Geomorphic controls on the abundance and persistence of soil organic carbon pools in erosional landscapes

Soils play a central role in the global carbon cycle and constitute a key component of natural climate solutions that require quantitative predictions of soil organic carbon (SOC) dynamics at local to regional scales. In hilly and mountainous terrain, variations in uplift and stream incision generate gradients in erosion and hillslope morphology that control soil properties that impact the abundance and persistence of SOC. Here we use topographic and soil biogeochemical analyses to show that across 16 sites in our study region, total SOC stocks and the typically slower-cycling mineral-associated fraction of SOC decrease exponentially with modelled erosion rate from 21.0 to 0.2 kg m^–2 and 12.0 to 0.1 kg m^–2, respectively. Along the greater than order-of-magnitude erosional gradient, radiocarbon (Δ¹⁴C), soil thickness and texture data trend younger, thinner and coarser, respectively, such that fast-eroding sites have much less SOC than slow-eroding sites and are dominated by faster-cycling SOC pools. By coupling these erosion-driven soil and SOC trends with high-resolution topographic data, hilltop convexity and other erosion rate metrics can be readily applied to estimate SOC abundance and persistence in diverse landscape settings, facilitating our ability to predict carbon dynamics across a range of spatiotemporal scales.