Soil Organic Carbon Stabilization in the Three Subtropical Forests: Importance of Clay and Metal Oxides

Abstract Field observations show that subtropical forests can accumulate soil organic carbon (SOC) at high rate; however, the key mechanisms of SOC accumulation in subtropical forests remain unclear. Here we selected three typical subtropical forests with soil originated from same parent material under same climate condition in Dinghushan Biosphere Reserve, southern China: coniferous forest, mixed forest, and broadleaved forest with very different rates of SOC accumulation over the last three decades. To quantify the relative importance of physical versus chemical process on SOC stabilization in these forests, we used regression analysis to evaluate the relationship between the soil physical (clay, silt, and sand) and chemical (pH, Fe, and Al oxides) properties and SOC, and explored their respective contributions to SOC stabilization. Results showed that SOC stabilization was more strongly influenced by soil clay fraction through sorptive protection or microaggregate formation with SOC in the coniferous forest ( r 2 = 0.89, p < 0.05), and by chemical protection through forming organo‐mineral complexes in the mixed or broadleaved forest. Increasing carbon input was likely the main cause for SOC accumulation in the coniferous forest, but was not in the broadleaved forest. The strong linear relationships between SOC and Fe/Al oxides indicate that both the mixed ( r 2 = 0.63 and 0.57 with Fe o and Al o , p < 0.05) and broadleaved forests ( r 2 = 0.96 and 0.78 with Fe d and Al o , p < 0.05) on acid soils in southern China still have a great potential for further carbon sequestration in the future.