A trade-off between plant and soil carbon storage under elevated CO2

Abstract

Terrestrial ecosystems remove about 30% of the CO2 emitted by human activities each year, yet the persistence of this carbon sink partly depends on how plant biomass and soil carbon stocks respond to future increases in atmospheric CO2. While plant biomass often increases in elevated CO2 (eCO2) experiments, soil carbon has been observed to increase, remain unchanged, or even decline. The mechanisms driving this variation across experiments remain poorly understood, creating uncertainty in climate projections. Here, we synthesized data from 108 eCO2 experiments and found that the effect of eCO2 on soil carbon stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO2, soil carbon accrual declines; conversely, when biomass is weakly stimulated, soil carbon accumulates. This trade-off appears related to plant nutrient acquisition, whereby enhanced biomass requires mining the soil for nutrients, which decreases soil carbon accrual. We found an increase in soil carbon stocks with eCO2 in grasslands (8±2%) and no increase in forests (0±2%), even though plant biomass in grassland responded less strongly (9±3%) than in forest (23±2%). Ecosystem models do not reproduce this trade-off, which implies that projections of soil carbon may need to be revised.