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, W15N10 treatments (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.