Nitrogen and Water Additions Affect N2O Dynamics in Temperate Steppe by Regulating Soil Matrix and Microbial Abundance

Elucidating the effects of nitrogen and water addition on N2O dynamics is critical, as N2O is a key driver of climate change (including nitrogen deposition and shifting precipitation patterns) and stratospheric ozone depletion. The temperate steppe is a notable natural source of this potent greenhouse gas. This study uses field observations and soil sampling to investigate the seasonal pattern of N2O emissions in the temperate steppe of Inner Mongolia and the mechanism by which nitrogen and water additions, as two different types of factors, alter this seasonal pattern. It explores the regulatory roles of environmental factors, soil physicochemical properties, microbial community structure, and abundance of functional genes in influencing N2O emissions. These results indicate that the effects of nitrogen and water addition on N2O emission mechanisms vary throughout the growing season. Nitrogen application consistently increase N2O emissions. In contrast, water addition suppresses N2O emissions during the early growing season but promotes emissions during the peak and late growing seasons. In the early growing season, nitrogen addition primarily increased the dissolved organic nitrogen (DON) levels, which provided a matrix for nitrification and promoted N2O emissions. Meanwhile, water addition increased soil moisture, enhancing the abundance of the nosZ (nitrous oxide reductase) gene while reducing nitrate nitrogen (NO3−-N) levels, as well as AOA (ammonia-oxidizing archaea) amoA and AOB (ammonia-oxidizing bacteria) amoA gene expression, thereby lowering N2O emissions. During the peak growing season, nitrogen’s role in adjusting pH and ammonium nitrogen (NH4+-N), along with amplifying AOB amoA, spiked N2O emissions. Water addition affects the balance between nitrification and denitrification by altering aerobic and anaerobic soil conditions, ultimately increasing N2O emissions by inhibiting nosZ. As the growing season waned and precipitation decreased, temperature also became a driver of N2O emissions. Structural equation modeling reveals that the impacts of nitrogen and water on N2O flux variations through nitrification and denitrification are more significant during the peak growing season. This research uncovers innovative insights into how nitrogen and water additions differently impact N2O dynamics across various stages of the growing season in the temperate steppe, providing a scientific basis for predicting and managing N2O emissions within these ecosystems.