Model-based assessment shows that soil texture determines the impact of soil organic carbon depletion on plant-available water

The depletion of soil organic carbon (SOC) during the Anthropocene may impact soil hydrological functioning and agricultural water security. Although relationships between SOC and soil water retention have been investigated, quantitative assessments isolating the impacts of SOC depletion across soil textural classes remain limited. Here, we evaluated the effects of SOC depletion on the available water capacity (AWC) of California’s surface soils (0–––30 cm). Using a pedon dataset ( n = 2,109) representing 11 soil textural classes and SOC levels (0 % − 4.6 %), random forest models were developed to estimate field capacity (θ FC ) and permanent wilting point (θ PWP ), achieving robust model strengths (R 2 > 0.77) with low prediction errors (RMSE < 0.05 cm 3 cm −3 ). Intrinsic soil properties, specifically clay and sand fractions along with SOC content, primarily controlled θ FC , whereas clay fraction exerts a stronger control on θ PWP . AWC simulations were conducted by progressively depleting SOC in 0.5 percentage-point increments down to 3.5 %. Repeated-measures analysis of variance revealed that the AWC response to SOC decline was nonlinear and strongly dependent on soil texture ( P < 0.05). Fine-textured surface soils exhibited minimal sensitivity of AWC to SOC depletion, whereas medium- and coarse-textured soils showed significant decreases in AWC. Progressive SOC depletion reduced AWC in surface soil by 0.4–––1.2 mm in medium-textured soils and by 0.4–––1.5 mm in coarse-textured soils, corresponding to losses of 4.0–––12.0 and 4.0–––15.0 m 3 ha −1 of plant-available water storage, respectively. These findings demonstrate that SOC depletion disproportionately reduces water retention near field capacity, potentially increasing irrigation demand and reducing agroecosystem resilience under environmental change.