ABSTRACT
Conservation agriculture (CA) has emerged as a sustainable farming practice aimed at preserving soil organic carbon (SOC) fractions. However, most extant literature has focused on short-term (less than 10 years) impacts and top-soil layers, 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, sub plots, sub-sub plots, and sub-sub-sub plots were allocated to tillage - no-tillage (NT) and conventional tillage (CT), crop rotations maize-fallow-maize (MFM), maize-soybean-maize (MFS), maize-wheat-maize (MWM), and maize-wheat-soybean (MWS), and crop residue management practices residue retention (R+) and residue removal (R-) as well as soil depths  0 – 20 cm, 20 – 40 cm, and 40 – 60 cm. Analysed soil samples were collected in the 2023/24 cropping cycle. The study found that no-tillage (NT) had a greater impact on SOC and carbon sequestration at the topsoil (0 – 20 cm) compared to conventional tillage (CT), owing to the presence of organic material from undisturbed soil. Particulate organic matter (POM) was increased by crop rotation and residue retention practices, particularly in the 0 – 20 cm, and 20 –40 cm soil layers. C: N ratios varied across treatment combinations, with higher ratios observed under NTR- x MFM and lower ratios under NTR- x MFS in topsoil. Interestingly, CT had higher humic acid concentrations, while NT showed elevated fulvic acid levels in the 0 – 20 cm layer. The humification ratio was greater under residue retention (R+) than under residue removal (R-). Principal components 1 and 2 analysis highlighted SOC, soil organic carbon, carbon sequestration, and humification ratio as key factors in determining SOC stocks, explaining 46.47% of variability. Although CA shows promise as a sustainable practice, the study suggests that subsoil fractions require long-term assessment to fully understand its impact beyond the initial 10 years of trial data.
Keywords: Soil degradation, farming practices, carbon sequestration, and subsoil.