Effects of iron/aluminum mineral phases on soil organic carbon storage in different clay soils of subtropical acidic forests

Abstract

Mineral carbon pump (MnCP) is the dominant mechanism for stabilizing soil organic carbon (SOC) in acidic forests. However, the influence of iron (Fe)/aluminum (Al) mineral phases on SOC storage mediated by soil clay remains unclear. Here, we selected 22 subtropical acidic forests with a wide range of clay content, and quantified the organic carbon (OC) associated with organo-Fe/Al complexes, crystalline minerals, and short-range-order (SRO) minerals by sodium-pyrophosphate (PP), dithionite-HCl (DH), and HCl-hydroxylamine (HH) extractions, respectively, and then constructed a structural equation model (SEM) for SOC accumulation. The results showed that the Fe/Al phase content was followed by crystalline minerals, Fe/Al-complexes, and short-range-order (SRO) minerals, with a clear increasing trend along the clay content gradient. Associated OC of organo-Fe/Al complexes, crystalline, and SRO minerals were 6.25 mg g⁻¹, 6.11 mg g⁻¹, and 1.14 mg g⁻¹, and accounted for 26.98 %, 26.31 %, and 4.95 % of SOC, respectively. The average dissolved molar OC to metals (C/M) ratios were 5.13 (ranging from 1.57 to 19.23) and suggested the binding mechanisms between Fe/Al phases and OC were governed by adsorption and co-precipitation. SEM revealed that metal-bound OC (MOC), cation exchange capacity (CEC), and mean weight diameter (MWD) jointly explained 62 % of the variability in SOC, with CEC being the most important variable. Clay particles enhanced CEC and MWD by providing more cation binding sites and acting as cementing agents in aggregation through Fe/Al phases, and combined with soil water content (SWC) and soil pH collectively contributed to SOC storage. These findings suggest that Fe/Al phases may control a substantial fraction of soil carbon in acidic forests, and Fe/Al complexes and pedogenic minerals may play more important roles in SOC accumulation.