Effects of chemical composition of plant residues on nitrogen release and crop uptake
To supplement high cost of inorganic fertilizers, smallholder farmers in the tropics are likely to increase the use of plant residues as a suitable source of plant nutrients especially nitrogen (N) and phosphorus (P). Management of these organic N - sources demand that their N - release patterns coupled with synchronization of the released N with crop growth be fully understood. Consequently this study was undertaken to evaluate the effect of chemical composition of various residues on N - release, improve the N release pattern of low quality organic materials by mixing them with those of high quality and synchronize the N released with crop production. These objectives were achieved through a series of controlled experiments. The experiments involved incubation, in the laboratory for 12 weeks, of six selected plant residues which involved leaves of Leucaena leucocephala, Croton macrostachyus, Calliandra calothrysus, Tithonia diversifolia, Sorghum bieolor and husks of Oryza sativa. Parallel to this, mixtures .of C macrostachyus (Cm) and O. sativa (Os)in various ratios were also incubated. Finally maize .wasgrown in the glasshouse in pots whose soil had been amended with C macrostaehyus, O. sativa, T. diversifolia(Ts) and Liz: LI:! mixture of Croton maerostaehyus and Oryza sativa. In the laboratory incubation, soil samples were taken after every 2 weeks for analysis of ammonium nitrogen (NH4"" -N) and nitrate nitrogen (N03·-N), while in the glasshouse experiment, harvesting of maize tops was done at 2 week intervals 8 weeks and the shoots- dry matter as well as N content determined. Two patterns of N (N03--N + NH4+-N) release were observed during the 12 week incubation period: Leaves of C macrostachyus, L. leucocephala, T. diversifolia and C calothrysus had a net release throughout the incubation period while S. bicolor leaves and O. sativa husks showed significant net immobilization. Due to concurrent nitrification over the 12 week incubation period, 65 - 80% of the accumulated mineral N was in nitrate form. The dynamics of N - mineralization of the vanous mixture of Croton macrostachyus (em) and Oryza sativa (as) were in general not significantly (p ~ 0.05) different from those predicted from the isolates of Oryza sativa (Os) and Croton macrostachyus (Cm) alone with the exception of the 3/4 Cm + 1/40s having depressed the incubated N - release in weeks 6 - 8 significantly while the 1I4Cm + 3/40s stimulated the incubated N-release at weeks 2 and 12 weeks of incubation respectively. Addition of plant residues increased maize biomass in the glasshouse significantly (P < 0.05) throughout the .growth period. However, the study showed a sharp contrast of maize response from the results of the incubation. Mixture of plant residues in the pots had the highest contribution to maize dry matter yield and N uptake. It could be speculated that high C:N ratio Oryza sativa material in the mixture stimulated microbial activity in the rhizosphere leading to high organic material decomposition. Of the chemical variables studied, initial contents of N and P as well as C:N and polyphenol:N ratios were significantly correlated with cumulative N mineralized. Nitrogen- release was best correlated with C:N ratio having r = - 0.84 to -0.90 for most of the sampling periods. Polyphenol: N ratio also gave high correlation with cumulative N mineralized with correlation coefficient (r) ranging from -0.65 to -0.95. Initial N and P contents showed positive significant correlation but not as high as those of the two ratios. These results show that the best predictors for N mineralization were residue CN and Polyphenol.N ratios.