<p>&#160; With the increasing of nitrogen(N) deposition and changing of precipitation patterns worldwide, large amounts of N are loaded in terrestrial ecosystem, resulting in soil nutrient imbalance and soil nitrous oxide(N<sub>2</sub>O) flux change. Nitrification and denitrification in soil are two major sources of N<sub>2</sub>O emission mediated by microorganisms. However, It is still unclear how the soil N<sub>2</sub>O flux and the abundance of nitrifiers and denitrifiers might change under the addition of N and water(W) in temperate semi-arid steppe. In this study, we established a one-year-long field experiment investigating how soil N<sub>2</sub>O flux, the abundance of nitrifiers and denitrifiers, and environmental properties, including soil pH, soil moisture, soil dissolved organic carbon content(DOC) and soil available N content responsed when N(NH<sub>4</sub>NO<sub>3</sub>&#160;was applied at a rate of 4 g N&#183;m<sup>-2</sup>&#183;yr<sup>-1</sup>, which is equivalent to one time the annual nitrogen deposition) and/or W(water was applied at a rate of 112.5 mm&#183;yr<sup>-1</sup>, which is equivalent to 30% of the annual rainfall) were added to temperate semi-arid steppe in northern China with the natural condition without any treatment as control. Quantitative PCR was used to analyze the abundance of ammonia oxidizers(ammonia-oxidizing bacteria and archaea amoA) and denitrifiers(nirS/nirK and nosZ). Our experimental results demonstrated that soil N<sub>2</sub>O emission decreased when W was added and W and N were added in temperate semi-arid steppe in northern China. The abundance of nirS and nosZ genes increased when W and N were added. Compared with AOA/AOBamoA and nirK genes, the abundance of nirS and nosZ genes is more sensitive to the addition of N and W. Soil N<sub>2</sub>O flux was negatively correlated with the abundance of nirS-denitrifier. The nirS gene abundance, soil pH and DOC were the main controls on soil N<sub>2</sub>O flux and totally explained 78.2% of the variation of soil N<sub>2</sub>O flux. The results of this study provide a theoretical basis for N cycle mechanism mediated by microorganisms and have practical significance for the prediction of N<sub>2</sub>O flux change in temperate semi-arid steppe under the background of global change.</p>
The composition, richness, and evenness of seedlings from the soil seed bank of a semi-arid steppe in northern China are affected by long-term stocking rates of sheep and rainfall variation