Effects of nitrogen addition on soil methane uptake in global forest biomes

Xia, Nan; Du, Enzai; Wu, Xinhui; Tang, Yang; Wang, Yang; Vries, Wim de


Nitrogen (N) deposition has been conventionally thought to decrease forest soil methane (CH4) uptake, while the biome specific and dose dependent effect is poorly understood. Based on a meta-analysis of 63 N addition trials from 7 boreal forests, 8 temperate forests, 13 subtropical and 4 tropical forests, we evaluated the effects of N addition on soil CH4 uptake fluxes across global forest biomes. When combining all N addition levels, soil CH4 uptake was insignificantly decreased by 7% in boreal forests, while N addition significantly decreased soil CH4 uptake by 39% in temperate forests and by 21% in subtropical and tropical forests, respectively. Meta-regression analyses, however, indicated a shift from a positive to a negative effect on soil CH4 uptake with increasing N additions both in boreal forests (threshold = 48 kg N ha−1 yr−1) and temperate forests (threshold = 27 kg N ha−1 yr−1), while no such shift was found in subtropical and tropical forests. Considering that current N deposition to most boreal and temperate forests is below the abovementioned thresholds, N deposition likely exerts a positive to neutral effect on soil CH4 uptake in both forest biomes. Our results provide new insights on the biome specific and dose dependent effect of N addition on soil CH4 sink in global forests and suggest that the current understanding that N deposition decreases forest soil CH4 uptake is flawed by high levels of experimental N addition.