Temporal patterns of soil carbon emission in tropical forests under long-term nitrogen deposition

Soil represents the largest terrestrial carbon pool, and it liberates massive amounts of carbon dioxide (CO₂) to the atmosphere via respiration, which can influence global carbon cycle. In recent decades, anthropogenic activities have dramatically increased the rates of atmospheric nitrogen (N) deposition worldwide, but our current understanding of soil respiration dynamics in anthropogenic N-deposition environments remains poor. Here we monitored soil CO₂ emission rates monthly following 9–13 years of N-addition treatments in three tropical forests in southern China. We found a three-phase pattern of soil CO₂ emission (insignificant changes–dramatic decline–insignificant changes) in three tropical forests and across three N-addition gradients. During the course of the experiments, N addition reduced a total cumulative amount of 6.53–9.06 MgCO₂ ha^–1 with the efficiency of 5.80–13.13 MgCO₂ Mg N^–1. The mechanisms underlying the temporal patterns of soil respiration were related to the lack of plant and microbial responses (phase 1), the reduction in fine root and microbial biomass due to soil acidification (phase 2) and the reorganization of plant and microbial community (phase 3). These findings advance our understanding of soil respiration dynamics and support prediction of long-term soil C fluxes in tropical forests in the context of N deposition.