Tree legumes in medium-term fallows: Nz fixation, nitrate recovery and effects on subsequent crops

Abstract

Fallows improved with tree legumes have a potential role in soil fertility management in the sub-humid tropics. In addition to N2 fixation, deep rooting species can be used to redistribute leached nutrients to the topsoil while producing useful woody by-products. Such fallows are not very effective in improving phosphorus availability, however, and supplying P from other sources may be required to take full advantage of the improved fallows. Busumbu rock P is a potential source of P, but of low reactivity. Our objectives were to compare three tree or shrub legumes with Mucuna [Mucuna pruriens (L.) DC. var. utilis], to determine their rotation effects on subsequent crops and on P availability from Busumbu P rock. Legume effects on the fixation of atmospheric N and nitrate recovery were determined in a sub-humid, bi-modal rainfall system. Fallows improved with sesbania (Sesbania sesban) and tephrosia (Tephrosia vogellii) produced more biomass and fixed more N than those fallows improved with pigeonpea (Cajanus cajan) or mucuna. Pigeonpea derived only about 11% of plant N from the atmosphere compared to 49% for sesbania and tephrosia. The maize-bean [Zea mays (L.)]-[Phaseolus vulgaris (L.)] intercrop was most productive following sesbania and tephrosia fallow and least productive after the fallow improved with pigeonpea. All legumes and food crops failed to acquire significant amounts of P from the P rock. Sesbania and tephrosia were most efficient in recovery of nitrates from below 50 cm depth. The carbon:N ratios were low enough for the biomass of all fallows to allow early mineralisation of N, but the C:P ratios were within the critical level range for a net immobilisation of P following application. Generally, recovery by the subsequent maize-bean intercrops of N supplied in the biomass of tephrosia and sesbania was poor and the results indicate that much of the biomass N was lost. Recovery of N may be improved by transfer of a major part of the legume biomass to manure adjacent land which was continuously cropped.



Key Words: Cajanus cajan, cover crops, fallow, green manure, Mucuna pruriens, Sesbania sesban, Tephrosia vogellii, Uganda



RÉSUMÉ


Les jachères améliorées avec des arbustives ont un rôle potentiel dans la gestion de la fertilité du sol dans la région sous humide tropicale. En plus de la fixation de l'azote , des espèces à racines profondes peuvent être utulisées pour recycler des élements lessivés de la partie supérieure du sol en même temps produisant du bois utile. De telles jachères ne sont pas effectives dans l'amélioration de la disponibilité du phosphore, cependant, en fournissant du P des autres sources peuvent être nécessaires pour donner avantages aux jachères améliorées. La roche à P Bususmbu est une source potentielle du P mais à faibe réaction. Nos objectifs étaient de comparer trois arbres ou arbustes avec Mucuna [Mucun a prureins (L)DC. Var. utilis], déterminer leurs effets de rotation sur les cultures subséquentes et sur la disponibilité du P de.


(African Crop Science Journal 8(3) 2000: 263-272)