Consistency, variability, and predictability of on-farm nutrient responses in four grain legumes across East and West Africa

Grain legumes are key components of sustainable production systems in sub-Saharan Africa, but wide-spread nutrient deficiencies severely restrict yields. Whereas legumes can meet a large part of their nitrogen (N) requirement through symbiosis with N₂-fixing bacteria, elements such as phosphorus (P), potassium (K) and secondary and micronutrients may still be limiting and require supplementation. Responses to P are generally strong but variable, while evidence for other nutrients tends to show weak or highly localised effects. Here we present the results of a joint statistical analysis of a series of on-farm nutrient addition trials, implemented across four legumes in four countries over two years. Linear mixed models were used to quantify both mean nutrient responses and their variability, followed by a random forest analysis to determine the extent to which such variability can be explained or predicted by geographic, environmental or farm survey data. Legume response to P was indeed variable, but consistently positive and we predicted application to be profitable for 67% of farms in any given year, based on prevailing input costs and grain prices. Other nutrients did not show significant mean effects, but considerable response variation was found. This response heterogeneity was mostly associated with local or temporary factors and could not be explained or predicted by spatial, biophysical or management factors. An exception was K response, which displayed appreciable spatial variation that could be partly accounted for by spatial and environmental covariables. While of apparent relevance for targeted recommendations, the minor amplitude of expected response, the large proportion of unexplained variation and the unreliability of the predicted spatial patterns suggests that such data-driven targeting is unlikely to be effective with current data.

Phylogeographic distribution of rhizobia nodulating common bean (Phaseolus vulgaris L.) in Ethiopia

Rhizobia are soilborne bacteria that form symbiotic relations with legumes and fix atmospheric nitrogen. The nitrogen fixation potential depends on several factors such as the type of host and symbionts and on environmental factors that affect the distribution of rhizobia. We isolated bacteria nodulating common bean in Southern Ethiopia to evaluate their genetic diversity and phylogeography at nucleotide, locus (gene/haplotype) and species levels of genetic hierarchy. Phylogenetically, eight rhizobial genospecies (including previous collections) were determined that had less genetic diversity than found among reference strains. The limited genetic diversity of the Ethiopian collections was due to absence of many of the Rhizobium lineages known to nodulate beans. Rhizobium etli and Rhizobiumphaseoli were predominant strains of bean-nodulating rhizobia in Ethiopia. We found no evidence for a phylogeographic pattern in strain distribution. However, joint analysis of the current and previous collections revealed differences between the two collections at nucleotide level of genetic hierarchy. The differences were due to genospecies Rhizobium aethiopicum that was only isolated in the earlier collection.

Inoculation and phosphorus fertilizer improve food-feed traits of grain legumes in mixed crop-livestock systems of Ethiopia

Grain legumes play an important role as source of food and feed in smallholder mixed systems. They also contribute to soil fertility improvement through biological nitrogen fixation. Although rhizobium inoculation and phosphorus fertilizer are known to improve grain yield of legumes, information is limited on the effect of this practice on the yield and fodder quality of the haulm. This study was conducted to evaluate the effects of rhizobium inoculation (I) and phosphorus fertilizer (P) on yield and nutritional quality of grains and haulms of grain legumes (faba bean, chickpea, common bean and soybean) on farm across diverse agroecological locations in the Ethiopian highlands. The crops were subjected to four treatments [+I, +P, -I + P and a negative control (-P-I)] at multiple locations on farm during the main cropping season in 2016. Yield data was recorded during grain harvesting, and subsequently representative samples of grains and haulms were collected and analyzed for quality variables. Effects of the treatments were significant (P < 0.05) with 30% increase on grain yield for all studied crops and 28% increase on haulm dry matter yield for faba bean, common bean and soybean. Crude protein (CP) and in vitro organic matter digestibility (IVOMD) values of faba bean, common bean and soybean haulms were higher (P < 0.05); and neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents were lower (P < 0.05) for the treatments than the control. The haulm CP content and IVOMD of chickpea also responded positively (P < 0.05) to the treatments. The current results demonstrated the possibility of improving both yield and quality of grains and haulms of grain legumes with the application of efficient rhizobium inocula and P fertilization. This practice offers an opportunity for smallholders in the crop-livestock system to improve the food-feed traits of grain legumes with minimal input and environmental footprint.

Additive yield response of chickpea (Cicer arietinum L.) to rhizobium inoculation and phosphorus fertilizer across smallholder farms in Ethiopia

The impacts of rhizobium inoculation on growth and yield of chickpea have mainly been tested in experiments conducted in greenhouses or on research stations. We report the response of the crop to inoculation (I) and phosphorus fertilizer (P) application across a large number of smallholder's farms over four regions of Ethiopia, covering diverse soil fertility and agro-ecological conditions. Increased grain yields due to the soil fertility treatments was evident for 99% target farmers. On average, I and P increased grain yield by 21% and 25% respectively, while the combined application of I and P resulted in a 38% increase. However, observed grain yields on control plots and responses to the treatments on individual farms varied greatly, and relative yield responses (%; yield of P and/I minus control yield, divided by control yield) ranged from 3% to 138%. With the exception of a few extremely poorly yielding locations, average responses to P and I were high across a wide range of control yields, indicating the possibility of boosting chickpea productivity for smallholders with P fertilizer and inoculant technology. Variation in response to rhizobium inoculation was mostly independent of agro-ecology and soil type although it was found to be low on a number of farms with extremely high N contents (%). Assuming that a relative yield increase of 10% due to treatment effects is required to be visible, 71%, 73% and 92% of the farmers observed a yield benefit by applying P, I, and P + I, respectively. The results are discussed with respect to the additive benefits of P fertilizers and rhizobial inoculation and their implications for wide scale promotion of inoculant technology to smallholders.

