Impacts of Erosion Rates on Organic Carbon Inventories and Carbon-Mineral Interactions in Steep Mountains.

Hillslope geomorphic processes not only erode and redistribute organic carbon (C) but also affect the thickness and coarseness of soils storing C in rugged landscapes. In order to better understand direct and indirect links between erosion and the soil C sequestration in rugged soil-mantled landscapes, we attempt to (ⅰ) quantify inventories of organic C in soil and (ⅱ) identify and quantify geomorphic and biogeochemical mechanisms regulating organic C inventories and carbon sorption on mineral surface. We determined soil organic C concentrations and inventories, BET mineral specific surface area (SSA), and stable carbon isotope (δ¹³C) ratios in soil and saprolite along three soil-mantled hillslope transects with drastically different erosion rates (POMD, 30 mm kyr^-1; FTA, 130 mm kyr^-1; and BRC, 300 mm kyr^-1) within the Feather River, California. All erosion rates had been previously determined using ¹⁰Be. Our results show that organic C concentrations in fine (<2 mm) fractions vary little despite the ~10 fold increase in erosion rates, which is in agreement with erosion-independent SSA of the same size fraction. However, organic C concentrations and SSA in the saprolite decreased with increasing erosion rates. Organic C inventories in the POMD and FTA soils were significantly greater than those of BRC soils, although no significant difference was observed between the POMD and FTA soils’ C inventories. This trend occurs largely because higher erosion rate results in soils with higher coarse fractions and subsequently lower amounts of SSA. The result further indicated that nonlinear negative relationships exist between erosion rates and organic C inventories in soils. Overall, enhanced erosion reduces the soils’ potential to store C in steep landscapes largely by constraining weathering that breaks down coarse rock fragments and generates SSA. This highlights that the coupling between weathering and erosion processes plays a key role in biogeochemical cycle in steep landscapes.