Demand-Driven Technologies for Sustainable Maize ... - IITA

161Results and DiscussionTotal and inorganic nitrogenThe incorporation of soybean and cowpea residues increased the totalsoil N by 30 and 11%, respectively (Table 3). Residue of each legumesignifi cantly increased the inorganic N. The low C/N ratio of thelegume residues (Swift 1987), the rapid rate of residue decompositionin this location (Wild 1972), and the direct contact of the residue withthe soil by incorporation will ensure a fast rate of decomposition of theresidue resulting in increased soil N within a short period of time, asobserved in this study.Thonnissen (1996) reported that N in soybean residue wasquickly released into the soil and almost totally mineralized for thefollowing crop within a short period of time. Some other researchershave also reported improved soil N status after legumes in the Guineasavanna zone (Carsky et al. 1997; Oikeh et al. 1998). Similarly, Tianet al. (1993) reported increased soil inorganic N with incorporationof residue with low C/N ratio. It has also been found that soybeanincreases mineralization of soil N and prevents N immobilization(Gentry et al. 2001).In the present study, the incorporation of fallow residue slightlyincreased the soil total N by 6% to 0.53 gkg -1 but did not change thelevel of the soil inorganic N. The inorganic N did not change probablybecause of N immobilization in the soil, as previously reported byCarsky et al. (1998).Grain yieldThe effect of the previous legumes and fallow on maize grain yield wasstatistically signifi cant (P