Al-/Fe-(hydr)oxides–organic carbon associations in Oxisols — From ecosystems to submicron scales

Al-/Fe(hydr)oxides accumulation upon weathering favors soil organic carbon (SOC) protection into mineral-organic associations (MOAs). Paradoxically, tropical ecosystems on highly weathered soils are characterized by high turnover rates of SOC. Our objective was to propose an explanation for such apparent contradiction by inferring the chemical composition of MOAs in Oxisols. To this end, we compiled a large data set from 43 published articles providing chemical and physical properties of 179 Oxisols/Ferralsols and their SOC content in the A horizon. Thermal analysis (TA) coupled to evolved gas detection (EGD) was used to assess mineral dehydroxylation and SOC oxidation in two soils with contrasting SOC content. Scanning transmission electron microscopy (STEM) was used to probe MOAs within the soil fraction <53μm. At large scales, SOC content was strongly correlated to Al extractable by both ammonium oxalate (AO) (r=0.71) and dithionite-citrate-bicarbonate (DCB) (r=0.41). Weaker, but significant correlations occurred for SOC against Fe-AO and Fe-DCB (r=0.38, and r=014, respectively). At micro-scale, SOC also was found associated to Al-/Fe-(hydr)oxides, as inferred from the STEM imaging. TA-EGD indicated that with increasing SOC content, proportionally more organic matter was oxidized following the dehydroxylation of Al-/Fe-(hydr)oxides at temperatures <400°C. Within the first 20 cm of the topsoil of the Oxisols included in our data set, Al-AO explained 62% of the total variation in SOC. However, while SOC content varied from 0.8 up to 145.8gkg−1 soil, Al-AO varied between 0.2 and 15.3gkg−1 soil. From the perspective of mineral control on SOC retention, reactive species of Al-(hydr)oxides should interact with a disproportionally large amount of organic matter. Because mineral protection is seemingly reduced upon increasing C content in MOAs, SOC in Oxisols may be more vulnerable to environmental changes than currently recognized.