ISSN:
1573-5036
Keywords:
decomposition
;
isotope dilution
;
15N
;
mineralization
;
nodulation
;
N use efficiency
;
residue management
Source:
Springer Online Journal Archives 1860-2000
Topics:
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
Notes:
Abstract 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 N2 symbiotically with Rhizobium. Measurements of biological N2 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 N2, 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 N2. The long-term evaluation of the potential or actual amounts of N2 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 N2 fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of 15N procedures and the selection criteria for appropriate reference plants have been discussed. Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the 15N 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 N2 fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N2 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 N2 to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on 15N labelling techniques or 15N natural abundance measurements are outlined. These would lead to estimates of N2 fixation and N saving resulting from the management of N2 fixation in alley cropping systems.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00032244
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