Association between CH₄ uptake and N₂O emission in grassland depends on nitrogen inputs

Increasing nitrogen (N) input has been recognized as one of the important factors influencing methane (CH₄) uptake and nitrous oxide (N₂O) emission in arid and semiarid grasslands. Numerous studies have examined the spatiotemporal variations of CH₄ and N₂O fluxes in various ecosystems; however, the variation of the interplay between CH₄ uptake and N₂O emission with increasing N has not yet been well understood. This study explored the relationship between CH₄ uptake and N₂O emission in a semiarid grassland in Inner Mongolia, northern China, under a gradient of 12-year N additions. We found a synergistic relationship at low-N levels, where CH₄ uptake and N₂O emission are positively correlated. Conversely, an antagonistic interaction emerged with a negative correlation between CH₄ uptake and N₂O emission observed at high-N levels, which was evidenced by a 33.62% decrease in CH₄ uptake and a 264.91% increase in N₂O emission. Further independent analysis, covering at least five N addition levels across grassland ecosystems in China, confirmed the general pattern: three of four cases showed a synergistic relationship at low-N levels and an antagonistic relationship at high-N levels. Given the increasing N deposition in the future, the dynamics between CH₄ uptake and N₂O emission are critical for understanding the impact of external N input on net greenhouse gas emission and consequent global climate change.