Potential of common extraction procedures to evaluate organic carbon association with different species of metal(loid)s in soils

Abstract. A strong association of organic carbon (OC) with metal oxides, especially iron (Fe) and aluminium (Al) oxides has been reported in several studies. However, the role of various species of Fe, Al, manganese (Mn) and silicon (Si) in the stabilisation of OC in Fe-rich and phosphorus (P) deficient soils, common in tropical and sub-tropical regions, remains poorly understood. To address this, we collected topsoil (0–20 cm) and subsoil (20–40 cm) samples from 37 sites across agricultural regions of New South Wales, Australia. Soil samples were subjected to three separate chemical extractions, i.e., sodium pyrophosphate (PP) to extract organo-metal complexes, ammonium oxalate (OX) to extract poorly crystalline and short-range order (SRO) minerals and dithionite citrate bicarbonate (DCB) to extract total metal oxides, to determine OC associated with each of the extractions. Soil organic carbon (SOC) was extracted in the sequence: C_PP>C_DCB>C_OX, with a mean of 62±0.16 %, 41±0.19 % and 28±0.21 %, respectively, of the total C extracted from soils. The extraction sequence for Fe was: Fe_DCB>Fe_OX>Fe_PP, with a mean of 49±0.18 %, 9±0.05 % and 3±0.03 %, respectively, of the total Fe extracted from soils. Aluminium was extracted in the same sequence as Fe, with a mean value of 4.0±0.02 % for Al_DCB, 3.9±0.02 % for Al_OX and 2.0±0.01 % for Al_PP of the total Al. Manganese extraction occurred in the sequence - Mn_OX>Mn_DCB>Mn_PP, with an average of 78±1.58 %, 65±1.10 % and 43±1.23 %, respectively, of the total Mn extracted. All extractants dissolved <1 % of the total Si. All extractable forms of Fe and Al showed significant positive correlations with extractable C, which suggested their potential role in the preservation of SOC. The large fraction of OC extracted by PP suggested that organic-Fe/Al complexes constituted a large fraction of the total SOC in the studied soils. The significant amount of OC associated with OX extraction also indicated that a substantial portion of the SOC was associated with SRO and poorly crystalline Fe oxides, while a relatively small proportion of the total OC was extracted by DCB suggested a limited role of crystalline Fe/Al oxides in the OC stabilisation.