The effect of conservation agriculture practices on soil quality and crop productivity under contrasting smallholder farming systems in western Kenya and eastern Uganda

Smallholder farming systems that dominate the tropical highlands of East Africa are faced with declining soil fertility, soil erosion and low crop productivity, threatening food security in the region. Hence, sustainable farming practices are required to restore soil fertility, control soil loss and enhance crop productivity. Conservation agriculture (CA) has been proposed as such an alternative with the potential of sequestering soil carbon and improving soil quality while sustaining higher crop productivity. This study assessed the impact of CA practices on total and labile soil carbon and nitrogen pools, aggregate stability, hydraulic properties and soil erosion control under four sites on an elevation gradient compared with an eight-year long-term trial. Tillage systems included no-till (NT), reduced or minimum tillage (RT or MT) compared with conventional tillage (CT), all combined with crop rotations and a cover crop or crop residue retention. Our results indicate that in the long-term trial, RT combined with crop residue retention enhanced soil aggregate stability and infiltration capacity. Nevertheless, soil organic carbon (SOC), microbial biomass, mineral nitrogen and crop yield were all higher under CT with crop residue retention probably due to enhanced mineralization of the incorporated crop residues. Whereas in the short-term study, SOC and total nitrogen were consistently higher under MT across all sites except Kapchorwa, though significant effects of tillage and cropping system interaction were only noted in Tororo. Penetration resistance and bulk density tended to decrease with tillage and were in the order NT> MT> CT, whereas infiltration rate was not significantly influenced by tillage. The Revised Universal Soil Loss Equation (RUSLE2) model and GIS were used to predict the potential impact of CA practices on soil erosion in Kapchorwa and Trans Nzoia catchments. Soil loss was highest under CT without crop residue retention, averaging 206 and 78 Mg ha-1 yr-1 and lowest under NT+50% crop residue at 19.7 and 7.5 Mg ha-1 yr-1 in Kapchorwa and Trans Nzoia respectively. Overall, soil loss was in the order CT+0% residue> CT+30% residue> MT+30% residue> NT+30% residue> MT+50% residue> NT+50% residue. However, final soil loss was still higher than allowable T value in Kapchorwa, indicating a need for alternative management or change of land use. Overall, CA practices showed potential of enhancing sustainability under smallholder farming systems, but long-term studies are required to ascertain trends and adjust to local conditions.