Soil respiration and nitrous oxide (N₂O) emission have attracted great concern for their contribution to global warming.The biological processes particularly microbial processes play a dominant role in soil respiration and N₂O emission processes.Southeast China is a region with high nitrogen deposition.The effect of nitrogen deposition on carbon and nitrogen cycling varies enormously in different soils.The purpose of this study was to determine soil biochemical factors which hold key roles in soil respiration and N₂O emission under simulated nitrogen deposition.Thirteen types of soil from different land use systems in Jiangsu province were collected and their biochemical characteristics were analyzed,then an indoor soil incubation experiment with ammonium nitrate (NH₄NO₃) input was carried out at constant soil temperature (25℃) and soil moisture (0.30 g·g^-1,simulated upland).The results showed that,when nitrogen addition significantly promoted CO₂ emission,the soils had the properties of low clay composition,low ratio of soil microbial carbon to nitrogen (MC/MN),high soil basic respiration,high soil available nitrogen value,and high abundances of bacterial and fungal.When nitrogen addition significantly promoted N₂O emission,the soils had the properties of high basic respiration,low endogenous N₂O emissions,low total nitrogen content as well as available nitrogen value,and low abundances of actinomycetes and fungi.Stepwise regression analysis suggested that soil respiration could be quantitatively determined by a linear combination of the abundance of soil bacteria and pH value.At the absence of the exogenous nitrogen,N₂O emission was mainly dependent on the values of soil bacteria and ammonium nitrogen.When the exogenous nitrogen was added,N₂O emission was only dependent on the value of soil bacteria.No significant correlation was found between endogenous N₂O emission and soil basic respiration (P>0.05),but there was a significantly positive correlation between N₂O emission and soil respiration (P<0.01) after exogenous nitrogen addition.Moreover,the CO₂ emission per gram exogenous nitrogen was positively correlated with soil organic carbon,total nitrogen contents and the abundance of soil fungi (P<0.05),and negatively correlated with the ratio of MC/MN (P<0.05),while the N₂O emission factor was significantly positively correlated with the abundance of soil bacteria (P<0.01).Stepwise regression analysis showed that the CO₂ emission per gram nitrogen could be quantitatively determined by the ratio of MC/MN,and the N₂O emission factor was mainly dependent on the abundance of soil bacteria.In conclusion,the ratio of MC/MN and soil bacterial value were the determinants of soil respiration and N₂O emissions under simulated nitrogen deposition,respectively.