Abstract: Experiments were conducted under greenhouse conditions to study I) the dynamics over time of mineral N, mineralizable N and humus N during decomposition in soil of leguminous and non-leguminous plant material and ii) the impact of these changes on growth and N nutrition of wheat fertilized with ¹⁵N-labelled ammonium sulphate. Finely ground straw of wheat, maize and sesbania was allowed to decompose in soil for 0, 2, 4 and 8 weeks. Sub-samples of soil were analyzed for the content of mineral N, mineralizable N and humus N. The bulk soil was sown to wheat and the plants harvested at maturity. Nitrogen as ¹⁵N-labelled ammonium sulphate was applied in two split doses of 40 and 60 mg kg^-1 soil. Accumulation of mineral N in soil during 8 weeks of residue decomposition 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 incubation but a substantially higher loss of NO₃¯+NO₂¯ -N during subsequent incubation under submerged conditions. The loss of NO₃¯+NO₂¯ -N was more where residues were given less time for decomposition. Plant residues applied immediately before sowing wheat or those allowed shorter period of decomposition had a negative effect on plant growth. Of the three residue types, wheat straw caused a higher reduction in plant growth. This was attributed mainly to significantly reduced availability to plants of soil-N, while uptake of fertilizer N was affected almost similarly by the three types of residues. Residues of maize and sesbania had a positive effect on grain yield and total biomass of wheat. As a whole, ca 30% of the fertilizer N applied was taken up by the plants and >40% was unaccounted in the soil-plant system, fertilizer N remaining in soil being <30% of the applied. Relatively higher proportion of the applied N remained in amended than unamended soil after crop harvest.