Soil carbon redistribution and organo-mineral associations after lateral soil movement and mixing in a first-order forest watershed

Abstract We test the hypothesis that erosion driven soil movement on hillslopes results in an increase in new organomineral associations and overall organic matter storage in colluvial deposits within a forested hillslope. We measured mineral specific surface area (SSA), organic carbon (OC), meteoric radioisotopes ( 210 Pb, 137 Cs, 10 Be), soil physical properties, C/N, δ 15 N, δ 13 C, and ∆ 14 C in bulk soil and density fractions in a hillslope transect of soil pits. The quantity of OC per unit of mineral surface area (OC/SA) and OC inventories increased by a factor of 2–3 in depositional sites as result of soil mixing due to erosional movement as confirmed by 210 Pb, 137 Cs, and 10 Be profiles and inventories. Soil mixing systematically decreased C/N and enriched stable isotopes of δ 13 C and δ 15 N, revealing that formation of organomineral associations instead of microbial processing was responsible for depth trends in organic matter composition. Our findings indicate that the processes that associate organic matter and minerals are fundamentally linked with organic matter composition, and OC/SA, C/N, δ 13 C, and δ 15 N provide proxies for organic matter stabilization by soil minerals.