Abstract
We investigated Bambara groundnut, groundnut, mung bean, cowpea and black gram for use as biofertilizers in cropping systems. The 15N natural abundance technique was used to measure N2 fixation in this study. The percent N derived from fixation by mung bean (Vigna radiata L. Wilczek), Bambara groundnut (Vigna subterranea L. Verdc.), cowpea (Vigna unguiculata L.Walp.), black gram (Vigna mungo L.) and groundnut (Arachis hypogaea L was 98, 83, 79, 66 and 45% respectively. Nitrogen contribution by these legumes was 83, 67, 39, 36 and 32 kg.ha-1 respectively for Bambara groundnut, groundnut, mung bean, black gram and cowpea. Maize grain yield without N fertilizer was 2449, 2291, 2204, 2046 and 1671 kg.ha-1 respectively for maize following groundnut, Bambara groundnut, cowpea, mung bean, black gram and maize. Grain yield increase of maize after legumes without N fertilizer was 47, 46, 37, 32 and 22% respectively for groundnut, Bambara groundnut, cowpea, mung bean, and black gram. Supplying N0 up to N60 to maize plants increased shoot DM from 3264 to 4279 kg.ha-1, yield grain from 2184 to 3586 kg.ha-1, and whole-plant DM from 5448 to 7865 kg.ha-1, which represented 31, 64 and 44% increase with N fertilizer supply from 0 to 60 kg N ha-1. Symbiotic N benefit of preceding legumes to maize without N fertilizer was 20 to 40 kg N. ha-1 in fertilizer equivalents. The preceding legumes increased maize grain concentrations of P, Ca, S, Fe, Mn and Zn in zero-N plots relative to maize after maize. There was 225, 222, 180, 154 and 108% increase in marginal returns of maize after groundnut, Bambara groundnut, cowpea, mung bean and black gram respectively without N fertilizer.
Highlights
Maize is a major staple food crop in Africa, especially in Southern Africa, and is the most important component of smallholder cropping systems in the continent
The results showed that shoot δ15N was significantly greater (p ≤ 0.05) in groundnut, followed by Bambara groundnut, black gram, cowpea, and mung bean (Table 3)
A comparison of the effect of symbiotic N contribution by the five food legumes to the following maize crop in this study revealed a significant increase in plant growth, shoot biomass, and grain yield of maize planted after legumes relative to maize after maize
Summary
Maize is a major staple food crop in Africa, especially in Southern Africa, and is the most important component of smallholder cropping systems in the continent. Maize production in Africa is often constrained by drought and low rainfall, as observed during the 2018 drought in Southern Africa. While the use of mineral fertilizers could improve maize yields in Africa, they are inaccessible to resource-poor farmers due to their high cost. The low or lack of fertilizer use is currently the major factor limiting increased crop yields (Sinclair and Vadez, 2012) as, on average, only about 8.8 kg NPK fertilizer is applied per hectare by smallholder farmers in Africa (Henao and Baanante, 2006). The inclusion of N2-fixing legumes in traditional cropping systems can improve soil N fertility and increase crop yields for enhanced food/nutritional security (Walley et al, 2007; Lithourgidis et al, 2011; Ngwira et al, 2012)
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