Potential of Rhizobia and Arbuscular Mycorrhizal Fungi to Enhance Nitrogen Fixation and Growth of Cowpea Genotypes in Eastern Kenya

Abstract

Semi-arid areas cover about 35% of the global landmass and support over 20% of the global population. Cowpea is an important crop in these areas due to its drought tolerance. However, its productivity is generally low owing to soil infertility, mainly due to nitrogen (N) and phosphorus (P) deficiency. Thus, cowpea associates with arbuscular mycorrhizal fungi (AMF) and rhizobia to supplement its P and N needs. However, low population of effective native rhizobia exists in these areas, which necessitates inoculation while the amount of native infective AMF propagules in the soil is highly variable. At the same time, the effective native rhizobia that can be used as cowpea inoculants in these areas remain unexplored, and the influence of rhizobia inoculation on native AMF association with cowpea remains unclear. The aim of this study was to determine the amount of AMF infective propagules and the influence of native rhizobia nodulating local cowpea genotypes on cowpea growth, production and AMF colonization on the smallholder fields. Soil samples were collected from fifteen smallholder farms across three semi-arid areas of Kenya (Embu, Kitui, and Tharaka Nithi counties) and assessed for physicochemical characteristics. The AMF infective propagules in the soil were estimated using the most probable number technique with Bermuda grass (Cynodon dactylon) as the trap host. Native rhizobia were trapped in the selected farms using three cowpea varieties (KVU 27-1, M-66 and K-80). Molecular identification of the isolates was done via the 16S rRNA gene sequencing using the universal primers 1492R and 27F. The symbiotic efficiency of the isolates was assessed relative to a commercial inoculant (Biofix), nitrogen supplemented control and uninoculated control. In the field, four rhizobial treatments, including native rhizobia, Biofix, native rhizobia + Biofix, and uninoculated control were tested. Field and greenhouse data were subjected to analysis of variance at a 5% level of significance. Rhizobial sequences were characterized based on bioinformatics tool, BLASTn and analysis of molecular variance and genetic differentiation computed using the Arlequin software version 3.5.2.2. The MPN values were related to the soil physicochemical characteristics using Redundancy Analysis. Results showed that the amount of AMF infective propagules varied in numbers (MPN values) across the fifteen farms. Based on the redundancy analysis, sand, clay and P were the most important soil parameters affecting the AMF-MPN values. Isolates were placed in twenty groups based on morphological characteristics. Thirteen groups initiated nodulation during authentication and belonged to the genus Rhizobium. Further assessment of their molecular diversity revealed a significant variation in the same population (County) but not among populations. In the greenhouse, 53.8% of the native isolates had a symbiotic efficiency of >80%, categorizing them as the most efficient isolates in nitrogen fixation. In the field, rhizobia inoculation significantly (P < 0.05) increased nodulation and shoot dry weight compared to the uninoculated controls. Native isolates led to the highest yield increase of 22.7% and 28.6% in season one and two, respectively. The performance of the cowpea genotypes was the same across all the rhizobia inoculants, although, it varied across different seasons and regions. Additionally, rhizobia inoculation significantly influenced AMF colonization only in the second season with no significant effect on individual cowpea genotypes in both seasons. The obtained native rhizobial isolates provide potential cowpea inoculants in the semi-arid areas compatible with the native AMF that can be used for sustainable cowpea production. Therefore, farmers in these areas should adopt rhizobia inoculation to boost the native rhizobia population and enhance cowpea production.