Co-deployment of legume nodulating bacteria and arbuscular mycorrhizae fungi for improved performance of common bean in an acid soil
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Nitrogen and phosphorus are by far the most limiting nutrients for crop production in sub-Saharan Africa, yet low-input natural systems are potentially the most dependable sources of these elements among the small-scale farmers. A study was conducted at Makerere University to evaluate the simultaneous exploitation of two biological mechanisms known to enhance plant access to N and P. Rhizobia and Arbuscular Mycorrhizae fungi were tested using common bean (Phaseolus vulgaris L.) as test plant and soil from Nyamagabe and Huye districts of Rwanda. The rhizobial populations in the soil varied from relatively high numbers in Nyamagabe (3.55 x 103 cells. g-1) to very low numbers in Huye (0.14 x 103 cells.g-1); Mycorrhizae population was highly different in Nyamagabe soil compared to Huye soil. Soil pH differences (4.5 for Huye and 5.0 for Nyamagabe) possibly explained variations in mycorrhizal populations. Nitrogen fixing bacteria were isolated from root nodules of common beans, inoculated with diluents from the test. A total of 27 bean rhizobial isolates were obtained and later arranged in one group based on colony morphology and biochemical characteristic. All bean isolates were fast growers, consistent with the knowledge that the test host often nodulates with Rhizobium spp. revealed that all isolates showed fast growth although bean is characteristically nodulated by isolates described as Rhizobium. Cluster analysis divided the strains into two distinct groups. The highest similarity coefficient (0.89) was between isolates B1LL, B1YY, B1E and B1 3D, all from Huye soil, suggesting close genetic makeup. There was though no 100% similarity between any two groups. Isolate B1A had the lowest similarity coefficient (0.67) from Huye district. The above groups were related to CIAT899, the commercial strain developed by CIAT. Among 27 bean Rhizobium isolates, 13 of them significantly had higher N content than the standard strain and N control application, and were ranked effective. The two bean isolates (B2AA and B2W) showing the highest dinitrogen fixing capacity and N content were combined with mycorrhizal spores for enhancing legume nodulation and production. Using a complete randomized design (CRD) with ten treatments i.e. T1: Control (no inoculation, no other treatments); T2:1 mL of B2AA; T3: 1 mL of B2W; T4: 50 g of AMF; T5: 882 mg of TSP; T6: 500 mg of Urea; T7: 1 mL of B2AA + 50 g of AMF; T8: 1 mL of B2W + 50 g of AMF; T9: 1 mL of B2AA + 882 mg of TSP; T10: 1 mL of B2W + 882 mg of TSP replicated three times under screenhouse conditions. Plants inoculated with Rhizobium or AMF alone, or in combination nodulated, leading to higher shoot N and P content compared to the control, and with or without triple superphosphate (TSP) addition. Thus, the effective Rhizobium (B2AA and B2W) isolates combined with arbuscular mycorrhizal fungi (AMF) are potential bean inoculants in the Rwanda acid soils. A positive correlation existed between number of nodules and N content (r = 0.74); shoot dry weight and N content (r = 0.81), total P and P content (r = 0.86) and between P content and AMF colonisation (r = 0.71). Evidently, dual inoculation of bean plant with Rhizobium and AMF increases nodulation, and increased N and P contents in the host bean plants.