Effects of hand-hoe tilled conservation farming on soil quality and carbon stocks under on-farm conditions in Zambia

Conservation farming (CF) has been promoted in Zambia since the 1980s. Despite long-term practice of CF in Zambia, its effect on soil fertility, including the storage of soil organic matter (SOM), on smallholder farms are inconclusive. Here, we assess the effect of CF as compared to conventional tillage on soil quality parameters on smallholder farms in the Eastern province (EP, 20 sites, two to six years of CF) and Central province (CP, 20 sites, four to twelve years of CF) in Zambia. Soils under CF (minimum tillage hoe basins, crop rotation and residue retention) were compared with adjacent conventional farms (hoe ridges in EP and overall digging or ridge splitting in CP). Only small differences were observed in the soil quality parameters between the CF basins and adjacent conventional plots after maximum 12 years since CF adoption. The concentration of soil organic carbon (%SOC) and carbon (C) stocks did not differ significantly between management practices, with C stocks in CF basins and conventional plots in EP amounting to 4.41 and 4.63 kg m−2, respectively, while this is 3.37 and 3.57 kg m−2, in CP. Likewise, the %SOC did not differ significantly between soils in the basins and in-between the basins. Both observations indicate that either the annual net accumulation of SOC is very small, or that on-farm surveys involve significant year-to-year changes in the position of the basins. However, the latter is not supported by plant available phosphorus (Bray P) data, which are significantly greater in CF basins than in-between them (12.7 vs 8.3 mg kg soil−1 in CP and 8.5 vs 5.2 mg kg soil−1 in EP), indicating significant Bray P accumulation in CF basins, due to annual fertilizer addition. Amounts of Bray-P in CF basins did not significantly differ from that under conventional management. Overall, our results show small differences in the soil quality parameters between the CF and conventional practices at smallholder farms after maximum 12 years since adoption of CF.