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 tree prunings by manipulating the quality of the prunings, a low-quality legume tree pruning (Peltophorum dasyrrachis) was mixed at various proportions with a high quality legume tree pruning (Gliricidia sepium). N mineralization behaviour of the pruning mixtures was tested upon application to the soil in the laboratory under controlled leaching and non-leaching conditions over a 14-week period. Mixing the two species resulted in complex, non-linear interactions in quality, particularly with regard to the activity of extractable polyphenols. N mineralization rate of the pruning mixtures decreased with increasing proportion of Peltophorum prunings in the mixtures, indicating that N mineralization rate of prunings can be manipulated by mixing different quality materials. The initial lignin-to-N ratio and the (lignin + polyphenol)-to-N ratio were strongly correlated with the N mineralization rate constant of the prunings mixtures under both leaching and non-leaching conditions. Under leaching conditions, however, the N mineralization rate constant was best correlated with the initial protein-binding capacity of the pruning mixtures. In agreement with patterns of N mineralized from the pruning mixtures, the N recovery by maize from the pruning mixtures increased with increasing proportion of Gliricidia prunings in the mixtures. The strong relationship between N recovery and the protein-binding capacity suggested that protein-binding by polyphenols was the cause of the reduced N recovery of individual prunings in the mixtures. 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 reduced, probably because of the high protein-binding capacity of polyphenols in the Peltophorum prunings.