Soil-atmosphere exchange of CH4 in response to nitrogen addition in diverse upland and wetland ecosystems: A meta-analysis

Reactive nitrogen (N) addition may profoundly impact the global CH4 budget through its substantial effects on soil CH4 uptake and emission. However, the magnitude and direction of soil CH4 uptake and emission rates in response to N addition on a global scale are still unclear. Here, to investigate the effects of N addition on soil CH4 uptake and emission rates in various upland and wetland ecosystems, we synthesized a large dataset comprising 878 paired observations from 178 studies. Across these studies, we found that N addition significantly reduced soil CH4 uptake rate in upland ecosystems but significantly increased soil CH4 emission rate in wetland eco-systems. The magnitude of the effects was ecosystem-type dependent. Following N addition, reduction of soil CH4 uptake rate and increase in CH4 emission rate were significantly higher in natural ecosystems (with the exception of grassland ecosystems) than in agricultural ecosystems. However, reduction in soil CH4 uptake rate increased with N addition rate in only natural ecosystems. Moreover, the soil CH4 uptake rate in N-limited ecosystems (cold temperate zone and Tibet Plateau) was less sensitive to N addition compared to N-rich ecosystems (subtropical and tropical zones). Additionally, organic N addition had a lower reduction effect on soil CH4 uptake in upland ecosystems and a lower stimulatory effect on soil CH4 emission rate in wetland ecosystems compared to the addition of inorganic N forms. Overall, our results shed light on the magnitude and direction of the effect of N addition on soil CH4 uptake and emission rates in diverse upland and wetland ecosystems and will help improve ecosystem models for predicting soil CH4 flux caused by N addition.