Aggregate-associated soil organic carbon dynamics as affected by erosion and deposition along contrasting hillslopes in the Chinese Corn Belt

Soil erosion and deposition processes act as key drivers of soil organic carbon (SOC) translocation and transformation, resulting in a spatially heterogenous distribution of SOC in terms of its quantity and composition. Soil aggregation and its interactions with geomorphic processes exert significant control over SOC dynamics, particularly in landscapes that are undergoing accelerated soil erosion. Here, we investigated the combined effects of erosion-induced soil redistribution and pedogenesis on aggregate-associated SOC dynamics in the sloping croplands of the seriously eroded, yet under-studied, Chinese Corn Belt, by conducting aggregate size fractionation and size-specific stable C isotope analyses. Soil samples from three depth intervals were taken at Summit, Mid-slope and Bottom-slope positions of two contrasting hillslopes, one located in Jiutai (JT) characterized by SOC-rich Chernozems and the other in Helong (HL) with poorly-structured sandy Luvisols. Results show that erosion-induced preferential redistribution of fine particles was found consistently in JT and HL, resulting in an elevated percentage of clay + silt sized particles and concurrent enrichment in SOC content at the Bottom-slope locations. As revealed by the delta C-13 signature, the loss of SOC at Mid-slope was accompanied by a higher degree of C-12 depletion in soil organic matter, particularly in the micro- and macroaggregate fractions, indicating that young and labile SOC previously associated with these fractions was released during the breakdown of soil aggregates by erosive forces and subsequently redistributed to the Bottom-slope position. Comparing the two soil types, the JT site had a hierarchical aggregate structure and increasing SOC concentration with aggregate size, while the opposite pattern was found at the HL site due to the high percentage of primary sand particles in the coarser fractions. This highlights the important control of pedogenesis over soil aggregation and that its interactions with erosion processes greatly affect SOC storage in eroding landscapes, requiring targeted conservation practices to prevent excessive SOC loss.