Assessing the symbiotic dependency of grain and tree legumes on N2 fixation for their N nutrition in five agro-ecological zones of Botswana

Abstract

To assess the symbiotic dependency of grain and shrub/tree legumes within five agro-ecological zones of Botswana, fully expanded leaves of the test species were sampled from about 26 study sites within Ngwaketse, Gaborone, Central, Ghanzi and Kalahari agro-ecological zones. Isotopic analysis revealed significant differences in 1)15N values of the grain legumes [cowpea (Vigna unguiculata L. Walp), Bambara groundnut (Vigna subterranea L. Verde.), and groundnut (Arachis hypogaea L.)] from the 26 farming areas in both 2005 and 2006. Estimates of %Ndfa of leaves also showed significant differences between farming areas, with cowpea deriving more than 50% of its N nutrition from symbiotic fixation. In terms of distribution, many more symbiotic shrub/tree species were found in the wetter Ngwaketse agro-zone compared to the fewer numbers in the drier Kalahari region. Acacias were the more dominant species at all sites. Leaf 1)15N values of shrub/tree species also varied strongly across Botswana, with 11 out of 18 of these legumes deriving about 50%, or more, of their N from symbiotic N z fixation. Acacia caffra, in particular, obtained as much as 93.6% of its N nutrition from symbiotic fixation in the wetter Ngwaketse agro-zone. This study has shown that grain legumes sampled from farmer's fields in Botswana obtained considerable amounts of their N from symbiotic fixation. We have also shown that shrub and tree legumes probably play an important role in the N economy of the savanna ecosystems in Botswana. However, the decline in the number of functional Ny-fixing shrub/tree legumes along an aridity gradient suggests that soil moisture is a major constraint to N z fixation in the tree legumes of Botswana.

Symbiotic and Phenotypic Characteristics of Rhizobia Nodulaing Cowpea (Vigna Unguiculata L. Walp) Grown in Arid Region of Libya (Fezzan)

“…However, rare reports indicated that legumes can nodulate by fast-growing rhizobia [33,37]. Based on cultural and symbiotic characteristics, the test isolates seemed to be similar to slow-growing rhizobia and therefore, this result support the findings of other researchers in West Africa [25], South Africa, Ghana and Botswana [26]. Some reporters claimed occurrences of both slow and fast-growing rhizobia inside nodules of cowpea [20][21][22].…”

“…However, it is currently classified into genus Bradyrhizobium [8,19]. Phenotypic and genetic studies of cowpeas rhizobia were aim of researchers in some parts of Africa [20][21][22][23][24][25][26]. Contradictory reports about the microsymbionts of these plants were reported.…”

“…Analysis of 16S rRNA gene sequences of fast-growing nodule occupancy showed that they are similar to coliform bacteria (Enterobacter, Klebsiella) and Rhizobium, while, the slow-growing one is close to Bradyrhizobium genus [24]. Others [25,26] only revealed the presence of slow-growing rhizobia related to B. Alkanii and B. japonicum which had cross nodulation with Faidherbia albida Nevertheless, the diversity of cowpea rhizobia is more diverse in arid regions [25,27]. Some researchers related this diversity to geographical origin [23], which differs in the environmental characteristics like soil type and pH, temperature, climate and moisture.…”

An exotic Australian Acacia fixes more N than a coexisting indigenous Acacia in a South African riparian zone

“…Schulze et al 1991;Pule-Meulenberg and Dakora 2009). Our estimates are within the range for African and Australian acacias reported in previous surveys (Schulze et al 1991;Forrester et al 2007;Pule-Meulenberg and Dakora 2009;Drake 2011). Although alterations to nutrient cycling by Australian acacias have been demonstrated previously (Witkowski 1991; Yelenik et al 2004), relative contributions of N 2 -fixation to plant N stocks have, until now, not been compared with a co-occurring native N 2 -fixing acacia.…”

“…Many published estimates include a similarly high degree of variation in the proportion of N 2 -fixed when using the natural abundance method (e.g. Schulze et al 1991;Pule-Meulenberg and Dakora 2009). Our estimates are within the range for African and Australian acacias reported in previous surveys (Schulze et al 1991;Forrester et al 2007;Pule-Meulenberg and Dakora 2009;Drake 2011).…”