Soil microstructure as affected by tillage, rotation and residue management in a sweet sorghum-based cropping system in soils with low organic carbon content in South Africa

Abstract

Soil aggregation is regarded as a key indicator of soil quality and behaviour that is mainly influenced by land use and management. There is lack of information on the effects of conservation agriculture (CA) on soil aggregation in low soil organic carbon (SOC) soils of South Africa. This study therefore aimed to quantify the effect of tillage, crop rotation and crop residue management on aggregate stability, binding agents and the resulting aggregate microstructure in a sweet sorghum-based cropping system in a low organic carbon soil. Two tillage levels; no-till (NT) and conventional tillage (CT), two crop rotations, i.e., sweet sorghum-grazing vetch-sweet sorghum (S-V-S) and sweet sorghum-fallow-sweet sorghum (S-F-S) and three crop residue retention levels, i.e., 0%, 15 % and 30 % were tested. The aggregate microstructure was quantified on aggregates of ∼5 mm diameter with micro-focus X-ray computed tomography (μXCT) and image analysis was done using VGstudio MAX 3.2. Aggregate stability, soil organic carbon and glomalin related soil protein content (GRSP) were statistically higher under NT than in CT. GRSP was also enhanced by 30 % residue retention compared to other residue management practices. NT and CT accompanied by 30 % residue retention had the highest total porosity. NT + S-V-S + 30 % treatment combination generally had higher observed total porosity, higher frequency of pores within each pore class and denser microstructure visualisation. This study showed that tillage is the main factor that influences soil aggregation followed by residue management. Thus, application of NT + S-V-S+ 30 % enhanced soil quality in low SOC soils after three years. The study also demonstrated that aggregate stability is linked to aggregate microstructure and regular pores are dominated in degraded soils with low SOC.