Changes in soil aggregate stability and aggregate-associated organic carbon during old-field succession in karst valley

Soil is the largest carbon pool whose change will have an impact on the terrestrial carbon cycle in the terrestrial ecosystem. Old-field succession on abandoned farmland, which usually has a noticeable effect on soil status, is a common phenomenon in karst valley where human activity alters frequently. In order to understand the changes in the accumulation of organic carbon (OC) in aggregates and bulk soil in different stages of old-field succession on abandoned farmland in the karst valley area, soil samples were collected at 0–10-cm and 10–20-cm depth representing three typical stages of old-field succession, i.e., abandoned farmland, secondary grass, and secondary shrub in Qingmuguan karst valley area, Chongqing City, Southwest China. Results displayed that during old-field succession (1) the mean weight diameter and geometric mean diameter of the aggregates increased and the fractal dimension decreased; (2) OC content within aggregates and bulk soil had no significant change in topsoil (0–10 cm); OC content within microaggregates and bulk soil had a significant reduction in subsoil (10–20 cm); the OC content within silt and clay fractions was significantly higher than that within the other two kinds of aggregates; (3) bulk-soil OC storage had no significant change but its accumulation relied more on the increase in the number of larger aggregates. It is concluded that the old-field succession in karst valley was beneficial to protect against soil erosion by improving soil aggregate stability, but had a limited effect on soil organic carbon sequestration.