Effects of nitrogen deposition on growing-season soil methane sink across global forest biomes

Abstract. Anthropogenic alteration of global nitrogen (N) deposition has resulted in profound impacts on soil fluxes of greenhouse gases in terrestrial ecosystems. However, the response of soil methane (CH₄) flux to N deposition remains poorly quantified in global forest. Based on a synthesis of experimental results from literature, we evaluated the effects of N deposition on growing-season soil CH4 flux across forest biomes. A distinction was made between low-level N addition that is comparable with the worldwide range in N deposition (< 60 kg N⁻¹ yr⁻¹) and high-level N addition (> 60 kg N⁻¹ yr⁻¹. The results showed that growing-season soil CH₄ flux was significantly affected by N additions, the value being dependent on the N addition level and forest biome. Low-level N addition significantly increased growing-season soil CH₄ uptake in boreal forest, while an opposite effect occurred in temperate and subtropical forests. However, high-level N addition significantly decreased growing-season soil CH₄ uptake across boreal, temperate, and subtropical forests. At biome scale, current N deposition was estimate to increase growing-season soil CH₄ sink by 0.029 Tg CH₄ in boreal forest, while it decreased growing-season soil CH₄ sink by 0.025 Tg CH₄ and 0.051 Tg CH₄ in temperate and subtropical forests, respectively. This work improves our understanding of biome-specific effect of N deposition on soil CH₄ uptake and identifies knowledge gaps in the effect of N deposition on soil CH₄ flux in tropical forest.