Shifts in Above‐ Versus Below‐Ground Carbon Gains to Terrestrial Ecosystems Carbon Sinks Under Excess Nitrogen Inputs

Abstract Although elevated atmospheric nitrogen (N) deposition can increase terrestrial carbon (C) sinks, the persistence of this C acquisition depends partly on the responses of plant biomass and soil organic carbon (SOC) to long‐term exogenous N input. Experiments with N‐additions show increases in above‐ground biomass (AGB), although C dynamics between above‐ and below‐ground under N enrichment are unconnected in many studies. Here, we meta‐analyzed data from 362 N‐addition experiments globally and found that both AGB (+34%) and below‐ground biomass (BGB, +13%) positively but root‐to‐shoot ratios negatively (−15%) responded to N addition. These responses were reversed, however, when the experimental duration was over 20 years, wherein BGB increased and AGB was unchanged or even declined, during which time plants invested more C into roots. The effects of increased AGB were weak and negative, whereas those of increased BGB were robust and positive on SOC. Under chronic N deposition, the significant increase in SOC (5%) was associated with the increased root C inputs and decreased soil C losses rather than with increased above‐ground C inputs. The results suggest that the sequestration of below‐ground C exceeds that of above‐ground C to sustain terrestrial C gain during long‐term N deposition.