Response Of Grassland Ecosystem CO₂ Exchange To N Addition At Different Levels In The Agro-pastoral Ecotone Of Northern China

The influence of the structure and function on the ecosystems with the increased nitrogen deposition in the future,and thus affecting the carbon cycle of the ecosystem,and especially the grassland ecosystem in the agro-pastoral ecotone which is named as ecologically fragile areas,due to the increasing atmospheric nitrogen?N?deposition caused by industrialization and human activities.In this study,the experiment was conducted in the grassland ecosystem in Youyu County,Shanxi Province,which located in the agro-pastoral ecotone of Northern China.In order to explore the response mechanism of short N addition on CO₂ exchange in semiarid grassland ecosystem.we conducted a situ experience about different N addition levels in a semiarid grassland in agro-pasture ecotone of the northern Shanxi,this study used randomized block design,and made eight N addition levels,included 0,1,2,4,8,16,24 and 32 g N m-2 a-1?represented as N0,N1,N2,N4,N8,N16,N24 and N32?,respectively.We measured the net ecosystem CO₂ exchange?NEE?and ecosystem respiration?ER?using the static box method to dynamically monitor the grassland in 2017-2018,and the total ecosystem gross primary productivity of ecosystem?GEP?was calculated,and researched soil temperature,soil moisture and aboveground biomass in the growing season.The results showed that:?1?The short-term addition of different levels of nitrogen in the agro-pastoral ecotone system was a “carbon sink”.?2?The cumulative amount of NEE was between 53 to 89 g C m-2 a-1 in the growing season after the addition of nitrogen at different levels in the short term,?3?The higher NEE,ER and GEP was preserved under nitrogen addition that between 16 to 24 g N m-2 a-1 in the agro-pastoral ecotone system.?4?There was a significantly affected between precipitation with CO₂ exchange in grassland ecosystems of the agro-pastoral ecotone system?P < 0.05?.?5?Soil temperature was the mainly affected ER and GEP,and ER increased exponentially with increased soil temperature?r2 > 0.1,P < 0.05?.The correlation between soil water content and ecosystem CO₂ exchange is not significant.In conclusion,increased of the net carbon uptake under short-term different levels of nitrogen addition that were a certain reference significance to the “carbon source/sink” function of the grasslandecosystem in the agro-pastoral ecotone of northern China.The impact of ecosystem CO₂ exchange is significantly aggravated by the annual difference of precipitation in two years.The higher NEE,ER and GEP was preserved between 16 to 24 g N m-2 a-1 in the agro-pastoral ecotone system,which rational nitrogen addition.There were not clear that the response sensitivity of NEE,ER and GEP to nitrogen,because the only lasted two years of nitrogen addition.Therefore,long-term and in-depth study is needed to explore the response and mechanism of carbon sources and sinks to nitrogen deposition in the grassland ecosystem of the agro-pastoral ecotone in the future,in order to provide technical support for the scientific management of this special grassland ecosystem.