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, whi le conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability t o 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 ca n 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 l ess than 20 kg N ha(-1) yr(-1). Other tree species such as Senna siame a 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 s ize, the great difficulty in obtaining representative samples and appl ying reliable methodologies for measuring N-2 fixed. Strategies for ob taining representative samples (as against the whole tree or destructi ve plant sampling), the application of N-15 procedures and the selecti on criteria for appropriate reference plants have been discussed. Litt le is known about the effect of environmental factors acid management practices such as tree cutting or pruning and residue management on BN F and eventually their N contribution in alley cropping. Data using th e N-15 Labelling techniques have indicated that up to 50% or more of t he tree's N may be below ground after pruning. In this case, quantific ation of N-2 fixed that disregards roots, nodules and crowns would res ult in serious errors and the amount of N-2 fixed may be largely under estimated. 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 r eported. The low N recovery in maize (Zea mays) is partly caused by la ck of synchronization between the hedgerow trees N release and the ass ociated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedger ow trees and thus remain in the system. This N can also be lost from t he system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and n odule 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 prunin g may allow for some management of underground transfer of fixed Na to associated crops. However many aspects of root dynamics in alley crop ping 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 r esulting from the management of N-2 fixation in alley cropping systems .