Soyabean response to rhizobium inoculation across sub-Saharan Africa: Patterns of variation and the role of promiscuity

Improving bacterial nitrogen fixation in grain legumes is central to sustainable intensification of agriculture in sub-Saharan Africa. In the case of soyabean, two main approaches have been pursued: first, promiscuous varieties were developed to form effective symbiosis with locally abundant nitrogen fixing bacteria. Second, inoculation with elite bacterial strains is being promoted. Analyses of the success of these approaches in tropical smallholder systems are scarce. It is unclear how current promiscuous and non-promiscuous soyabean varieties perform in inoculated and uninoculated fields, and the extent of variation in inoculation response across regions and environmental conditions remains to be determined. We present an analysis of on-farm yields and inoculation responses across ten countries in Sub Saharan Africa, including both promiscuous and non-promiscuous varieties. By combining data from a core set of replicated on-farm trials with that from a large number of farmer-managed try-outs, we study the potential for inoculation to increase yields in both variety types and evaluate the magnitude and variability of response. Average yields were estimated to be 1343 and 1227 kg/ha with and without inoculation respectively. Inoculation response varied widely between trials and locations, with no clear spatial patterns at larger scales and without evidence that this variation could be explained by yield constraints or environmental conditions. On average, specific varieties had similar uninoculated yields, while responding more strongly to inoculation. Side-by side comparisons revealed that stronger responses were observed at sites where promiscuous varieties had superior uninoculated yields, suggesting the availability of compatible, effective bacteria as a yield limiting factor and as a determinant of the magnitude of inoculation response.

An assessment of plant growth and N2 fixation in soybean genotypes grown in uninoculated soils collected from different locations in Ethiopia

Achieving food and nutritional security is a major challenge in Ethiopia, especially with increasing human population and low crop productivity. Legumes offer an alternative choice to chemical fertilizers for increasing crop yields. The aim of this study was to assess, under glasshouse conditions, plant growth and symbiotic performance of uninoculated soybean genotypes planted in soils collected from different locations in Ethiopia. The results showed significant differences in plant growth and symbiotic performance among the soybean genotypes planted in different soils. There was a location-specific effect of soil on plant growth and symbiotic N nutrition of soybean. Whole-plant biomass was highest in soil from Amaro, followed by Boricha, Dorebafano, Pawe, and Mambuk. The δ15N values ranged from +0.82‰ for Pawe to +5.11‰ at Dorebafano. However, %Ndfa of soybean was greater in plants grown in Mambuk soil, followed by Pawe with the lowest %Ndfa being in Amaro soil. The amount of N-fixed followed similar pattern as %Ndfa. The significant interaction found between soil type and soybean genotype for plant DM, shoot N concentration, δ15N, %Ndfa, N-fixed and soil N-uptake clearly indicated the effect of soil factors. This study revealed the presence of native rhizobia in Ethiopian soils that are compatible with soybean. The N contribution of the soybean genotypes was variable, and strongly influenced by the soil factors.

Phylogenetic diversity of Rhizobium strains nodulating diverse legume species growing in Ethiopia

The taxonomic diversity of thirty-seven Rhizobium strains, isolated from nodules of leguminous trees and herbs growing in Ethiopia, was studied using multilocus sequence analyses (MLSA) of six core and two symbiosis-related genes. Phylogenetic analysis based on the 16S rRNA gene grouped them into five clusters related to nine Rhizobium reference species (99-100% sequence similarity). In addition, two test strains occupied their own independent branches on the phylogenetic tree (AC86a2 along with R. tibeticum; 99.1% similarity and AC100b along with R. multihospitium; 99.5% similarity). One strain from Milletia ferruginea was closely related (>99%) to the genus Shinella, further corroborating earlier findings that nitrogen-fixing bacteria are distributed among phylogenetically unrelated taxa. Sequence analyses of five housekeeping genes also separated the strains into five well-supported clusters, three of which grouped with previously studied Ethiopian common bean rhizobia. Three of the five clusters could potentially be described into new species. Based on the nifH genes, most of the test strains from crop legumes were closely related to several strains of Ethiopian common bean rhizobia and other symbionts of bean plants (R. etli and R. gallicum sv. phaseoli). The grouping of the test strains based on the symbiosis-related genes was not in agreement with the housekeeping genes, signifying differences in their evolutionary history. Our earlier studies revealing a large diversity of Mesorhizobium and Ensifer microsymbionts isolated from Ethiopian legumes, together with the results from the present analysis of Rhizobium strains, suggest that this region might be a potential hotspot for rhizobial biodiversity.

Metabolic and genomic diversity of rhizobia isolated from field standing native and exotic woody legumes in southern Ethiopia

Eighty-seven rhizobial strains isolated from root nodules of field standing native and exotic woody legumes in southern Ethiopia were characterized using the Biolog method and AFLP fingerprinting technique. Cluster analysis of the metabolic and genomic fingerprints revealed 18 and 25 groups, respectively, demonstrating considerable diversity in rhizobial population indigenous to Ethiopian soils. While 25 strains (29%) were linked to members of Agrobacterium, Bradyrhizobium, Mesorhizobium, Rhizobium or Sinorhizobium, the bulk of the strains formed several distinct groups in both methods and did not relate to reference species included in the study. In contrast to exotic species which formed symbiosis with strains of only one specific genomic group, indigenous host species nodulated by metabolically and genomically diverse groups. The results in this study support the view, that long-term association between the symbionts allows gradual differentiation and diversity in compatible rhizobial population resident in native soils. Lack of significant metabolic and genomic relatedness to the reference strains in our results suggested that test strains in our collection probably included 'unique' types, which belong to several yet undefined rhizobial species.