Influence of crop residues on nitrogen mineralisation in an acidic nitosol.

Organic matter contents and nutrient supplying power of most cultivated soils in Ethiopia are low. This is mainly due to many years of cultivation with little or no external input and total removal of crop residues from cropped fields at harvest. Incorporation of crop residues supplies plant nutrients through mineralization and increases soil organic matter. The amount of nutrients returning to the soils through mineralization of residues very depending on type of crop residue incorporated. A laboratory incubation experiment was conducted to determine the mineral-N contribution of different residues (Vicia fabae, L., Leucaena leucocephala, Zea mays L. and Sesbania sesban) to the soil. The treatments included the four types of residues at four rates of application (0.0, 1.0, 2.0 and 4.0%) on air-dry soil weight basis. Incorporation of sesbania residue resulted in immediate net N mineralization which increased significantly with increasing levels of residue added to the soil. The highest mineral N content was observed at the application rate of 4.0% sesbania residue. Nitrification was not suppressed due to the application of higher rate of sesbania residue. In the case of leucaena residue, net N mineralization started after 60 days of incubation. With faba bean residue, immobilization of N increased with increasing rate of application up to 60 days after application, whereas in the case of maize straw, the amount of mineral N was significantly less than the control throughout the study period. It was only at 120 days after incubation and at the highest application rate of maize straw that net mineral N was obtained. The overall results of the experiment indicated that for plant residues of low C:N ration and high N content like sesbania and leucaena, the N content in the residues in enough to meet the immobilization needs of soil microorganism and the surplus is released as mineral N into the soil. The reverse is true for residue with high C:N ratio, like maize straw.