Responses Of CH₄ Uptakes,CO₂ And N₂O Emissions To Nitrogen Addition In Growing Season Of Grasslands In Northern China:A Meta-analysis

The emission of anthropogenic-source reactive nitrogen leads to the intensification of atmospheric nitrogen deposition,nitrogen goes into the terrestrial ecosystems,which affects the plant community characteristics,soil physicochemical properties and characteristics of soil microbial community,and then affects the process of greenhouse gas flux between the atmosphere and soil.Grassland is an important ecosystem,in recent years,China has carried out a large number of studies on the effects of nitrogen addition on CH₄ uptakes,CO₂ and N₂O emissions in growing season of grasslands in northern China,however,the experimental sites of these studies are distributed in different geographical locations,and the experimental designs such as nitrogen addition methods are also different,resulting in often contradictory results,so the general law of the effects of nitrogen addition on CH₄ uptakes,CO₂ and N₂O emissions in growing season of grasslands in northern China is still controversial.To clarify the general law of the effects of nitrogen addition on CH₄ uptakes,CO₂ and N₂O emissions during the growing season of grasslands in northern China,in this study,Meta-analysis method was used to retrieve the databases of China National Knowledge Infrastructure（CNKI）,Wanfang,VIP and Web of Science,and integrate the independent research results on the effects of nitrogen addition on CH₄ uptakes,CO₂ and N₂O emissions during the growing season of grasslands in northern China.The effects of nitrogen addition methods（Nitrogen rates,Nitrogen forms and Nitrogen durations）on CH₄ uptakes,CO₂ and N₂O emissions in the growing season of grasslands in northern China,and the effects of climate factors（Mean annual temperature,Mean annual precipitation and Elevation）on the responses of the above three greenhouse gas fluxes to nitrogen addition were analyzed.The results showed that nitrogen addition significantly increased the CO₂ and N₂O emissions in the growing season of grasslands in northern China by 6.11% and 65.33%,respectively.CH₄ uptakes showed a decreasing trend,but it was not significant.In the regions of different mean annual temperature,mean annual precipitation and elevation,the differences of CO₂ emissions in grasslands of northern China during growing season under nitrogen addition were significant.In the regions of mean annual temperature of <0℃ and ≥0℃,nitrogen addition significantly increased and decreased CO₂ emissions by 14.11% and 9.82%,respectively.In the arid regions,nitrogen addition showed a trend to inhibit CO₂ emission,but it was not significant.In the humid regions,nitrogen addition significantly increased CO₂ emission by 12.24%.In the regions of low-medium and ultra-high elevations,nitrogen addition showed a trend of inhibiting and promoting CO₂ emissions,but not significantly.In the regions of high elevations,nitrogen addition significantly increased CO₂ emissions by 6.02%.There were significant differences of CH₄ uptakes in the growing season of grasslands in northern China with different nitrogen durations and the regions of different elevations.Nitrogen addition of 3⁴ years showed a tendency to promote CH₄ uptakes,but not significantly.Nitrogen addition of 1²,5⁶ and >6 years showed a tendency to inhibit CH₄ uptakes,but not significantly.Nitrogen addition significantly inhibited CH₄ uptakes in the regions of low-medium elevations by 13.36%,while in the regions of high and ultra-high elevations,nitrogen addition showed a trend to promote CH₄ uptakes,but not significantly.There were significant differences in N₂O emissions in the growing season of grasslands in northern China under different nitrogen rates and nitrogen durations.N₂O emissions were significantly increased by44.35%,55.72% and 290.63%,respectively,with low,medium and high nitrogen additions.Nitrogen addition of 1²,3⁴,5⁶ and >6 years significantly increased N₂O emissions by 48.16%,47.74%,61.01%and 207.78%,respectively.The results also showed that environmental variables influenced the response of CH₄ uptakes,CO₂ and N₂O emissions to nitrogen addition in the growing season of grasslands in northern China.Response ratio for CO₂ emissions was significantly negatively correlated with mean annual temperature.Response ratio for CH₄ uptakes was significantly negatively correlated with nitrogen rates and mean annual temperature,and significantly positively correlated with elevation.Response ratio for N₂O emissions was significantly positively correlated with nitrogen rates and nitrogen durations.In summary,when evaluating the responses of CH₄ uptakes,CO₂ and N₂O emissions to nitrogen addition in the growing season of grasslands in northern China,the effects of nitrogen addition methods（Nitrogen rates,Nitrogen forms and Nitrogen durations）and local climate factors（Mean annual temperature,Mean annual precipitation and Elevation）should be fully considered.This conclusion provides theoretical and scientific bases for the assessment of greenhouse gas flux of grasslands under the background of the increase of atmospheric nitrogen deposition.