Effects of freeze-thaw action and nitrogen input on soil nitrogen transformation and nitrous oxide emissions in grassland ecosystem

Freeze-thaw is known to affect soil nitrogen transformation and N₂O emission by changing the physicochemical and biological characteristics of soil. Exogenous nitrogen input during the freeze-thaw period further alters soil nitrogen availability, which may result in the generation of more complex effects on the soil nitrogen transformation process and N₂O emissions. However, the coupling effects and functional mechanism of soil active layer freeze-thaw and nitrogen input on nitrogen transformation and N₂O emission in grassland soils remain unclear. This study systematically analyzed the features of freeze-thaw action, the roles of exogenous nitrogen input, and their coupling effects on soil nitrogen transformation and N₂O emission in the grassland ecosystem. The results showed that the process of nitrogen transformation is closely related with soil freeze-thaw pattern, and soil N₂O emission mainly occurs during the freeze-thaw period in spring, with the amount making a large contribution to annual emissions. Changes in the freeze-thaw pattern had positive effects on soil N mineralization, with the corresponding increase in freeze-thaw frequency, freezing intensity, and freezing duration giving rise to increased N mineralization. Exogenous nitrogen input promotes nitrogen mineralization, nitrification, and N₂O emission by increasing nitrogen supply and enhancing microbial activity. The coupling effects of freeze-thaw and exogenous nitrogen affect soil nitrogen and N₂O emission flux under the differential influences of factors, including climatic conditions, freeze-thaw patterns, and the exogenous nitrogen content and types. These findings provide theoretical insights into the effects of soil freeze-thaw and nitrogen input on nitrogen biogeochemical cycling in grassland ecosystems, as well as acting as a useful reference for optimizing nitrogen input strategies in seasonal freeze-thaw areas, reducing N₂O emissions, and maintaining the healthy development of grassland ecosystems.