Effect Of Nitrogen And Phosphorus Addition On Key Process Of Ecosystem Carbon Cycling In Hulunbuir Meadow Steppe

Atmospheric nitrogen（N）deposition and large amount of phosphorus（P）fertilizer increased the available N and P in terrestrial ecosystems rapidly,which had important effects on the carbon cycle processes such as net CO₂ exchange（NEE）,gross photosynthesis（GEP）and respiration（ER）,and thus affected the carbon source/sink functions of ecosystems.Grassland carbon pool,as an important part of the terrestrial carbon pool,is sensitive to climate change and nitrogen and phosphorus deposition.However,the degree of response may be regulated by soil microenvironment,plant productivity and other factors,with great uncertainty.Therefore,it is of great significance to explore the response of key carbon cycle processes of grassland ecosystem to N and P addition and the regulatory mechanism of soil microenvironment and plant productivity to the response to N and P addition,for exploring the function of grassland carbon source/sink and predicting the dynamic change of grassland carbon under the intensification of global change in the future.To this end,we investigated the effects of N and P addition on the diurnal and seasonal dynamics of key processes of carbon cycling through a 3-year manipulative experiment in Hulunbuir meadow grassland,Inner Mongolia,from 2018 to 2020.The effects of N and P addition on CO₂ fluxes in the key processes of carbon cycling were studied via two-way repeated measures ANOVA and structural equation modeling,and the regulatory effects of soil temperature and moisture,soil ammonium nitrogen content,soil nitrate nitrogen content and aboveground net primary productivity of plant communities were analyzed.The main results were as follows:（1）2018-2020 growing season,Hulunbuir meadow steppe ecosystem carbon exchange process had obvious unimodal or bimodal dynamic and seasonal variation characteristics,and its variation characteristics mainly affected by the change of temperature,precipitation and seasonal distribution of precipitation,N and P addition has not changed its dynamic and seasonal dynamic variation characteristics.（2）The Hulunbuir meadow steppe ecosystem in Inner Mongolia under the addition of N and P was as a carbon source in 2018,2019 and at the overall level of three years,and as a carbon sink in2019.Due to the different effects of N addition on GEP and ER in different years,N addition significantly promoted the ecosystem carbon emission in 2018,but significantly promoted the ecosystem carbon absorption in 2019,2020 and at the overall level of three years.P addition significantly inhibited GEP and ER in all years and at the overall level of three years.However,it significantly promoted the ecosystem carbon emission in 2019 and at the overall level of three years,but had no significant effect on NEE in 2018 and 2020.N and P addition had significant interaction in most years.（3）Soil microenvironment and plant productivity were the key factors which regulate the response of key processes of the carbon cycle to N and P addition.Soil temperature significantly inhibited GEP and ER,promoted ecosystem carbon emissions,and had a significant negative correlation with soil moisture,while soil moisture significantly promoted GEP and ER,and promoted ecosystem carbon absorption.Soil nitrate nitrogen content significantly promoted ER via promoted plant growth,while soil ammonium nitrogen content directly promoted GEP and thus promoted ecosystem carbon emission.N and P addition had significant effects on key processes of carbon cycling in Hulunbuir meadow steppe,but the extent of these effects was controlled by abiotic factors such as the seasonal distribution of precipitation,soil microenvironment and biological factors such as plant productivity.N addition weakens the ecosystem carbon emissions in most years,indicated that the intensification of nitrogen deposition in the future may enhance the carbon sink capacity of the Hulunbuir meadow steppe ecosystem.