In tropical agricultural systems with limited access to fertilizers, plant residues are often used to meet the N requirements of annual crops. Although plant residues may carry large quantities of N, under field conditions, the contribution of plant residue N to crop nutrition is of ten small and crop recoveries of residue N are less than 20 per cent. The reason for low recoveries is a lack of synchrony between N release and crop N demand. Nitrogen release from residues can be regulated by altering or manipulating the factors that influence the mineralization of N from the residues, such as residue quality, environmental factors and management factors. In a given situation (location, soil type), environmental factors like temperature and water cannot usually be altered. Altering the method or timing of application can improve synchrony to some extent but cannot regulate N release to achieve a desired pattern of release. The only practical option left is to alter the quality of residues. The concentrations of nitrogen, lignin and polyphenols are the major determinants of plant residue quality. Strategies for management of residue quality may include choosing plant species and plant parts having a desired quality, and the manipulation of residue quality by producing residues of desired quality or by mixing residues of different quality to obtain a desired pattern of N release. While there are several avenues for manipulating plant residue quality, mixing residues of different qualities is by far the simplest and most practicable strategy for regulating the rates of decomposition and N release. When a mixture of high and low quality residues is applied, the high quality residue can meet the peak demand from young crops while that of low quality can extend the supply through the growing season. By the same analogy, a combination of plant residues and chemical fertilizers is a pragmatic approach for supplying N to crops commensurate with their demand. N use efficiency can be maximized by strategic conjunctive use of plant residues and chemical fertilizers in appropriate proportion and at appropriate time during crop growth.
