MANIPULATION OF QUALITY AND MINERALIZATION OF TROPICAL LEGUME TREE PRUNINGS BY VARYING NITROGEN SUPPLY

The effect of N supply on the quality of Calliandra calothyrsus and Gl iricidia sepium prunings was studied in a glasshouse over a 7-month gr owing period. Increasing the concentration of N supplied from 0.625 to 10.0 mM NO3-N resulted in increased N concentration but decreased pol yphenol concentration, protein-binding capacity and C:N ratio of pruni ngs from both species. Lignin concentration was not consistently alter ed by the N treatment. Mineralization of N from the prunings was measu red over a 14-week period under controlled leaching and nonleaching co nditions. The results indicated a strong interaction between legume sp ecies and concentration of N supply in their influence on N mineraliza tion of the prunings applied to the soil. Differences in the %N minera lized were dictated by the quality of the prunings. The (lignin + poly phenol):N ratio was the pruning quality factor which could be used mos t consistently and accurately to predict N mineralization of the legum e prunings incubated under leaching conditions, and the relationship w as best described by a linear regression. Under non-leaching condition s, however, the protein-binding capacity appeared to be the most impor tant parameter in determining the patterns of N release from the pruni ngs studied. The relationship between the N mineralization rate consta nt and the protein-binding capacity was best described by a negative e xponential function, y = 0.078 exp(-0.0083x). The present study also i ndicated that the release of N from legume prunings containing a relat ively high amount of polyphenol could be enhanced by governing the N a vailability conditions under which the plant is grown, for example whe ther or not it is actively fixing nitrogen. Estimates of pruning N min eralization after 14 weeks with the difference method averaged 6% (lea ching conditions) and 22% (nonleaching conditions) more than with the N-15 method for all legume prunings studied. The recovery of pruning b y maize (4-38%) was well correlated with the % pruning N mineralized s uggesting that incubation data closely reflect the pruning N value for a given catch crop under non-leaching conditions.