Effects Of Nitrogen Addition And Moisture Change On N₂O Emissions At Different Soil Depths

Nitrous oxide（N₂O）is one of the major greenhouse gases in the atmosphere,and its accumulation contributes to global warming.Soil nitrogen cycling is the primary source of N₂O emissions into the atmosphere and is strongly influenced by global changes,including nitrogen deposition and changes in precipitation.In recent years,numerous experiments simulating nitrogen deposition and precipitation have been conducted worldwide to investigate the response of soil N₂O emissions to climate change.However,these studies have typically focused on single or a series of nitrogen addition experiments,and the combined effects of nitrogen deposition and precipitation changes on N₂O emissions remain unclear.Even minor changes in deep soil can significantly alter N₂O concentrations in the atmosphere,but current knowledge of the response of N₂O emissions to climate change mainly focuses on surface soil,with little understanding of N₂O emissions from deep soil layers.Therefore,in this study,soil samples were collected from different soil depths（0-140 cm）in the Loess Plateau,Ningxia,China.An indoor simulation experiment was conducted,incorporating nitrogen additions(0,10,100μg N g^-1)and variations in soil moisture（30%,60%,and100%of field capacity）,to investigate the response of N₂O emissions from different soil depths to nitrogen additions,changes in soil moisture,and their interactions.This research aims to provide scientific evidence for the assessment and prediction of the primary ecological effects under the combined influence of future precipitation changes and atmospheric nitrogen deposition.The main conclusions are as follows:（1）The nitrate nitrogen（NO₃^--N）content in all treatments was significantly higher than the ammonium nitrogen（NH₄⁺-N）content,and the NO₃^--N content significantly increased with increasing nitrogen addition.High nitrogen addition significantly increased the soil net mineralization rate.The enhanced nitrogen mineralization promoted the nitrification process,thereby increasing soil N₂O emissions.The net mineralization rate in deep soil was significantly lower than that in shallow soil.The soil net nitrification rate and mineralization rate were significantly negatively correlated with soil p H,and significantly positively correlated with NO₃^--N,DOC,SOC,and TN.（2）Nitrogen addition,water content,soil depth,and their interactions significantly effected on soil microbial characteristics.High water content reduced microbial biomass nitrogen.High water content alleviated soil carbon-nitrogen imbalance,while nitrogen addition promoted this imbalance in the 0-20cm and 80-100cm soil layers.High water content increased carbon acquisition enzyme activity,but decreased nitrogen acquisition enzyme activity.Soil carbon and nitrogen acquisition enzyme activities decreased with increasing soil depth.（3）The combined effect of high moisture and high nitrogen significantly increased the soil N₂O emission rate.The peak time of N₂O emission rate in deep soil was delayed compared to shallow soil.The soil N₂O emission rate was highest in the surface soil layer and decreased significantly with increasing soil depth.Increasing water content enhanced the promoting effect of high nitrogen on the cumulative N₂O emissions from the soil.NH₄⁺-N and NO₃^--N had the greatest influence on soil N₂O flux,and in addition,net mineralization rate,soil enzyme C:N ratio,carbon acquisition enzyme activity,dissolved organic carbon（DOC）,and net nitrification rate also had a significant impact on soil N₂O flux.