Organic Carbon and Aggregate Stability of Three Contrasting Soils as Affected by Arable Agriculture and Improved Pasture in Northern KwaZulu-Natal, South Africa

Abstract

Changes in soil organic carbon (SOC) and aggregate stability as a result of agricultural practices have been extensively studied, but the extent of these changes is site and crop specific. In this study, SOC and aggregate stability were evaluated under undisturbed grassland, cultivated pasture and arable land uses in northern KwaZulu-Natal, South Africa. The soils in this study were sampled from the 0–5, 5–10 and 10–20 cm depths of a Nitisol, Stagnosol and Fluvisol, and analysed for SOC content and stocks and aggregate stability. In all soils, SOC was significantly lower under arable cultivation (range from 1.4 to 2.1%) compared to the grassland and pasture (3.4 to 4.2%) in the top 10 cm. The soil carbon stocks followed a similar trend to the SOC in all soils and under all land uses. The overall aggregate stability expressed as mean weight diameter (MWD) was also significantly lower under the arable land use (1.03 to 1.82 mm) compared to grassland (2.44 to 2.96 mm) at all depths in all the soils, except at 5–10 cm in the Stagnosol. The MWD values under grassland and pasture were equal at all depths in all the soils. The MWD increased with SOC up to a threshold of 3.0–3.9% indicating the importance of SOC in soil aggregation. The weak correlation in the Stagnosol suggests that other factors are important in this soil. The loss of SOC and decline in aggregate stability over a period of 38 years of continuous arable cultivation in the near surface layers of two of the soils are potentially problematic for soil sustainability in the longer term.