Effect Of Stocking Rate On Ecosystem Carbon Exchange Of Stipa Breviflora Desert Steppe

As the largest terrestrial ecosystem in China,grassland ecosystem plays an important role in carbon budget dynamics.Carbon exchange of grassland ecosystem is affected by climate change,grazing method and intensity at varying levels.In this study,the Stipa breviflora desert steppe was taken as the research object,and a 12-year continuous grazing control experiment was used as the research platform.Using a completely randomized block experimental design,four different stocking rates were set:no grazing（CK）,lightly grazing（LG）,moderately grazing（MG）and heavily grazing（HG）.In the growing season from 2016 to 2017（May to October）,portable optical LI-6400（LI-COR,USA）and the method of static chamber were used to measure the ecosystem carbon exchange.During this period,we also measured aboveground standing crop,underground biomass,air temperature,precipitation,soil temperature,soil moisture,air-saturated vapor pressure difference（VPD）and ecosystem evapotranspiration（ET）.It was aiming to reveal the characteristics of ecosystem carbon exchange in desert steppe ecosystem and the main driving factors affecting ecosystem carbon exchange under different stocking rates.The main results are as follows:1.In 2016（wet year）,aboveground standing crop showed a trend of decreasing with the increase of stocking rate.There were significant differences between four stocking rates（P<0.05）.In 2017（drought year）,the total amount of aboveground standing crop in LG block was the highest,which had no considerably different from that in CK block.HG block had the lowest total amount（P<0.05）.2.Stocking rate and annual change significantly impacted the net ecosystem carbon exchange（NEE）,ecosystem respiration（ER）and gross ecosystem productivity（GEP）,their interactions posed a significant influence on ER and GEP（P<0.05）.With the growth of stocking rate,ecosystem carbon exchange reduced dramatically（P<0.05）.There was a notable correlation between NEE,ER,and GEP（P<0.0001）.GEP explained NEE better than ER.The LG block was able to convert more than half of the GEP into NEE,net carbon absorption efficiency was also higher than other treatments.3.In the Stipa breviflore desert steppe,CO₂ flux（NEP）and ET had strong coupling during the wet year（P<0.0001）.However,drought decoupled the carbon-water relationship.There was a conspicuous correlation between ecosystem carbon exchange and ET,WUE,precipitation,soil moisture,aboveground standing crop（P<0.0001）.On the interannual scale,aboveground standing crop was the largest contribution to NEE in all environmental factors,followed by soil moisture and then the soil temperature.On the seasonal scale,soil temperature had higher contribution rate and relative importance to NEE than soil moisture.