Investigating the hydrological co-benefits of soil carbon sequestration using a land surface model

Soil carbon sequestration is an important strategy for climate change mitigation. Soil carbon stocks on agricultural lands can be augmented through sustainable land management practices such as organic manures addition, cover cropping, mulching, conservation tillage and agroforestry. Soil carbon sequestration has several co-benefits, of which increased water holding capacity and infiltration are often named. However, a global scale quantification of these hydrological co-benefits for water availability is still lacking. In this study, we aim to quantify how soil carbon sequestration impacts soil water budget and availability, to identify potential hydrological co-benefits. We use the Community Land Model (CLM) version 5.2 in land-only mode with prescribed phenology to conduct idealized experiments simulating present-day climate conditions with altered soil carbon stocks after 20 years of sequestration. Three scenarios of carbon sequestration are investigated, based on spatially explicit soil organic carbon input maps. These include two scenarios with high and medium sequestration rates focused on cropland. Additionally, an aspirational scenario with a 0.4% annual increase in soil organic carbon stocks is conducted, which follows the "4 per mille" initiative target. Upon analyzing the simulations at subgrid level for the crop fraction of the grid cell, our findings indicate that, overall, soil carbon sequestration enhances the water holding capacity by increasing the field capacity and reducing the permanent wilting point of the soil. This increase in water holding capacity predominantly arises from augmented porosity, which in turn surpasses the rise in actual water content. Consequently, the saturated fraction of soils across most regions decreases. Furthermore, CLM simulations consistently demonstrate that elevated carbon stocks reduce the surface runoff and subsurface drainage, and increase soil evaporation. The upper soil layers, corresponding to the layers with elevated soil carbon, exhibit increased water content, whereas lower layers indicate either negligible or slight water content reduction. This is particularly accentuated in arid regions, which leading to an overall decline in water content in these areas. Finally, water stress is found to be decreasing, which indicates improved water retention in carbon-sequestered soil and enhances the soil sponginess. Overall, despite remaining modelling uncertainties, particularly linked to soil hydrological parametrizations and their dependency on soil carbon fractions, these sensitivity experiments reveal the potential of carbon sequestration to increase water availability and counteract water scarcity.