Symbiotic performance of grain and wild herbaceous legumes in the Okavango Delta and Tswapong region of Botswana

Abstract

The low inherent soil fertility, especially nitrogen (N) constrains arable agriculture in Botswana. Nitrogen is usually added to soil through inorganic fertilizer application. In this study, biological nitrogen fixation by legumes is explored as an alternative source of N. The objectives of this study were to measure levels of N2 fixation by grain legumes such as cowpea, Bambara groundnut and groundnut in farmers’ fields as well as to estimated N2 fixation by indigenous herbaceous legumes growing in the Okavango Delta. Four flowering plants per species were sampled from the panhandle part of the Okavango Delta and Tswapong area. Nitrogen fixation was measured using the 15N stable isotope natural abundance technique. The δ15N values of indigenous herbaceous legumes indicated that they fixed N2 (−1.88 to +1.35 ‰) with the lowest value measured in Chamaecrista absus growing in Ngarange (Okavango Delta). The δ15N values of grain legumes growing on farmers’ fields ranging from −1.2 ‰ to +3.3 ‰ indicated that they were fixing N2. For grain legumes growing at most farms, %Ndfa were above 50% indicating that they largely depended on symbiotic fixation for their N nutrition. With optimal planting density, Bambara groundnuts on farmers’ fields could potentially fix over 90 kg N/ha in some parts of Tswapong area and about 60 kg N/ha in areas around the Okavango Delta. Results from this study have shown that herbaceous indigenous legumes and cultivated legumes play an important role in the cycling of N in the soil. It has also been shown that biological N2 on farmer’s field could potentially supply the much needed N for the legumes and the subsequent cereal crops if plant densities are optimized with the potential to increase food security and mitigate climate change.

Diversity of wild herbaceous legumes in Southern Africa, their associated root nodule bacteria, and insect pests

“…1 and 2). Bernard et al (2017) have shown that in Botswana, the legumes in Table 1 fi xed N 2 in their locales. Furthermore, results from the current study indicated that the diversity of legumes decreased with increasing aridity in Botswana.…”

“…In Namibia, plant surveys were carried out in Kavango Province near Rundu and Mashare, in the Kunene region near the Grootberg Pass, and in the Omaheke region near Gobabis. All of the sites in the three countries where legume surveys were undertaken are covered by Kalahari aeolian sand deposits, and hence the soils are very sandy with very low fertility, low soil moisture, and high soil temperatures (Schulze & Kruger, 2007;Bernard et al, 2017).…”

“…In Botswana, studies on the amount of N 2 fi xed per hectare of herbaceous legumes are scarce. In the study by Bernard et al (2017), an estimation of N-fi xed per plant was made using the natural abundance technique. For example, Chamaecrista absus fi xed 134.8 mg N/plant while Tephrosia sp could fi x 143.4 mg N/plant.…”

Potential of underutilized crops to introduce the nutritional diversity and achieve zero hunger

“…An indigenous African legume bambara groundnut (Vigna subterranea) is known for its adaptability in poor soils of hot and arid environments where other crops fail to grow. It also fixes an enormous amount of nitrogen to the soil (~ 90 kg/h) which boosts production of intersessional crops (Bernard et al 2018). Excessive diversity of the traditional farming system reflects human innovation, multifarious food preferences, and cultural diversity across the regions.…”