Abstract: Laboratory and greenhouse experiments were conducted to study the changes in mineral N, humus N and plant available N during following decomposition of plant residues (wheat straw, maize straw and sesbania straw) for different time periods. Accumulation of mineral N in soil was found to depend on the chemistry of plant residues, more mineral N being released in soil amended with plant residues with narrow C/N ratio i.e., maize and sesbania. These residues also contributed more to humus N and maintained a higher content of potentially mineralizable N. Wheat straw not only caused a net immobilization of N during 8 weeks of aerobic but a substantially higher loss of NO₃ +NO₂ -N during anaerobic incubation. The loss of N under these conditions appeared to depend on the length of time the residues were allowed to decompose in the soil, more losses being recorded for residues at early stages of decomposition. Undecomposed or partially decomposed plant residues had a negative effect on plant (wheat) growth; the effect was positively related to N uptake by plants. The negative effect was eliminated by increasing the time of residue decomposition to 8 weeks at which point maize and sesbania had a positive effect on grain yield and total biomass of wheat. Since N availability could be the main yield determining factor, sufficient time for residue decomposition will be required to achieve net N mineralization and thus improved plant growth especially for plant residues with a wide C/N ratio. However, the N released during aerobic incubation (or during land preparation prior to planting) may indeed be lost at first irrigation from the soil-plant system depending upon the content of easily oxidizable organic C.