Decomposition and Nitrogen Release From Leucaena leucocephala,senna siamea, and Flemingia macrophylla Litter in managed alleys for maize in Zambia:Infuence of inorganic-N and litter Quality

The effects of litter quality and inorganic nitrogen (N) on rate of decomposition and N release pattern by three leguminous woody species were investigated under field conditions by the litterbag technique. The species studied in 1994/1995 were Leucaena leucocephala, Senna siamea and Flemingia macrophylla. This study was superimposed on an existing alley cropping trial. In the on-going experiment , maize (variety MM 603,3-way cross), was the companion crop and the leguminous species comprised the alley hedgerows. Nylon litterbags with 4 mm mesh size were each filled with 20 g of dry leaf prunings of each species. Five litterbags of each pruning type were randomly buried in a subplot of each treatment. At each sampling time, one bag representing each pruning type was removed from each subplot. The samples were cleaned , dried , weighed and analysed for N, polyphenol and lignin contents. Decomposition rate constants (kD) and N release rate constants (kN) were calculated from exponential decomposition equations. The mean(kD) of the plant litter ranged from 0.30 to 0.12wk-1, decreasing in the order , L. leucocephala,>S. siamea> F. macrophylla. Analysis of variance of the mean kD values showed that L. leucocephala, and S. siamea decomposed significantly (P<_0.01) faster than F. macrophylla. It was also observed that the level of inorganic N had a significant effect on decomposition rate. Residues in plots which received 68 and 112 kg N ha-1 had similar decomposition rate which were faster((P<-0.05) than those for 0 and 34 kg N ha-1. Negative partial correlations were recorded between kDs, on one hand and lignin and polyphenol contents(P<-0.01), C:N ratio (P<-0.01) and lignin + polyphenol:N ratio (P<-0.01) and inorganic N level (P<-0.01). These results show that both the chemical composition of plant residues and level of inorganic fertilizer N applied increased the rate of plant residue decomposition. The mean (kN) ranged from 0.151 to 0.114wk-1, decreasing in the order L. leucocephala,>S. siamea> F.macrophylla. There were however , no significant species on N level effects on kN. Generally, net N release was obtained in the first 2-3 weeks followed by immobilization thereafter in all species. Positive partial correlations(P<-0.05)were recorded with initial N- content ( P<-0.05) suggested that it affects decomposition rates. Negative partial correlations were recorded between initial lignin and polyphenol contents and kN. Lignin and polyphenols are thought to have caused N immobilization through the formation of resistant complexes. There was no significant species effect on maize yields . A significant N fertilizer effect on yield was obtained at 68 kg N ha-1. This result probably indicates the need for moderate fertilizer N addition to incorporated residues in alley cropping.