Nitrogen mineralization from legume tree prunings of different quality

A study on legume tree prunings iof different quality, in terms of N, lignin and polyphenol contents, was conducted under laboratory, growth room and humid tropical field conditions to investigate the role of quality of legume tree prunings on decomposition, N release, and N uptake by crops. Prunings from four legume tree species; Calliandra calothyrsus, Peltophorum pterocarpa, Gfiricldia sepium and Leucaena leucocephala, were selected for this study. A polyphenol protein-binding capacity assay was introduced as a further new approach to determine the involvement of polyphenols in the decomposition and N mineralization processes. Results of field and laboratory experiments showed that the patterns of decomposition and N release varied between incubation methods and species. The inclusion in experiments of legume tree species with widely varying quality parameters, especially polyphenol content, revealed that the widely used parameter of C:N ratio alone was not adequate to predict N release from plant residues. Instead, the protein-binding capacity of the prunings played an important role in N release from the prunings where there was no leaching. Under leaching conditions, however, this effect was reduced due, presumably, to the removal of soluble active polyphenols. This was confirmed as leaching of pruning materials with water reduced the original polyphenol content which resulted in reduced protein-binding capacity of the prunings. Extraction of prunings with cold water further reduced polyphenol content and thereby, enhanced N mineralization of the prunings, especially during the early stages of decomposition. Thus the (lignin + polyphenol):N ratio was consistently among the best quality descriptions to predict N released from the prunings incubated under leaching conditions. N mineralization of legume tree prunings can be controlled by mixing low and high quality prunings. Results of laboratory experiments, however, suggested that attempts to regulate the N release by mixing together prunlngs~ of different species may result in complex, non-linear interactions between.. prunings of different quality. The quality of prunings can also be altered by growing legume tree species with varying rates of N supply. By increasing the concentrations of the N supply during the growth of legume trees it was not only possible to alter the N content of the legume tree prunings but also to manipulate the polyphenol content and protein-binding capacity. Results of this study also indicated that the release of N from legume prunings containing a relatively high amount of polyphenol, could be enhanced by governing the N availability conditions under which the plant is grown. Growth room experiments were conducted to study the effects of application of legume prunings of different quality on N uptake and growth of maize. Application of high quality prunings, Gliricidia and Leucaena, resulted in a better maize production and N recovery confirming results of laboratory studies. The % N recovery by maize was more strongly correlated with the p . rotein-binding capacity of the prunings than with other quality parameters since no leaching took place. In comparison with N mineralization data of incubation experiments, there was a high utilization of the N that was estimated to have been released from the legume prunings to the maize in the absence of losses. Results of experim ents on the effect of applications of various mixtures of GlIficidla and Peltophorum prunings on N uptake by maize showed that N recovery by maize increased with increasing proportion of Gfific1dia to Peltophorum prunings. It was observed that more than 60% of N recovered in maize was contributed by Gliricidia, especially when proportion of Gliricidia was more than 50% in the mixtures. It can be concluded that quality of the legume tree prunings can impose a significant effect to the rate of N release and N uptake by crops. A proper choice of legume tree prunings in terms of quality is thus a prerequisite towards and adequate N supply to meet crop demands for N. In order to be applicable in recommendation for the management of humid tropical crop production systems, simulation modelling of the action between pruning quality, leaching conditions, decomposition, N release and N uptake by crops would seem to be a useful next step.