Chemical stabilization of organic carbon in agricultural soils in a semi-arid region (SE Spain)

Land use and management, together with soil properties, determine soil organic carbon (SOC) concentration and its stabilization mechanisms. Four soils (0–30 cm depth) were studied in a semi-arid region with different uses and management regimes: two soils with olive cultivation, both under a non-tillage regime and one with a cover crop (OCC) and the other without (ONT); a fluvial terrace soil (FT) with cereal–sunflower–fallow rotation; and an unaltered soil under natural vegetation (oak trees; OT). The OT soil had a higher SOC concentration than the agricultural soils (OCC, ONT and FT), followed by the FT soil without significant differences. The olive grove soils had a lower SOC concentration but the two types of management differed significantly, with higher concentrations due to the cover crop. Hydrofluoric acid (HF)-soluble, hydrochloric acid (HCl)-resistant, and non-oxidizable (sodium peroxodisulphate; Na₂S₂O₈) SOC fractions were determined at different depths (0–5, 5–10, 10–20 and 20–30 cm). The relative HCl-resistant and non-oxidizable SOC fractions increased with depth, whereas the relative HF-soluble SOC fraction varied slightly among the four soils considered. Differences in the SOC-stabilization mechanism were found according to the chemical SOC fractionation. In the FT and OT soils, where HF-soluble SOC and soil respiration rates were higher, the intense biological activity rapidly degraded the plant debris, being partially fixed and stabilized by the fine mineral-soil fraction as the principal stabilization mechanism of SOC. The olive grove soils had lower biological activity but higher SOC resistance to oxidation with Na₂S₂O₈, thus suggesting that chemical recalcitrance of soil organic matter was a relevant stabilization mechanism in these soils.