Nitrogen dynamics in tropical soils of Mali, West Africa

Soil samples were collected from a loamy sand and a clayey soil near Cinzana, Mali, for the purpose of documenting the seasonal dynamics of soil inorganic N after 9 years under five crop-management systems. The cropping systems were: continuous grain sorghum (Sorghum bicolor) or millet (Pennisetum glaucum) without residue return, continuous grain with stalk residue returned to the field every second year, grain in rotation with cowpea (Vigna unguiculata), and grain in rotation with the green manure crops, sesbania (Sesbania rostrata) and dolichos (Dolichos lablab). A sharp increase in soil N was observed early in the rainy season in both soils. Extractable N concentration in loamy sand and clayey soils, respectively, peaked between 15-22 kg and 33-51 kg N ha(-1) in the upper 10 cm of soil. In the clayey soil, the higher soil N concentrations associated with the early season flush lasted 8 weeks after the onset of rain. Nitrogen addition through rotational crops and crop residue was low. Significant improvement of cereal grain yield may not be possible solely by rotation with sesbania and dolichos green manure or cowpea without additional nutrient input. Earlier cereal planting, where feasible, is recommended to improve synchrony of soil N mineralization and crop demand.