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

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 (²¹⁰Pb, ¹³⁷Cs, ¹⁰Be), soil physical properties, C/N, δ¹⁵N, δ¹³C, and ∆¹⁴C 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 ²¹⁰Pb, ¹³⁷Cs, and ¹⁰Be profiles and inventories. Soil mixing systematically decreased C/N and enriched stable isotopes of δ¹³C and δ¹⁵N, 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, δ¹³C, and δ¹⁵N provide proxies for organic matter stabilization by soil minerals.