Effect of straw incorporation on the yield and nitrogen balance in the sandy soil-pearl millet cropping system of Senegal

Summary In a lysimetric experiment conducted in a sandy soil of Senegal, nitrogen fertilization (¹⁵N) and straw incorporation, were combined factorially, the soil being left bare or cropped with millet. On the one hand, yields were estimated, and on the other hand nitrogen absorption, immobilization, and losses were estimated in the soil-plant system. The depressive effect of straw incorporation on grain yield (32% on the average) was attributed mainly to the immobilization of fertilizer nitrogen in the rhizosphere. The depressive effect of such immobilization was alleviated by additional applications of nitrogen. Increasing the quantity of straw incorporated beyond the average amount resulted in a decrease of straw yield but had no effect on grain yield. Straw incorporation was thought to impede the plant growth during early stages but to promote it afterwards. Moreover, when the yield expressed in a fertilizer nitrogen unit basis was the highest, more than half of the plant nitrogen was nevertheless provided by the soil. The authors infer from this fact that soil organic matter was important in the efficiency of nitrogen fertilizer on pearl millet. The reduction of fertilizer nitrogen absorption following straw incorporation and not compensated by additional nitrogen fertilizer storage in the soil appeared to be related (cause or effect) to the increase of total fertilizer losses. Atmospheric losses significantly contributed to total losses (on the average 45%) of the fertilizer nitrogen applied to a planted soil. These losses can be mainly attributed to denitrification.