1.1 MAIZE

1.1.1 ••wwKasirayi, Makaza (1996). Decomposition and nitrogen release from Leucaena leucocephala, Senna siamea and Flemingia macrophylla litter in managed alleys for maize (Zea mays L.) in Zambia; Influence of inorganic -N and litter quality. (Supervisors: Dr. D. M. N. Mbewe and Mr. R. C. Nyemba).

Leucaena leucocephala

Flemingia macrophylla
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 ongoing experiment, maize (variety MM 603, 3-way cross), was the companion crop and the leguminous species comprised the alley hedgerows. Nylon li'tterbags 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 0.12/wk, 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 had similar decomposition rates which were faster (P< 0.05) than those for 0 and 34 kg N/ha. Negative partial correlations were recorded between kDs, on one hand and lignin and polyphenol contents (P<0.01), CN ratio (P<0.01), polyphenohN ratio (P< 0.01) and lignin + polyphenol:N ratio (P<0.01). Positive partial correlations (P<0.01) were recorded with initial residue N content (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 fertiliser N applied increased the rate of plant residue decomposition. The mean (kN) ranged from 0.151 to 0.114/wk, decreasing in the order L leucocephala > S. siamea > F macrophylla. There were however, no significant species or N level effects on kN. Generally, net N release was obtained in the first 2-3 weeks followed by immobilisation thereafter in all species. Positive partial correlations (P<0.05) were recorded with initial N-content (P<0.05) suggesting 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 immobilisation through the formation of resistant complexes. There was no significant species effect on maize yields. A significant N fertiliser effect on yield was obtained at 68 kg N/ha. This result probably indicates the need for moderate fertiliser N addition to incorporated residues in alley cropping.