The Effect of Tillage and Crop Residue Application On Total and Labile Soil Organic Carbon and Nitrogen Pools in a Long-Term Conservation Tillage Trial in Western Kenya.

The impact of conservation agriculture practices on soil quality under tropical smallholder farming conditions is not very clear often with conflicting results. The effects of tillage and crop residue (maize stover) application were investigated on a long-term conservation tillage trial on a tropical Oxisol in western Kenya. The experiment was established in 2003 as a split-split plot design involving a factorial combination of tillage systems in the main plots [reduced tillage (RT) and conventional tillage (CT)], and crop residue management in split plots (plus and minus crop residue). Cropping systems were superimposed in split-split plots (continuous cereal, rotation and intercropping) plots. After 16 cropping seasons in 8 years, maize grain yield ranged from 3.3 Mg ha^‑1 under reduced tillage without residue application to 5.4 Mg ha^‑1 under conventional tillage with crop residue application, and was significantly affected by tillage. However, soil organic carbon and total nitrogen whose values averaged 22.9 Mg ha^‑1 and 1.7 Mg ha^‑1 respectively in the topsoil (0-10 cm) were not significantly influenced by tillage, crop residue management or cropping system. Potentially mineralizable carbon was lowest under reduced tillage without crop residue application (76.4 mg kg^-1) and highest under conventional tillage with crop residue application (187.1 mg kg^-1) and was significantly influenced by tillage and crop residue application. Microbial biomass carbon averaged 342 mg kg^-1 under reduced tillage and 405 mg kg^-1 under conventional tillage being significantly influenced by tillage only. Tillage seems to play a major in regulating maize yield under limited crop residue retention as was the case in this system. Further investigations are necessary to determine how crop residue can be best managed under conservation tillage especially for the resource-constrained smallholder farming systems.