Organic Carbon-Metal-Mineral Complexes in Upland Vs. Riparian Soils

A large body of evidence demonstrates that the stability of organic carbon (OC) in soils increases when OC is complexed with Fe and Al or onto mineral surface. The response of OC-metal-mineral complexation to land use changes, however, remains poorly understood. We hypothesize that agricultural conversion of forests may significantly affect the complexation due to (1) pH-change via liming, (2) reduction of OC in soils, (3) increased mixing of organic matter and minerals within a profile due to plowing, and (4) accelerated soil erosion and deposition within watersheds. To test this hypothesis, we collected soil samples in a second-growth upland forest (~100 years old), an adjacent agricultural field, and a floodplain within a mixed-use watershed in the Piedmont region of southwest Pennsylvania. These soils have long history of human impacts starting with the agricultural expansion during the colonial era, and floodplains soils show buried A-horizon that presumably represent the pre-agricultural period. All samples were fractionated by the size (53 μm) to separate mineral-complexed vs. mineral-free OC pools. Both the bulk soils and the size fractions were analyzed for total OC and N and their stable isotope ratios, pH, CEC, degree of soil mixing and erosion, and the amounts of C-complexed Fe and Al ions, and mineral specific surface area. Here we report the preliminary data showing the differences in the degree of OC-metal-mineral complexation, soil carbon storage, and soil mixing/erosion among the forest, agricultural, and floodplain soils. The preliminary data supports our hypothesis that the agricultural conversion of forest affects OC-metal-mineral complexation not only by altering the chemical environments but also by accelerating physical mixing and erosion of the soils.