystem functions and processes,such as ecosystem productivity,species diversity and nutrient cycling.In the research,control（CK）（0 g N/m²/yr）,low nitrogen（LN,2.5g N/m²/yr）,medium nitrogen（MN,5 g N/m²/yr）and high nitrogen（HN,10 g N/m²/yr）were set up along the gradient of nitrogen concentration in meadow steppe and typical steppe of Inner Mongolia.Additionally,P treatment（10g P/m²/yr）and NP treatment（10g N/m²/yr,）10g P/m2/yr also were set up.The aboveground biomass,α,βandγdiversity index,functional group biomass,community phylogenetic diversity,community soil respiration and temperature sensitivity(Q₁₀)and other indicators were determined and analyzed under different N&P additions treatments from 2012 to 2017.The research will contribute to revealing the effects of nitrogen deposition worldwide on ecosystem structure,function and process,and the mechanism of ecosystem stability of grasslands.It also plays a positive role in dealing with global changes and grassland ecosystem management.The main findings and conclusions are as follows:1.The results showed that nitrogen deposition（LN,MN and HN）did not significantly change the aboveground total biomass of meadow steppe and typical steppe community in Inner Mongolia.Except 2017（the particularly dry year）,the aboveground total biomass in two grasslands in 2012-2016 had the highest response to NP addition,in particular,the coupling addition of nitrogen and phosphorus significantly improved the production function of typical grassland yet the lowest to P addition.Compared with controls,aboveground biomass under NP tended to increase community aboveground biomass.The results indicate that the effects of nitrogen deposition on community aboveground biomass mainly depended on the coupling of water,nitrogen and phosphorus.2.Simulated nitrogen deposition and nutrient addition significantly affected the Shannon-Wiener diversity index,Pielou evenness index,andαandγdiversity index in meadow steppe,but not in typical grassland.Shannon-Wiener index and Pielou index tended to decrease significantly under LN、HN、MN and NP treatments in meadow steppe.With the increasing N concentration,theαindex decreased gradually in the middle and late experiments in meadow steppe,especially the significant reduction inαandγindex under HN and NP treatments.The results indicated that nitrogen deposition mainly reduces the community species diversity in meadow grassland with better water conditions.P treatment has no significant effects on the four diversity indexes in two grasslands.The responses ofβindex in each year to all treatments were not significant in two grasslands,yet with the significant reduction in the average ofβindex under three N treatments in meadow steppe.In addition,all treatments did not change community phylogenetic diversity.3.Nitrogen deposition and nutrient addition mainly affected the composition of community functional groups.LN,MN and NP treatment significantly increase the proportion of PR In the meadow grassland.NP treatment significantly reduced the relative biomass of PB.In typical grasslands,PF increases significantly under MN treatment.NP treatment significantly increases the proportion of AB and reduces the proportion of PB.HN treatment significantly reduces the ratio of AB.The addition of N and P nutrients has a weak influence on hydrological ecological type in typical steppe.In meadow steppe,HN,P and NP treatments significantly reduced the proportion of Xero-mesophytes,and NP treatment significantly increased the proportion of Xerophytes.4.In the year with significant response,the response of soil respiration rate to different nitrogen additions differed between two grassland types.Soil respiration rate was inhibited under LN treatment and promoted under HN treatment in meadow steppe,without significant effects in typical steppe.Phosphate treatment showed inhibitory effect on soil respiration rate in two grassland types,in which the effect of NP treatment was not obvious.The results indicated that the effects of nitrogen deposition and nutrient addition on soil respiration are closely related to soil moisture.5.During 2012-2017,the values of Q₁₀ fluctuated between 0.85 and 2.98 in two grasslands,with the highest value of 2.20±0.53 in meadow steppe and the value of 2.08±1.06 in typical steppe.Nutrient addition did not significantly reduce Q₁₀0 in two grasslands.Q₁₀0 tended to increase under LN,HN and P treatments in meadow steppe and typical steppe.Changes in Q₁₀ is largely not affected by soil moisture and soil temperature.Our study indicated that nitrogen deposition can play an important influence on ecosystem structure,function and process in the grassland with higher water content or the period with higher precipitation.The effect is more dependent on the coupling of water-nitrogen-phosphorus.Nitrogen deposition has a weak influence on the above-ground total biomass of grassland ecosystems,mainly affect functional groups composition of communities,which maintains ecosystem production functions stability through the compensatory effects between functional groups.