Mineral and organic contribution to nutrient pool in black anthropogenic soils from the African Great Lakes Region

In the African Great Lakes Region, low crop yields due to widespread soil infertility involve difficulties to cope with demographic pressure. Improving the productivity and sustainability of the dominant banana-based cropping systems requires a better understanding of nutrient pools. Four sites differing in parent rock (granite, shale and other pelitic rocks) were selected from Rwanda and Burundi. In two or three homesteads per site, soils were sampled as a function of distance to farmer’s house. Two major types of soil properties discriminated the soils: (i) contents of organic matter and major plant nutrients (Ca, Mg, P), both linked to past and current soil management practices, (ii) nature of parent rock, linked to the abundance of iron and the occurrence of muscovite as residual mineral. All soils were deeply and strongly weathered. Typical anthropogenic soils similar to the Amazonian Dark Earths occurred under ancient banana cropping where frequent supply of manure, ash and kitchen residues was done. Close to the house, the thickness of man-made dark Ah horizons could exceed 160 cm. Accumulation of organic matter promoted root exploration, thereby enhancing uptake of water and nutrients, and banana plant biomass. Yet, the soil gradient was strong in some sites and smooth to absent in ancient banana cropping areas. Gradients were revealed by contrasted stocks of a.o. carbon (19-12; 29-15; 30-22 kg m-3), and total content of Ca, Mg and K (420-370; 570-290; 370-280 molc m-3). The organic contribution to the nutrient stock of Ca, K and Mg ranged between 33 and 12%, and was highest around farmer’s house in contrasted sites, but also in ancient banana cropping areas whatever the distance to the house. High population pressure and soil infertility require sound management practices, based on better insight into soil formation and soil management processes