Sugarcane cultivation reduces charcoal C and Al/Fe-bound OC in selected Ferralsols

Long-term sugarcane cultivation, with pre-harvest burning, may add recalcitrant charcoal carbon (C) to soil organic carbon (OC) in Ferralsols, whereas aluminium/iron-organic matter (Al/Fe-OM) complexes may dominate the mineral-associated OC in these soils. Therefore, the objective of this study was to examine the effect of sugarcane cultivation relative to wattle forest on soil OC in charcoal C form and Al/Fe-OM complexes on two selected Ferralsols. Total C, charcoal C, and Al/Fe-OC were analysed in samples collected from the two sites under forest and sugarcane cultivation to a depth of 100 cm, and the data were subjected to a two-way analysis of variance. At both sites, sugarcane cultivation reduced charcoal C and Al/Fe-bound OC when compared to forest. The Al/Fe-bound OC in sugarcane soils accounted for 48.7 and 72.2% of the total OC at Eston and Wartburg sites, respectively. In forest soils, the Al/Fe-bound OC accounted for 45.6 and 44.4% of the total OC at Eston and Wartburg, respectively. Charcoal C accounted for 8.42 and 4.07% of the total OC in sugarcane soils, at Eston and Wartburg, respectively. Overall, charcoal C concentration decreased with increase in soil depth, while its stocks only decreased with depth in soil under sugarcane. The Al/Fe-bound OC decreased with an increase in soil depth for both land uses, while Alp and Alp + Fep concentrations were not affected by sampling depth. These findings demonstrate that OC in Al/Fe-OC complexes and charcoal C contribute to the high OC concentrations, with Al/Fe-bound OC fraction being the primary mechanism of OC stabilisation in these Ferralsols, while sugarcane cultivation reduces these concentrations.