MANAGEMENT OF BIOLOGICAL N-2 FIXATION IN ALLEY CROPPING SYSTEMS - ESTIMATION AND CONTRIBUTION TO N BALANCE

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N-2 symbiotically with Rhizobium. Measurements of biological N-2 fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha(-1) yr(-1) from atmospheric N-2, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha(-1) yr(-1). Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N-2 The long-term evaluation of the potential or actual amounts of N-2 fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N-2 fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of N-15 procedures and the selection criteria for appropriate reference plants have been discussed. Little is known about the effect of environmental factors acid management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the N-15 Labelling techniques have indicated that up to 50% or more of the tree's N may be below ground after pruning. In this case, quantification of N-2 fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N-2 fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (> 300 kg N ha(-1) yr(-1)) but N contribution to crops is commonly in the range of 40-70 kg N ha(-1) season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5-10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25-102 kg N ha(-1) season(-1). Timing and severity of pruning may allow for some management of underground transfer of fixed Na to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on N-15 labelling techniques or N-15 natural abundance measurements are outlined. These would lead to estimates of N-2 fixation and N saving resulting from the management of N-2 fixation in alley cropping systems.