Control of active Al and Fe on mineral-associated organic carbon regulates soil organic carbon distribution in acidic to alkaline tropical and subtropical soils

Deciphering controls on soil organic carbon (SOC) is fundamental for predicting SOC distribution and sequestration potential. Mineral-associated organic carbon (MAOC) is recognized as the most stable, making it vital for modeling long-term SOC dynamics. However, its controlling factors and pathways remain unclear, particularly in tropical and subtropical soils. We used topsoil from 66 sites (non-volcanic, mainly natural vegetation) from India (arid and high pH), Tanzania, Cameroon, Japan (Okinawa), Indonesia (humid and low pH) with a wide range of soil pH (3.9 to 9.2) and moisture condition (represented by effective precipitation, EP = precipitation – potential evapotranspiration, −1740 to 2850 mm). Soils were categorized as strongly acidic (pH ≤ 5.5), weakly acidic (5.5 < pH ≤ 7), and alkaline (pH > 7). Soil fundamental physicochemical properties and mineral components that may influence MAOC, including active Al/Fe (oxalate-extractable Al and Fe: Al_o + Fe_o), clay, and exchangeable Ca and Mg (Ca_ex + Mg_ex), were determined. MAOC was measured as the carbon content in the fine heavy fraction (FHF, > 1.7 g cm⁻³ and <53 µm) obtained through density and particle size fractionations. The ratio of MAOC to FHF was used to indicate the carbon stabilization ability of FHF. Correlation analyses examined the influence of climate, vegetation, and soil properties on MAOC. Structural equation modeling (SEM) quantified the contributions of factors correlated with MAOC to its accumulation.

The fractionation results showed that MAOC accounted for 76 ± 14% of SOC, confirming it as the primary fraction regulating overall SOC. Correlation analysis identified pH, EP, net primary productivity, and active Al/Fe and clay contents as significant factors affecting MAOC. Notably, no significant relationship was found between MAOC and Ca_ex + Mg_ex, even in alkaline soils, suggesting that Ca and Mg ions play a minimal role in SOC stabilization. SEMs revealed active Al/Fe content as the primary factor across all pH categories, regulating most of MAOC (β > 0.48, R² > 0.80 for all, strongly acidic, and weak acidic soils; β = 0.62, R² = 0.61 for alkaline soils). Direct impact of EP was the secondary factor. Introducing clay content as a parallel factor to active Al/Fe reduced quality metrics (e.g., P < 0.05) of SEMs for all pH categories and showed no significant contribution to MAOC, indicating its less importance even in alkaline soils. Interestingly, the carbon stabilization ability of FHF was comparable in strongly and weakly acidic soils but significantly lower in alkaline soils. This difference is likely due to the lower active Al/Fe content in alkaline soils, where the intensified drying enhanced crystallization. The lower slope of MAOC to Al_o + Fe_o in alkaline soils further highlights the reduced carbon stabilization ability of active Al/Fe, likely due to the lowered positive charge in alkaline conditions and decreased hydroxyl groups from enhanced crystallization. In summary, active Al/Fe controlled MAOC, which constituted most of SOC, while soil pH and moisture conditions regulated its abundance and carbon stabilization ability, and higher moisture levels also directly enhanced MAOC.