Chapter 9 - Impacts of nitrogen deposition on soil methane uptake in global forests

Abstract

Forest soils are an important sink of atmospheric methane (CH4). Anthropogenic nitrogen (N) emissions have altered global N deposition and resulted in profound impacts on biogeochemical processes in terrestrial ecosystems. Based on a synthesis of experimental results in literature, we evaluated the responses of soil CH4 fluxes to N additions in global forest biomes. The results indicate that N additions within the global range of actual N deposition (≤60kgNha−1 yr−1) exerted positive effects on soil CH4 uptake in boreal forest while it decreased soil CH4 uptake in temperate, subtropical and tropical forests. On a biome scale, N deposition was estimated to increase soil CH4 uptake by 0.02Tg CH4 yr−1 in boreal forest, while it decreased soil CH4 uptake by 0.02Tg CH4 yr−1 in temperate forest and by 0.18Tg CH4 yr−1 in subtropical and tropical forests, respectively. Overall, N deposition decreased soil CH4 sink by 0.18Tg CH4 yr−1 in global forests. Based on the average rate of soil CH4 uptake in control plots in different forest biomes and their areas, the CH4 sink global forest soils is estimated to be 7.55Tg CH4 yr−1. Nitrogen deposition thus reduces the CH4 sink by 2% only. The response of soil CH4 flux to the low-level N additions showed an increase with higher soil pH and N additions, in line with insights of their impact on methane oxidizing bacteria. The observed biome-specific effects of N deposition on soil CH4 sinks in global forests vary with a poleward increase in N limitation from tropical forest to boreal forest.