Soil Respiration And Its Temperature Sensitivity In Response To Gradient Nitrogen Addition In Semi-arid Grasslands On The Loess Plateau

Nitrogen emissions and atmospheric nitrogen deposition have increased significantly over the past decades in many regions of the world,including our country.Grassland is the largest terrestrial ecosystem in China,and its plant growth,microbial activity,and soil respiration are often influenced by nitrogen availability.Increased nitrogen deposition will inevitably affect soil respiration in grassland ecosystems,so a systematic understanding of the characteristics of soil respiration changes,influencing factors and potential mechanisms is important to accurately estimate the carbon dynamics of terrestrial ecosystems in the context of future global changes.The semi-arid grassland ecosystem of Loess Plateau is sensitive to global changes,and how soil respiration and its temperature sensitivity in this grassland respond to multiple gradients of nitrogen addition still needs to be systematically studied.Based on a continuous N addition field control experiment set up in 2016 in a semi-arid grassland on the Loess Plateau,this paper investigates the response characteristics and mechanisms of soil respiration and its temperature sensitivity(Q₁₀).The experiment included four treatments:control（no nitrogen addition）,low N treatment(4 g N m^-2 yr^-1),medium N treatment(10 g N m^-2 yr^-1)and high N treatment(16 g N m^-2 yr^-1).The results of the study were as follows:（1）High N treatment significantly increased soil heterotrophic respiration rate（37.5%）and its cumulative carbon emissions（37.3%,p<0.05）in 2021.Cumulative carbon emissions from soil respiration under all three nitrogen application treatments were significantly different between years（p<0.05）.There was no significant effect of the three N application treatments on total soil respiration and autotrophic respiration rates.Temperature sensitivity of total soil respiration was significantly increased by all three N application treatments(Q₁₀,p<0.05),33.5%,23.9%and 23.9%by the low,medium and high N treatments,respectively.Medium and high N treatments were able to significantly increase the temperature sensitivity of soil heterotrophic respiration(Q₁₀,p<0.05),both by 49.3%and 34.8%,respectively.The three nitrogen application treatments had no significant effect on the temperature sensitivity(Q₁₀)of soil autotrophic respiration.（2）The three N application treatments significantly increased soil microbial biomass nitrogen content in the 0-20 cm soil layer（p<0.05）.The high N treatment significantly increased soil nitrate N and inorganic N content in the 0-20 cm soil layer and significantly decreased soil p H（p<0.05）.The nitrogen application treatment had no significant effect on other soil nutrients.（3）Structural equation modeling showed that total soil respiration in this study area was mainly influenced by soil inorganic nitrogen,p H and aboveground biomass,and soil heterotrophic respiration was mainly influenced by soil microbial biomass phosphorus.The nitrogen application treatment ultimately increased total soil respiration by decreasing p H and increasing inorganic nitrogen and aboveground biomass.The nitrogen application treatment eventually inhibited soil heterotrophic respiration by increasing inorganic nitrogen and microbial biomass phosphorus and by decreasing p H.The temperature sensitivity of total soil respiration in this study area was mainly influenced by soil microbial biomass nitrogen,the nitrogen application treatment ultimately increased the temperature sensitivity of total soil respiration by increasing soil microbial biomass nitrogen.The temperature sensitivity of soil heterotrophic respiration was not significantly affected by available phosphorus,p H,C/N ratio,soil microbial biomass nitrogen and microbial biomass phosphorus.In this paper,we found that nitrogen addition promoted the response of soil respiration and its temperature sensitivity to soil physical and chemical properties,and also clarified that total soil respiration in the study area was mainly influenced by soil inorganic N,p H and aboveground biomass;its temperature sensitivity(Q₁₀)was mainly influenced by soil microbial biomass nitrogen;and soil heterotrophic respiration was mainly influenced by soil microbial biomass phosphorus.It provides reliable parameters for the modeling of carbon cycling processes in the grassland ecosystem of the region in the context of future nitrogen deposition.