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

Nitrogen (N) deposition has been conventionally thought to decrease forest soil methane (CH) 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 CH uptake fluxes across global forest biomes. When combining all N addition levels, soil CH uptake was insignificantly decreased by 7% in boreal forests, while N addition significantly decreased soil CH 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 CH uptake with increasing N additions both in boreal forests (threshold = 48 kg N ha yr) and temperate forests (threshold = 27 kg N ha yr), 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 CH uptake in both forest biomes. Our results provide new insights on the biome specific and dose dependent effect of N addition on soil CH sink in global forests and suggest that the current understanding that N deposition decreases forest soil CH uptake is flawed by high levels of experimental N addition.