Exploiting Belowground Biodiversity of Rhizobia and Arbuscular Mycorrhizal Fungi to Upscale Cowpea Production in the Semi-Arid Zones of Eastern Kenya
Muindi, Mercy Martha
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Use of microbial symbionts to promote legume production in the dry lands is of great importance to agriculture. Most soils in dry lands are characterized as being nutrient poor. Inoculation of legumes such as cowpea (Vigna unguiculata) with arbuscular mycorrhizal fungi (AMF) and rhizobia has the ability to avail nutrients and water to the plants inherently increasing crop productivity sustainably and enhancing food security. However, there is paucity of data on the identities and efficiencies of native symbiotic microorganisms from lower Eastern Kenya. Therefore, this study aimed at determining the morphological and genetic diversity of native rhizobia strains that nodulate cowpea, assess their symbiotic efficiency (SE), determine the effect of dual inoculation of native rhizobia and AMF on cowpea biomass as well as establish the effect of soil properties on the number of infective AMF propagules. Soil samples were obtained from 10 farms in Embu and Kitui Counties. Four cowpea varieties were used to trap native rhizobia in greenhouse bioassays. Rhizobia were isolated from cowpea nodules and their symbiotic nitrogen fixation efficiency assessed. 16S rRNA gene analysis was used to determine genetic diversity. The number of infective AMF propagules in the soils was estimated using most probable number technique with Cynodon dactylon as the trap host. Cowpea plants were dually inoculated with Rhizophagus irregularis, Funneliformis mosseae and 8 indigenous rhizobia strains. A randomized complete block design was used in the greenhouse experiments. Cowpea varieties differed significantly (p ˂ 0.0001) in nodule number where, the local landrace recorded the lowest nodulation compared to the open pollinated varieties. Ninety four bacterial nodule isolates obtained from cowpea root nodules and placed into 22 groups based on morphological and biochemical characteristics. Sequencing of the 16S rRNA region revealed the isolates closely resembled bacteria belonging to the genera Rhizobium, Paraburkholderia and non-rhizobial endophytes Enterobacter, Strenotrophomonas, and Pseudomonas. This study also reports for the first time in Africa the presence of an efficient native Beta-Rhizobia nodulating cowpea: Paraburkholderia phenoliruptrix BR3459a coded isolate M2. Symbiotic efficiency of the native rhizobia isolates varied (p < 0.0001) significantly. Isolates M2 and M3 had SE of 82.49 % and 72.76 % respectively which was higher than that of the commercial strain (67.68 %). Data obtained after dual inoculation of cowpea showed that there was significant variation in nodule number (p < 0.0001), nodule dry weight (p < 0.0001), shoot dry weight (p < 0.0001), and root dry weight (p = 0.0444) following rhizobia inoculation. AMF inoculation affected cowpea nodulation (p = 0.0005), shoot dry weight (p < 0.0001), and root dry weight (p = 0.0061) significantly. Cowpea plants treated with Rhizophagus irregularis had higher nodule number, shoot and root dry weights compared to those treated with Funneliformis mosseae. Based on redundancy analysis, the soil characteristics phosphorus, potassium, manganese, zinc, iron correlated positively with mycorrhizal colonization. There was a negative correlation between soil pH, nitrogen, calcium, sodium, soil organic matter and the number of AMF infective propagules. This study demonstrated that Kenyan soils harbor diverse and efficient cowpea nodulating rhizobia which after field trials can be developed into effective cowpea inoculums. When co-inoculated with rhizobia, Rhizophagus irregularis was established to be a better colonizer of cowpeas. Variations in soil properties influenced AMF populations suggesting that conservation agricultural practices should be used to increase crop production and protect beneficial microorganisms.