Nitrogen and Water Additions Affect N 2 O Dynamics in Temperate Steppe by Regulating Soil Matrix and Microbial Abundance

Elucidating the effects of nitrogen and water addition on N 2 O dynamics is critical, as N 2 O 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 N 2 O 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 N 2 O emissions. These results indicate that the effects of nitrogen and water addition on N 2 O emission mechanisms vary throughout the growing season. Nitrogen application consistently increase N 2 O emissions. In contrast, water addition suppresses N 2 O 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 N 2 O emissions. Meanwhile, water addition increased soil moisture, enhancing the abundance of the nosZ (nitrous oxide reductase) gene while reducing nitrate nitrogen ( N O 3 − -N) levels, as well as AOA (ammonia-oxidizing archaea) amoA and AOB (ammonia-oxidizing bacteria) amoA gene expression, thereby lowering N 2 O emissions. During the peak growing season, nitrogen’s role in adjusting pH and ammonium nitrogen ( N H 4 + -N), along with amplifying AOB amoA , spiked N 2 O emissions. Water addition affects the balance between nitrification and denitrification by altering aerobic and anaerobic soil conditions, ultimately increasing N 2 O emissions by inhibiting nosZ . As the growing season waned and precipitation decreased, temperature also became a driver of N 2 O emissions. Structural equation modeling reveals that the impacts of nitrogen and water on N 2 O 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 N 2 O dynamics across various stages of the growing season in the temperate steppe, providing a scientific basis for predicting and managing N 2 O emissions within these ecosystems.