Global quantification of the eco-hydrological co-benefits of soil carbon sequestration

Abstract. Soil carbon sequestration is an important strategy for climate change mitigation with several co-benefits, including increased water holding capacity and infiltration. However, a global-scale quantification of hydrological co-benefits for water availability to plants is still lacking. In this study, we investigate the effect of soil carbon sequestration on hydrology and water resources by conducting experiments with the Community Terrestrial Systems Model (CTSM). Using global experiments with spatially explicit soil organic carbon (SOC), we apply various carbon sequestration scenarios, including one aligned with the '4 per 1000' initiative, to investigate the effect on soil moisture and soil water balance variables with a focus on cropland regions. Our results show that soil organic carbon redistributes water within the soil profile, retaining moisture in the rooting zone and limiting percolation into deeper layers, which is particularly pronounced in arid regions with sandy soils. Under a scenario with a uniform SOC increase of gC kg^-1 soil, globally averaged total global soil liquid water content increases by 4 mm in the first 30 cm. Carbon sequestration also redistributes the soil water balance, with global mean reductions in surface runoff (–1 mm), subsurface runoff (–0.6 mm), and an increase in evapotranspiration (+2 mm), contributing to improved vegetation productivity. Water stress is modestly reduced across most regions, though effects vary spatially. Although the hydrological impacts of soil carbon sequestration are generally small in magnitude, they are consistent and systematic. The relative changes following realistic and policy-relevant SOC enhancement scenarios, such as those under the 4 per 1000 initiative, are limited due to the modest carbon additions involved. Nevertheless, these changes offer measurable eco-hydrological co-benefits that may support both climate mitigation and ecosystem resilience, particularly in water-limited environments.