Tillage and crop rotation effects on carbon sequestration and aggregate stability in two contrasting soils at the Zanyokwe Irrigation Scheme, Eastern Cape province, South Africa

Abstract

Intensive tillage and monocropping have adversely affected the quality of soils in South Africa through accelerated loss of soil organic matter. Two clay loam soils, a Bonheim at Burnshill and a Shortlands at Lenye, at the Zanyokwe Irrigation Scheme in the Eastern Cape province were used to evaluate the short-term effects of tillage and crop rotations on carbon sequestration and aggregate stability under sprinkler-irrigated crop production. A split-plot arrangement of treatments in a randomised complete block design was used with tillage as the main plots and crop rotations as subplots. Conventional tillage (CT) was compared to no-till (NT) under maize–fallow–maize (MFM), maize– wheat–maize (MWM) and maize–oat–maize (MOM) rotations. Carbon sequestration was monitored by measuring changes in soil organic carbon (SOC) and the stability index (SI) was used for monitoring aggregate stability. No-till had inconsistent effects on SOC relative to CT but resulted in improved soil SI on both soils, especially on the Shortlands soil. The MOM rotation enhanced SOC relative to the MWM and MFM rotations on both the Bonheim and Shortlands soils. Across tillage practices, the MOM rotation significantly increased the soil aggregate SI compared with the MWM and MFM rotations on the Shortlands and to a lesser extent on the Bonheim soils. Scanning electron microscopy revealed that soil aggregates under MOM had dense organic coatings and bridges compared with the MFM rotation, indicating the positive effect of carbon sequestration on aggregate stability. Generally, the results indicated that, in the short term, cover crops, especially oats, have greater influence on SOC accumulation and aggregate stability than tillage, irrespective of soil type.