Nitrification regulates the responses of soil nitrous oxide emissions to nitrogen addition in China: a meta-analysis...

China is a global hotspot of nitrogen (N) emissions and N deposition, which has implications for the N dynamics of ecosystems and stimulates the production of nitrous oxide (N₂O). Although many studies have explored the contributions of different processes to N₂O emissions, it remains unclear whether N addition affects the relative contributions of these different processes to N₂O emissions. We conducted a meta-analysis from 55 published works and 3 unpublished datasets in terrestrial and coastal ecosystems of China to generalize information about the effects of N addition on N cycling, from genes to processes. On average, N addition increased nitrification, denitrification, and N₂O emission rates by 165%, 37%, and 160%, respectively. There was a significant increase in the abundances of nitrification-related genes (i.e., amoA) but a significant decrease in the abundances of denitrification-related genes (i.e., narG, nirS, nirK, and nosZ) after N addition. The variance of nitrification genes caused by N addition was the most important factor explaining increases of N₂O emissions. The N₂O emissions increased exponentially with the N addition rates while decreased with the duration of experiments. The responses of the N₂O emissions to N additions were 2.89 times and 1.3 times stronger in farmland and wetland ecosystems than those in forest and grassland ecosystems, respectively. Additionally, this effect was stronger in regions with high temperature and precipitation. Overall, our meta-analysis reveals the response of soil N cycling to N addition and suggests that N addition promotes N₂O emissions by enhancing nitrification; these findings have major implications for N fertilizer management in China.