Molybdenum-Rhizobia-Phosphorus Management for Amelioration of Biological Nitrogen Fixation in Common Bean on a Ferralsol

Abstract

Common bean (Phaseolus vulgaris L.) is among the primary food security crops in Sub-Saharan Africa (SSA), whose production almost exclusively depends on natural Biological Nitrogen Fixation (BNF) as the source of nitrogen. The process is constrained by various biotic (e.g. low BNF capacity of indigenous rhizobia strains) and abiotic (e.g. inadequate levels of soil Mo and P) stresses. It is, therefore, imperative that BNF processes are enhanced through or revisiting rhizobia strains and amelioration of BNF relevant soil nutrient profiles. A study was conducted with the objectives of: (i) determining the response of BNF in common bean to different regimes of molybdenum application, treated with a local and imported (BioStacked) rhizobia inoculant in a Ferralsol; and (ii) determining the influence of intervention with P on BNF in common bean treated with Mo and rhizobia inoculants. A screenhouse study was conducted at Makerere University Agricultural Research Institute Kabanyolo (MUARIK), Uganda, during the December to March 2016; using a Ferralsol obtained from previously cropped fields with different types of legumes and cereals. The study was arranged in two sets namely; (i) molybdenum verses rhizobia inoculants and (ii) molybdenum x rhizobia inoculants x phosphorus. Specific treatments in (i) included Mo applied at rates of 0, 0.6, 1.3 and 2.5 mg kg -1 of soil, equivalent to 0, 0.5, 1 and 2 kg ha -1 ; and rhizobia inoculants namely BioStacked (code named “stress tolerant inoculant” by USA manufacturer) and Mak- Bio-fixer (indigenous from Makerere University). The second experimental setup, which tested the effectiveness of (ii) with P intervention, comprised of three treatments namely Mo and rhizobia inoculants as described above, and P at rates of 0, 38 and 76 mg kg -1 of soil, equivalent xito 0, 15 and 30 kg ha -1 . The experiment was laid out in a Completely Randomized Design (CRD) with 3 replicates and two repeats. The experimental crop was common bean, NABE 4 variety. Results revealed that joint application of Mo and the two types of rhizobia inoculants in setup (i) had a significant (P<0.005) effect on the number of effective nodules and their dry weights, total shoot N and shoot dry weight. Molybdenum rates of 0.5 to 1 kg ha -1 yielded the best overall. However, estimated values for biologically fixed nitrogen (total shoot N from rhizobia inoculated pots discounted for rhizobia uninoculated pots) were not significantly (P>0.005) influenced by Mo application, rhizobia inoculation and/or both treatments. Additionally, intervening with P application in the Mo-rhizobia inoculant setup (setup ii) had no significant (P>0.005) effect on the status of estimated biologically fixed nitrogen generated by the common bean; implying that energy requirement for biological fixation of N was not the key limiting factor for the process. Overall, the BioStacked rhizobia inoculant (imported) tended to perform better in terms of number of effective nodules and their dry weights without Mo and/or P application than its Mak Bio Fixer counterpart.