The impacts of nitrogen addition on upland soil methane uptake: A global meta-analysis.

Elevated nitrogen (N) addition from anthropogenic activities has great impacts on soil methane (CH4) uptake, which could interrupt the existing global CH4 balance and cause feedbacks to climate and biogeochemical processes. Previous studies have come to inconsistent conclusions on both the quantification of the response of CH4 uptake to N addition and understanding of its underlying mechanisms, probably due to the lack of experimental data. Here, we conduct a broad meta-analysis of 90 papers to quantify the responses of CH4 uptake to N addition in upland soil. The results show that N addition has a significant negative impact on soil CH4 uptake (-19.25%), which is termed the N inhibition effect. Soil pH is identified as the dominant factor, with the other factors affecting the CH4 uptake through the alteration of soil pH. The N inhibition effect is observed to be large and significant in forest and grassland, but small and insignificant in farmland, because of the distinct composition of their methanotrophic communities. A threshold of the N addition level is identified at about 68 kg N ha-1 year-1, which indicates the lowest N inhibition effect. Furthermore, the convex relationship between response ratio of CH4 uptake (negative) and N addition duration indicates that a medium level of N addition duration has the largest N inhibition effect, and longer or shorter durations will both reduce the effect. Our analysis of the N inhibition effect implies that controlling the N addition level could effectively reduce the CH4 concentration in the atmosphere and thus relieve global warming.