Authors: John O Ojiem Bernard Vanlauwe Nico de Ridder Ken E Giller
Publish Date: 2007/02/16
Volume: 292, Issue: 1-2, Pages: 119-135
Abstract
Nitrogen N deficiency is a major constraint to the productivity of the African smallholder farming systems Grain green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N2fixation The N2fixation of bean Phaseolus vulgaris soyabean Glycine max groundnut Arachis hypogaea Lima bean Phaseolus lunatus lablab Lablab purpureus velvet bean Mucuna pruriens crotalaria Crotalaria ochroleuca jackbean Canavalia ensiformis desmodium Desmodium uncinatum stylo Stylosanthes guianensis and siratro Macroptilium atropurpureum was assessed using the 15N natural abundance method The experiments were conducted at three sites in western Kenya selected on an agroecological zone AEZ gradient defined by rainfall On a relative scale Museno represents high potential AEZ 1 Majengo medium potential AEZ 2 and Ndori low potential AEZ 3 Rainfall in the year of experimentation was highest in AEZ 2 followed by AEZ 1 and AEZ 3 Experimental fields were classified into high medium and low fertility classes to assess the influence of soil fertility on N2fixation performance The legumes were planted with triple super phosphate TSP at 30 kg P ha−1 with an extra soyabean plot planted without TSP soyabeanP to assess response to P and no artificial inoculation was done Legume grain yield shoot N accumulation N derived from N2fixation N2fixation and net N inputs differed significantly P001 with rainfall and soil fertility Mean grain yield ranged from 086 Mg ha−1 in AEZ 2 to 030 Mg ha−1 in AEZ 3 and from 078 Mg ha−1 in the high fertility field to 048 Mg ha−1 in the low fertility field Shoot N accumulation ranged from a maximum of 486 kg N ha−1 in AEZ 2 to a minimum of 10 kg N ha−1 in AEZ 3 Based on shoot biomass estimates the species fixed 25–90 of their N requirements in AEZ 2 23–90 in AEZ 1 and 7–77 in AEZ 3 Mean N2fixation by green manure legumes ranged from 319 kg ha−1 velvet bean in AEZ 2 to 29 kg ha−1 jackbean in AEZ 3 For the forage legumes mean N2fixation ranged from 97 kg N ha−1 for desmodium in AEZ 2 to 39 kg N ha−1 for siratro in AEZ 3 while for the grain legumes the range was from 172 kg N ha−1 for lablab in AEZ 1 to 3 kg N ha−1 for soyabeanP in AEZ 3 Lablab and groundnut showed consistently greater N2fixation and net N inputs across agroecological and soil fertility gradients The use of maize as reference crop resulted in lower N2fixation values than when broadleaved weed plants were used The results demonstrate differential contributions of the green manure forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers’ needsThe authors thank the Rockefeller Foundation for providing the grant that enabled the implementation of this study and the Kenya Agricultural Research Institute for availing the facilities for the study including some of the legume seeds which were provided by the Legume Research Network Project LRNP The support of all the field assistants in the three trial sites is greatly appreciated We sincerely thank the farmers in Kakamega Vihiga and Bondo districts in Western Kenya for their willingness to participate in the onfarm trials Ken Giller and Nico de Ridder thank the European Union for funding through AfricaNUANCES
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