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 <i>nosZ</i> (nitrous oxide reductase) gene while reducing nitrate nitrogen (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">O</mi></mrow><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>-N) levels, as well as AOA (ammonia-oxidizing archaea) <i>amoA</i> and AOB (ammonia-oxidizing bacteria) <i>amoA</i> gene expression, thereby lowering N₂O emissions. During the peak growing season, nitrogen’s role in adjusting pH and ammonium nitrogen (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">H</mi></mrow><mn>4</mn><mo>+</mo></msubsup></semantics></math></inline-formula>-N), along with amplifying AOB <i>amoA</i>, 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 <i>nosZ</i>. 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.