Total organic carbon, aluminium and iron in bulk samples and aggregate size fractions of a sandy clay loam humic soil under sugarcane relative to native forest in northern KwaZulu-Natal, South Africa

The distribution of total aluminium (Al) and iron (Fe) and organic carbon (TOC) in aggregate fractions gives an insight into the dynamics of these elements in soil. This study determined the effects of over 30 years of sugarcane cultivation, compared to adjacent native forest, on Al, Fe and TOC in bulk soil and aggregate fractions within the 100 cm depth of a sandy clay loam humic soil. Samples were separated into large macro-aggregates: LM (>2000 μm), small macro-aggregates: SM (250–2000 μm), micro-aggregates: M (250-63 μm) and silt + clay: SC (<63 μm) fractions. The TOC was analyzed by dry combustion and total Al and Fe by X-ray fluorescence spectrometry. Sugarcane cultivation (i) reduced macro-aggregates and TOC and (ii) increased the SC fraction and total Al and Fe. The mean weight diameter declined from 1.32 mm (0–30 cm) to 1.06 mm (30–100 cm) under forest. Average (0–100 cm) Al and Fe contents (g kg(−1)) increased in LM (6–16 for Al; 6 to 9 for Fe), SM (7–11 for Al), M (5–14 for Al; 6 to 9 for Fe) and SC (7–16 for Al; 9 to 10 for Fe) under sugarcane relative to forest. The TOC (g kg(−1)) declined in the LM (13–7) and SM (7–6) but increased in the M (5–9) and SC (10–13) due to cultivation. These findings suggested that sugarcane cultivation decreases aggregate stability and TOC in macro-aggregates, and increases Al and Fe in all aggregates. Adoption of practices inclined to improve or maintain TOC as well as liming to increase pH are necessary management practices for sustainable production.