REGULATING N RELEASE FROM LEGUME TREE PRUNINGS BY MIXING RESIDUES OF DIFFERENT QUALITY

In an attempt to regulate the rate of N mineralization from legume tre e prunings by manipulating the quality of the prunings, a low-quality legume tree pruning (Peltophorum dasyrrachis) was mixed at various pro portions with a high quality legume tree pruning (Gliricidia sepium). N mineralization behaviour of the pruning mixtures was tested upon app lication to the soil in the laboratory under controlled leaching and n on-leaching conditions over a 14-week period. Mixing the two species r esulted in complex, non-linear interactions in quality, particularly w ith regard to the activity of extractable polyphenols. N mineralizatio n rate of the pruning mixtures decreased with increasing proportion of Peltophorum prunings in the mixtures, indicating that N mineralizatio n rate of prunings can be manipulated by mixing different quality mate rials. The initial lignin-to-N ratio and the (lignin + polyphenol)-to- N ratio were strongly correlated with the N mineralization tate consta nt of the prunings mixtures under both leaching and non-leaching condi tions. Under leaching conditions, however, the N mineralization rate c onstant was best correlated with the initial protein-binding capacity of the pruning mixtures. In agreement with patterns of N mineralized f rom the pruning mixtures, the N recovery by maize from the pruning mix tures increased with increasing proportion of Gliricidia prunings in t he mixtures. The strong relationship between N recovery and the protei n-binding capacity suggested that protein-binding by polyphenols was t he cause of the reduced N recovery of individual prunings in the mixtu res. The results also showed that by mixing with slow N release legume prunings, such as Peltophorum, possible losses of N mineralized from the rapid N release legume prunings (Gliricidia) can be minimized. In this study, when the proportion of Peltophorum prunings in the mixture was more than 50%, the N recovery from Gliricidia was strongly reduce d, probably because of the high protein-binding capacity of polyphenol s in the Peltophorum prunings. (C) 1997 Elsevier Science Ltd.