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

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.