Abstract
This study was aimed at quantifying soybean (Glycine max) nitrogen fixation under reduced tillage (RT) and conventional tillage (CT) in a tropical Ferralsol of the sub-humid zone of western Kenya, using the isotope 15N dilution method. Crop residue (CR) management was a superimposed treatment in soybean-maize rotation and intercropping systems. This study quantified N in abscised soybean leaves. Soybean-N derived from the atmosphere (%NDfA) ranged between 41–65%; it was higher (P < 0.05) in RT (55.6%) than in CT (46.6%). Total fixed-N under ‘RT + CR’ was more than in the other treatments by at least 55% in intercropping and 34% in rotation system. Nitrogen fixed in soybean aboveground parts was 26–48 kg N ha−1 with intercropping and 53–82 kg N ha−1 with rotation. Seasonal litter fall contained about 15 kg N ha−1, with 54% NDfA. Annual nitrogen balances with soybean and maize grain removed were better in RT (−9 to −32 kg N ha−1) than in CT (−40 to −60 kg N ha−1). Application of P increased nodule weight (P < 0.05) by 3 to 16 times over the control. Soybean residues should be returned to the field after harvest to reduce soil N mining. We conclude that ‘RT + CR’ increases biological nitrogen fixation in soybean, over CT, and that phosphorus application is needed for better soybean nodulation in western Kenya.
Highlights
Relying on the biological nitrogen fixation (BNF) of grain legumes is a strategy to ease the burden that commercial fertilizers exert on poor farmers
Nodulation was always better in conventional tillage (CT) than in reduced tillage (RT)
The greater %Nitrogen Derived from the Atmosphere (NDfA) observed in RT when Crop residue (CR) was applied, and the resultant higher amount of fixed N compared to disturbed soils has been attributed to the greater colonization by arbuscular mycorrhizae that increases P supply to the nodules [4]
Summary
Relying on the biological nitrogen fixation (BNF) of grain legumes is a strategy to ease the burden that commercial fertilizers exert on poor farmers. In sub-Saharan Africa (SSA), fertilizer use is estimated at 8 kg ha−1, which is only 10% of the worlds’ average [1]. The unique socio-economic conditions and wide-spread rural poverty in sub-Saharan Africa limit access to mineral fertilizers, inducing the search for cheap sources of fertilization, such as nitrogen fixation by grain legumes. Promiscuous soybean varieties (varieties that do not require inoculation with specific Rhizobium) have recently been introduced to western Kenya for soil fertility improvement, through the Tropical Soil Biology and Fertility institute of CIAT (TSBF-CIAT), and their performance is being tested in various agro-ecological zones
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