Soil Organic Carbon Stabilization Is Dominated by Non‐Sorptive Process Among the Subsoils From Different Parent Material.

Soil parent material can strongly influence soil's physical and chemical properties, significantly affecting soil organic carbon (SOC) stabilization. However, the importance of different soil parent materials on the amount and composition of subsoil carbon in subtropical soils remains poorly qualified. Subsoil SOC (below A horizon) is chosen for this study because of its higher proportion of stable carbon than surface soils in A horizon. Here, we investigated the dependence of soil mineral‐associated organic carbon (MAOC) and its three mineral‐stabilized fractions on soil physical and chemical parameters (clay content, cation exchange capacity (CEC) and metal oxides) in the subsoil soil (B and C horizons) originating from three contrasting parent materials (sandshale, granite, limestone). The results showed that MAOC accounts for an average of 88% of total SOC, of which organo‐metal complexes (extracted with sodium pyrophosphate) are the dominant form at all three sites. MAOC and its three fractions were significantly correlated with soil clay, CEC, sodium pyrophosphate extractable Fe and oxalate extractable Fe concentrations (P < 0.05), which varied significantly with soil parent material. Since the sodium pyrophosphate plus oxalate‐ extracted mineral‐associated carbon accounted for more than 70% of MAOC, and the molar ratio between sodium pyrophosphate or oxalate‐ extracted mineral‐associated carbon and Fe (C:Fe) ranged from 2.7 to 17.1. The formation of Fe‐associated organic complex is dominated by co‐precipitation when C:Fe is >1. Therefore we conclude that non‐sorptive interactions with the mineral phase through co‐precipitation are the dominant mechanism for SOC stabilization in subtropical forest soils. Plain Language Summary: Identifying the mechanisms for the accumulation and stabilization of forest soil organic carbon (SOC) is particularly important for increasing land‐based carbon mitigation as a part of the solution toward carbon neutrality by 2060 in China. However, our understanding of the mechanism of SOC stabilization is still limited, particularly how soil carbon stabilization is influenced by soil parent material, which has been shown to have strong influences on soil's physical and chemical properties, therefore on SOC stabilization. By analyzing the carbon content and composition of the soils developed from the different parent materials, we showed that soil parent material indirectly affected SOC through soil physical and chemical properties, of which soil clay content, CEC and soil Fe oxides played an important role in SOC stabilization. Both sorptive and co‐precipitation interactions between Fe and SOC contributed to SOC stabilization in subtropical forest soil with co‐precipitation as the dominant mechanism. Key Points: Mineral‐associated organic carbon accounts for >80% of soil organic carbon (SOC) in southern China forest soilsCo‐precipitation with Fe is the dominant SOC stabilization mechanismParent material is a significant contributor to the observed SOC variations [ABSTRACT FROM AUTHOR]