Effects of erosion on macroaggregation, aggregate associated organic carbon sources and compositions in a Mollisol agricultural landscape

Macroaggregation is widely recognized as an important soil carbon (C) stabilization mechanism. However, in eroding landscapes, the relationships among soil aggregation, organic carbon (OC) sources, and chemical composition are poorly understood. In this study, we aimed to explore the effects of erosion and deposition on macroaggregation, OC sources, and chemical compositions in topsoil (0-20 cm) versus subsoil (80-100 cm) in a Mollisol agricultural landscape. The bulk soil was fractionated into macroaggregates (0.25-2 mm), microaggregates (0.053-0.25 mm), and clay and silt (<0.053 mm) using a wet sieving procedure. The plant sources of soil OC were quantified based on the stable isotope C-13 (delta C-13), and the chemical composition of macroaggregate associated OC (MaOC) was determined by solid-state nuclear magnetic resonance techniques. The results indicated that macroaggregates and MaOC comprised 45-72% of soil mass and 50-66% of soil OC, respectively. There was a reduction in macroaggregate-associated C-3-derived C (C-3-C) in the eroding position compared to that in the non-eroding position. However, similar amounts of C-4-derived C (C-4-C) were detected among three topographical areas, indicating the re-macroaggregation of eroded materials in the depositional position and preferential protection of C-3-C. In the up-slope positions, topsoil MaOC was composed primarily of O-alkyl C (52%). Compared with the up-slope position, there was a considerable increase in the amount of aromatic C in the erosional macroaggregate fraction. However, at the eroding position, there was a marked increase in the proportion of substituted alkyl-C in the topsoil material, and deposition resulted in an increase in aromatic C in the subsoil. Overall, erosion and deposition change soil aggregation, and sources and composition of OC in different functional pools have important implications for the stabilization of organic matter along an eroding Mollisol hillslope.