Impacts of collapsing gullies on the dynamics of soil organic carbon in the red soil hilly region of southeast China

•SOC in the collapsing gully was significantly lower than that in the hillslope.•There was no organic carbon enrichment in the alluvial fan and check dam.•The loss of soil clay and iron oxides in collapsed soil intensified SOC erosion.•Collapsing gullies deteriorated soil properties and accelerated SOC loss. Collapsing gullies are one of the most serious soil erosion problems in the tropical and subtropical areas of South China. However, few studies have determined the effects of collapsing gullies on the redistribution and loss of soil organic carbon (SOC) in the red soil hilly region. This study investigated the variations in different kinds of SOC in a collapsing gully and a hillslope and the relationships between various soil properties and SOC. The results showed that the dissolved organic carbon (DOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROOC) and total organic carbon (TOC) contents on the hillslope were higher than those in the collapsing gully. Below the location of upper catchment in the collapsing gully, the DOC, POC, ROOC and TOC contents were all greatly reduced, and there was no organic carbon enrichment in the alluvial fan and check dam. The stability of SOC in the collapsing gully was lower than that in the hillslope. The average free iron oxides (Fed), organically bound iron oxides (Fep) and clay contents in the collapsed soil were much lower than those on the hillslope. SOC content was significantly and positively correlated with the soil Fed, Fep and soil clay contents but negatively correlated with soil pH. Among these soil parameters, the dominant factors influencing TOC in the red soil hilly region were soil ROOC and soil pH. The SOC density of 0–30 cm soil in the collapsing gully was equivalent to nearly 15.9% of that in the hillslope. These results demonstrate that collapsing gullies have highly deteriorated soil properties and are therefore subject to accelerated SOC loss in the red soil hilly region.