Effects and mechanism of freeze-thawing cycles on the soil N2O fluxes in the temperate semi-arid steppe

High nitrous oxide（N2O） emissions during freeze-thawing period（FTP） have been observed in many different ecosystems. However, the knowledge about the dynamic of soil N2O emissions and its main driving mechanism during the freeze-thawing processes in grassland ecosystem is still limited. An in-situ experiment was conducted during the FTP on the sites with 0 and 15% surplus of the average rainfall and two levels of N addition（0,10 g N/（m2·year）） during growing season（marked as W0N0, W15N0, W0N10, W15N10, respectively） to explore the effects of water and N background on soil N2O emissions during FTPs and the relationship between soil N2O emissions and environmental factors. The results indicated that water and N treatments conducted during growing season did not show significant effect on the N2O effluxes of FTP, but the soil mineral N contents of W0N10 treatment were significantly higher than those of W0N0, W15N0, W15N10treatments（p < 0.05）. The soil PLFA concentrations of microbial groups monitored during 2015 spring freeze-thawing period（2015S-FTP） were lower than those during winter freeze-thawing period of 2014（2014W-FTP）, while cumulative soil N2O emissions of 2015S-FTP were higher than those of 2014W-FTP. The correlations between soil N2O effluxes and most of the measured environmental factors were insignificant, multiple stepwise regression analysis indicated that the soil temperature, soil NH4+-N content and air temperature were the major environmental factors which significantly influenced the N2O effluxes during 2014W-FTP, and air temperature and soil water content were the significant influencing factors during 2015S-FTP.