Evaluating N2 fixation by food grain legumes in farmers’ fields in three agro-ecological zones of Zambia, using 15N natural abundance

A survey of N₂ fixation in farmers’ fields of Northern (>1,000 mm rainfall), Central (800–1,000 mm rainfall), and Southern (<800 mm rainfall) Zambia revealed some significant differences in plant growth and symbiotic performance of different food grain legumes. Of the three grain legumes (i.e., Bambara groundnut (Vigna subterranea L. Verdc.), cowpea (Vigna unguiculata L. Walp.), and groundnut (Arachis hypogaea L.)) grown in Southern Zambia, cowpea showed greater shoot biomass and significantly lower shoot δ¹⁵N values than groundnut and Bambara groundnut. The lower shoot δ¹⁵N resulted in greater %Ndfa (59%) in shoots and higher amounts of N-fixed, whether per square meters (6,394.0 mg N), per plant (650.8 mg N), or per hectare (63.9 kg N) relative to groundnut and Bambara groundnut, even though the number of cowpea plants per square meter was significantly lower than that of groundnut or Bambara groundnut. Although the shoot δ¹⁵N values of cowpea, Bambara groundnut and common bean (Phaseolus vulgaris L.) were significantly lower than those of groundnut in Central Zambia and their %Ndfa values, therefore, greater, the higher number of groundnut plants per square meter resulted in significantly greater shoot N content, as well as N-fixed per square meter and per hectare relative to the other species. Despite having similar plant density as cowpea in Central Zambia, common bean could fix only 6.0 kg N ha⁻¹ compared with 35.4 kg N ha⁻¹ by cowpea. In Northern Zambia, Bambara groundnut showed the lowest mean shoot δ¹⁵N value (0.54 ± 0.3‰), followed by groundnut (1.59 ± 1.0‰), and then common bean (the three grain legumes grown in that region). As a result, %Ndfa and N-fixed were significantly greater in groundnut (69.7% and 566.0 mg N per plant) and Bambara groundnut (62.9% and 440.1 mg N per plant) than in common bean (2.6% and 2.4 mg N per plant). In Northern Zambia, groundnut, Bambara groundnut and common bean fixed 78.7, 67.6, and 0.9 kg N ha⁻¹, respectively, even though the plant density per square meter of common bean (which fixed the lowest amount of N per hectare) was twice that of groundnut and Bambara groundnut. A species × site analysis showed that cowpea fixed relatively greater amounts of N per plant, per square meter, and per hectare in Southern than Central Zambia. Bambara groundnut and common bean also had significantly lower δ¹⁵N values and higher %Ndfa in Central than Northern Zambia.     

Farmers’ practices,utilization, conservation and marketing of Bambara groundnut (<Emphasis Type="Italic">Vigna subterranea</Emphasis> (L.) Verdc.) in Dosso Region,Western Niger

Bambara groundnut (Vigna subterranea (L.) Verdc.) is one of the important legume crop grown in marginal soils of sub-Saharan Africa. Despite its importance in food security and income generation for small scale farmers, it remains as a neglected and underutilized crop and the productivity is very low in the field due to the lack of improved varieties and lack adequate farming practices. Thus, the aim of this study is to investigate all aspects related to its production including source of seeds supply and farmers management practices, utilization, conservation and marketing. The results revealed that in this Region of Western Niger Bambara groundnut was mainly produced by female (95%) compared to male (5%). Farmers use their own recycled seed (80%) which is the main source followed by provision of recycled seeds by relatives (5%) and purchasing from the local market (15%). Also, this finding shows that there is a lack of adequate farming practices and the crop is mostly produced on inherited land without any inputs as reported by 80% of the respondents. Mono-cropping is mainly practiced by 97% of farmers while a few of them do rotation (2%) and most of pre and postharvest handling technologies are traditional. The most important traits preferred by Bambara farmers is seed colour (cream color), followed by high grain yield, early maturity and cooking ability according to the respondents with 98, 92, 88 and 72% respectively. Several features have to be taking into account in order to promote Bambara nut including its ability to do well in harsh conditions, its nutritional benefits, and its ability to fix nitrogen, thereby increasing soil fertility in mixed cropping systems.