Early Soil Nitrogen and Greenhouse-Gas Responses to Reduced Tillage in Smallholder Maize-Bean Systems Across Contrasting Agro-ecological Zones of Western Kenya

Abstract

Reduced tillage is increasingly being promoted in sub-Saharan Africa, yet early changes in soil nitrogen (N) cycling and greenhouse-gas (GHG) exchange remain poorly resolved under smallholder maize–bean systems. This three-year field experiment evaluated conventional tillage (CT), minimum tillage (MT), and no-till (NT) at Bungoma (lower altitude, warmer, two annual cropping seasons) and Trans-Nzoia (higher altitude, cooler, one principal cropping season) in western Kenya. Treatments were arranged in randomized complete blocks with four replicates. Potentially mineralizable nitrogen (PMN), mineral nitrogen, soil moisture, and chamber-based CO₂, CH₄, and N₂O fluxes were measured during long-rain, short-rain, and fallow periods. Site and season strongly structured biogeochemical responses. PMN and GHG fluxes were generally greater at Bungoma than Trans-Nzoia, and short-rain periods produced the largest CO₂ and N₂O fluxes. Reduced soil disturbance consistently lowered CO₂ flux. Relative to CT, NT lowered mean CO₂ flux from 79.5 to 64.5 mg CO₂-C m⁻² h⁻¹ at Bungoma and from 61.3 to 52.0 mg CO₂-C m⁻² h⁻¹ at Trans-Nzoia. In contrast, mean N₂O differences among tillage treatments were not statistically significant, indicating that mitigation responses were air and context-specific. CH₄ exchange was small and predominantly represented net uptake. The results show that early transition to reduced tillage can suppress soil CO₂ losses, while N₂O responses remain strongly conditioned by season and agro-ecological setting. These findings support locally adapted conservation agriculture strategies and caution against assuming uniform GHG benefits from reduced tillage alone.