| 25 Aug 2026
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: CPP>CDCB>COX, 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: FeDCB>FeOX>FePP, 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 AlDCB, 3.9±0.02 % for AlOX and 2.0±0.01 % for AlPP of the total Al. Manganese extraction occurred in the sequence - MnOX>MnDCB>MnPP, 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.
The manuscript presents results on the extraction of metals, Si, and C from (sub) tropical soils with frequently used methods. I agree that the (sub) tropical soils under study have not intensively investigated yet so the authors aimed to fill a gap in knowledge. However, there are some major issues that must be clarified prior to potential publication.
My main concerns relate to definitions, lab procedures and data interpretation.
Several times, the authors mentioned “Fe/Al oxides” without defining this phrase. Do they mean the sum of individual Fe and Al oxides? Or do they mean mixed oxides, which contain both Fe and Al so that the “pure” Fe and Al oxides, respectively, are not included? Clarification is necessary, as it determines the interpretation of extraction data.
Several times (see below), the authors claimed that Al oxides are dissolved (extracted by) DCB. This is wrong and shown by many studies/reviews, which the authors even cited. On the other hand, sometimes the authors interpreted DCB extraction of Al oxides correctly. Again, clarification and uniform correct interpretation is necessary.
Several times, the authors mentioned SRO and poorly crystalline minerals. Does it mean that they distinguished between SRO and poorly crystalline? If so, which minerals are SRO, but not poorly crystalline and vice versa?
The authors referred to Wagai et al. (2013), who suggested filtration of PP extracts with a 0.025 µm filter, but the authors used a 0.2 µm filter without explanation. Furthermore, they filtrated all extracts at 0.2 µm, except for DCB extracts, for which they applied a Whitman filter with approx. 2 µm pore size. Not only is the inconsistency of the filter pore size between the different extracts incomprehensible, but also that it does not exclude the presence of particulate species in the DCB extracts - dissolved species, and only those are at stake, are defined via a 0.45 μm filter. These are not the only examples where studies/reviews are cited but the statements in them are not considered.
More specifically and minor concerns:
Due to my above-mentioned objections, a substantial revision of the manuscript is necessary.