Low-level nitrogen and short-term addition increase soil carbon sequestration in Chinese forest ecosystems

Soil carbon (C) sequestration plays a vital role in mitigating global climate change. Human activities have vastly increased nitrogen (N) deposition rate in China, which in turn influences belowground C cycle processes. We performed a meta-analysis based on 61 published studies on N addition experiments, including 4072 observations across China, to quantify the responses of belowground soil C dynamics and sequestration to N enrichment in Chinese forest ecosystems. The results showed that, on average, N enrichment significantly enhanced C dynamics in boreal and temperate forests by an average of 24% and 10% while it reduced in subtropical and tropical forests by 11% and 19%. The response of C pools and C input to N addition were more pronounced in boreal forests with +17% and +10% while it was lower in subtropical forests with −0.4% and −19% respectively. The N enrichment enhanced soil C output by in boreal (6%) and temperate (7%) forests and a negative effect in subtropical (−30%) and tropical forests (−10%). This can be explained by the higher reduction levels in C respiration, despite the slightly lower litter and root-derived C inputs. Notably, N addition specifically increased belowground C sequestration, at low N addition rates (<30 kg N ha⁻¹ yr⁻¹) and during a short-term period (<1 year). These C sequestration effects were reversed at higher N deposition levels and /or during a longer time period. This implies that soil C sequestration is currently most likely not enhanced in (large) parts of China, as forests are characterized by long-term elevated N deposition levels.