Seasonal Variability in Greenhouse Gas Emissions Associated with Maize - Bean Intercropping Under Inversion Plowing in Western Kenya.

Smallholder farmers in tropical Sub-Saharan Africa (SSA) produce maize (Zea mays L.) intercropped with common bean (Phaseolus vulgaris L.) using inversion deep tillage and low rates of fertilizers. In order to produce sufficient amounts of food at low altitude areas, farmers grow maize/beans biannually during long and short rain seasons while in  high altitude farmers obtain proportionally greater yields during one long growing season only. Second planting during biannual cropping requires additional tillage and leads to low nutrient recovery from crop residues and gradual exhaustion of soil organic matter (SOM). Greenhouse gas (GHG) emissions are robust indices to assess SOM changes and hence, soil resilience. Little is known however, about carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes in response to different management practices in SSA. This study aimed to assess spatial and temporal variability in continuous annual and biannual maize/bean intercrop production on GHG emissions, soil C and N and crop performance at high and low altitude areas. Seasonal GHG emissions were monitored for a period of two years at two locations: low elevation site with biannual crop production (Bungoma) and high elevation site with one growing season (Trans-Nzoia). Our results suggest that growing second crop during the Short Rains (SR) resulted in high CO₂ and N₂O fluxes. In general N₂O fluxes in SR in Bungoma averaged 89.3 µg N₂O-N m^-2 h^-1 and accounted for an average 79% of cumulative amount of flux calculated for the entire year, which was 83% greater than N₂O flux in SR in Trans-Nzoia (48.7 µg N₂O-N m^-2 h^-1). Majority of N₂O originated from soils beneath bean plants and 108% more compared to soils associated with maize plants. High N₂O fluxes in SR at low altitude areas suggested rapid decomposition of crop residues from the first growing season.