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

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Introduction Conservation agriculture (CA) has emerged as a sustainable farming practice for improving soil health and preserving soil organic carbon (SOC) fractions. However, most studies in South Africa have focused on short-term impacts and surface soil layers, leaving the long-term effects of CA on SOC distribution throughout the soil profile poorly understood. This study evaluated the persistence and distribution of SOC fractions after 10 years of CA implementation under different tillage, crop rotation, residue management, and soil depth treatments. Methods The study was conducted on a long-term field experiment established in 2012 using a randomized complete block design with a strip-split-split plot arrangement. Treatments included no-till (NT) and conventional tillage (CT), four crop rotations (MFM, MFS, MWM, and MWS), crop residue retention (R+) and removal (R−), and three soil depths (0−20, 20−40, and 40−60 cm). Soil samples collected during the 2022 cropping season were analyzed for SOC, SOC stocks, particulate organic matter (POM), permanganate-oxidizable carbon (POXC), total extractable carbon (TEC), humic acid (HA), fulvic acid (FA), and related humification indices. Results No-till (NT) increased SOC by 0.64%, representing a 14.06% improvement over conventional tillage (CT), with greater carbon sequestration in the 0−20 cm soil layer. Crop rotation and residue retention enhanced particulate organic matter, particularly in the 0−20 and 20−40 cm depths. Conventional tillage increased humic acid concentrations by 25.40%, while NT increased fulvic acid concentrations by 41.67% in the surface layer. Principal component analysis identified SOC, HR, and TEC as the major drivers of SOC stock variability. Discussion Conservation agriculture, particularly NT combined with crop rotation and residue retention, improved SOC and carbon quality indicators in the topsoil. However, limited changes were observed in the 20−60 cm soil layers after 10 years, suggesting that longer-term implementation and monitoring are required to achieve substantial subsoil carbon sequestration and stabilization.