Aggregate dynamics and intra-aggregate carbon contents as influenced by long-term wheat production management in semi-arid South Africa

Soil aggregate dynamics have received renewed interest due to their relevance in carbon (C) stabilisation. This study evaluated changes in aggregate-size distribution and intra-aggregate C following 41 years of wheat production in semi-arid central South Africa. Management practices that have been applied for 41 years include two straw management protocols (unburned and burned), three tillage systems (no-tillage, stubble mulch and mouldboard ploughing) and two weeding methods (chemical and mechanical). Soil cores were taken at 0–10 cm and 10–20 cm depths and fractionated to aggregates of different sizes using wet sieving. All the fractions including bulk soil were analysed for C. Unburned plots had more small macro-aggregates (35% and 33%, respectively) in the 0–10 cm and 10–20 cm depths, but 26% and 27% lower large macro-aggregate and bulk C, respectively were found in the 10–20 cm depth compared to the burned plots. No-tillage stimulated re-aggregation, especially large macro-aggregates, by 68% in the 0–10 cm depth compared to stubble mulch. Small macro-aggregates, large micro-aggregates and bulk soil under no-tillage had on average 25% higher C compared to those under mouldboard ploughing in the 0–10 cm soil layer. Although signs of aggregate equilibrium state were evident, future studies focusing specifically on this subject are needed.