Tillage and Urea Fertilizer Application Impacts on Soil C Fractions and Sequestration

Conservation tillage has been considered a smart agriculture practice which preserves soil organic carbon (SOC). However, little work on the labile C fractions in South Africa has been documented. As such, this work focused on C fractions under different management systems. The objective of this study was to assess the impact of different tillage techniques and fertilizer application rates on soil C fractions along the soil profile. Samples from no-till (NT), conventional tillage after 5th season (CT-Y5), and annual conventional tillage, longer than 5 years (CT-ANNUAL) at 0, 60, 120, and 240 kg N ha−1 were taken at 0–10, 10–20, and 20–30 cm depths and analyzed for C fractions. The 30 cm depth was chosen as the sampling depth because of the 30 cm plough layer. At 0–10 cm, soil NT had higher total C, organic C, particulate organic C (POC), and permanganate oxidizable C (POxC) for all application rates, especially in the control treatment, compared to both the CT-Y5 and CT-ANNUAL treatments (p < 0.05). At the 10–20 cm soil depth, CT-Y5 had higher POC than both NT and CT-ANNUAL at 60 kg N ha−1 (p < 0.05). Greater C fractions in the surface soil under NT, and at deeper depths under CT, was due to litter availability on the surface under NT and incorporation to the subsoil on CT. Higher C sequestration in NT than in CT-Y5 and CT-ANNUAL was observed because of slower organic matter (OM) turnover in NT leading to the formation and stabilization of C. A larger input over output of OM, through high crop residue accumulation over decomposition, is the reason for the increase of C fractions in the fertilized treatments. Therefore, using conservation agriculture, particular NT, with 0 kg N ha−1 application rate in dryland agriculture is recommended.