Pathways Regulating Decreased Soil Respiration with Nitrogen Addition in a Subtropical Forest in China

Knowledge of nitrogen (N) impacts on soil respiration (R-s) and its components is critical to assess soil carbon (C) storage and C stability with accumulating N deposition. However, the abiotic and biotic mechanisms underlying the responses of R-s and its components to N enrichment are still far from clear. This study investigated the direct and indirect pathways of N enrichment on autotrophic respiration (R-a) and heterotrophic respiration (R-h) in a subtropical forest. The results showed that N addition significantly decreased R-s across the 2-year observation. The negative effects of N fertilization on R-s were the combination of suppressive effects of excessive N on R-a and R-h. On average, N enrichment decreased R-s, R-a, and R-h by 34%, 33%, and 35%, respectively across the 2years. The decrease in R-a was directly due to reducing fine root biomass and indirectly affected by N-induced soil acidification. The decline in R-h was directly controlled by reducing soil microbial biomass C and indirectly determined by N-induced soil acidification and the elevation of soil N availability. Nitrogen addition significantly increased temperature sensitivities of R-s and its components, suggesting a potential positive C-climate feedback in the future scenarios of global warming and aggravating N deposition. Our study highlights the direct and indirect pathways of N fertilization on soil respiration and its components, which could have implications for assessing forest C sequestration and C stability.