Distribution of soil organic carbon associated with Al and Fe minerals within alpine mountain soils in western Sichuan

The chemical protection mechanism of soil organic carbon (SOC) was explored to quantify the distribution of organic carbon protected by iron (Fe) and aluminum (Al) minerals in two alpine soils. Selective extraction methods were used in mountain podzolic (MP) soil and alpine meadow (AM) soil to investigate the distribution of SOC, as well as Fe and Al contents of SOC associated with crystalline metal oxides extracted by (dithionite-HCl, DH), short-range-order minerals extracted by (hydroxylamine-HCl, HH) and organo-mineral complexes extracted by (Na-pyrophosphate, PP). Results showed that the carbon concentrations extracted by DH, HH, and PP in MP soils were (10.91±6.23) g/kg, (5.92±1.66) g/kg, and (8.76±2.29) g/kg, respectively. These oxides, minerals, and complexes accounted for 20.92%, 12.07%, and 19.93% of SOC, respectively. Comparatively, the carbon concentrations for DH, HH, and PP in AM soils were (9.05±1.33) g/kg, (5.52±1.02) g/kg, and (9.12±3.21) g/kg, accounting for 21.04%, 12.47%, and 19.34% of SOC, respectively. The distribution of carbon contents and their proportions to SOC extracted through three extractants showed an order of PP > DH > HH in the A horizon and an order of DH > PP ≈ HH in the B horizon, for both MP and AM soils. An increase in soil depth, together with increasing abundance of secondary minerals and crystalline minerals in deeper soils, leads to organo-mineral associations changing from being dominated by organo-metal complexes to being dominated by the crystalline mineral-associated organic carbon. PP-extracted carbon contents and its relative proportion to SOC were higher in the A horizon than those in the B horizon, whereas DH- and HH- extracted carbon contents and their proportions to SOC were generally lower in the A horizon than those in the B horizon. These differences were more pronounced in MP because of its special soil property. The distribution of metal (Al+Fe) contents extracted by the three extractants showed the order DH > HH > PP in both soil types. However, the molar ratio of carbon-to-metal showed the order DH < HH < PP, suggesting that as the degree of mineral crystallization increases, the interaction between minerals and carbon changed from precipitation dominant to adsorption dominant. A correlation analysis of DH, HH, PP, and SOC suggested that the accumulation of SOC was driven to a certain extent by the association of crystalline Fe and Al minerals and organic matter in MP, as well as the association of short-range-order minerals and organic matter as well as organo-metal complexes in AM soils. The organomineral associations are, to some extent, driving the accumulation of SOC in alpine mountain soils in western Sichuan. The distribution of organic carbon associated with minerals and the accumulation ways of organic carbon is different across various soil types. © 2022, Science Press. All right reserved.