Persistence of soil profile carbon fractions after ten years of tillage, crop rotation, and residue management practice in a Haplic Plinthosol

Conservation agriculture (CA) has emerged as a sustainable farming practice aimed at preserving soil organic carbon (SOC) fractions. However, most existing literatures have focused on short-term impacts and top-soil layers in South Africa, leaving long-term effects on the soil profile less understood. This research evaluated the persistence of carbon fractions after 10 years of CA in a field trial initiated in 2012, laid out in a randomised complete block design with strip-split-split plot treatment structure. The main plots were allocated to no-till (NT) and conventional tillage (CT), sub plots; maize-fallow-maize (MFM), maize-fallow-soybean (MFS), maize-wheat-maize (MWM), and maize-wheat-soybean (MWS), and sub-sub plots; crop residue retention (R+) and crop residue removal (R−), it later incorporated the soil depths (0 – 20 cm, 20 – 40 cm, and 40 – 60 cm) in 2022 as the sub-sub-sub plots. Analysed soils were sampled in 2023/24 cropping cycle. The study found that NT increased SOC by 0.64% which was 14.06% more than CT. This has led to an increase of carbon sequestration at the topsoil (0 – 20 cm). Particulate organic matter was increased by crop rotation and residue retention practices, particularly in the 0 – 20 cm, and 20 – 40 cm soil layers. Carbon to nitrogen ratios varied across treatment combinations, with higher ratios observed under NTR- x MFM (14.71 g/kg) and lower ratios under NTR- x MFS (2.1 g/kg) in topsoil. Interestingly, CT increased humic acid concentrations by 0.63% which was 25.40% more than NT, while NT showed elevated fulvic acid levels of 1.2% which was 41.67% more than CT in the 0 – 20 cm layer. Principal component analysis identified SOC, carbon sequestration, and humification ratio as major contributors to SOC stock variability (46.47%). While conservation agriculture improves topsoil carbon, its impact on subsoil (20–60 cm) remains limited after 10 years, highlighting the need for longer-term monitoring.