Impact of Arbuscular Mycorrhizal Fungi Inoculation on Soybean (Glycine max) Production in the Semi-Deciduous Forest Zone of Ghana

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May, 2018
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Phosphorus (P) is an important but limiting major nutrient in crop production. Its availability in the soil and consequently for plant nutrition remains a major challenge. The overall aim of this study therefore was to assess the potential of four arbuscular mycorrhizal fungi (AMF) isolates in enhancing mineral P fertilizer use efficiency in soybean and their beneficial residual effects on succeeding maize crop. Two pot experiments were conducted under greenhouse conditions using sterile river sand and a non-sterile soil on which two soybean lines (TGx 1989-48 FN and TGx 1989-75 FN) were evaluated. The soybean lines were inoculated with the four mycorrhizal isolates (Glomus etunicatum, G. fasciculatum, G. mosseae and Rhizophagus irregularis) as a preliminary screening study to select the top 50% symbionts. A factorial arrangement using a completely randomized design was used. Furthermore, a two-year field experiment was also conducted following the screening study during the minor and major cropping seasons of 2016 and 2017 respectively to evaluate the potential of combined mycorrhizal inoculation and P mineral fertilizer in enhancing soil P availability, soybean growth and yield and economic profitability. The two aforementioned soybean lines used in the pot experiments were separately inoculated with the top 50% mycorrhizal isolates (G. mosseae and R. irregularis). An uninoculated control treatment was included. Triple superphosphate (TSP) was applied at three levels (0, 15 and 30 kg P ha-1). A split-split plot design with three replications was used; soybean lines as the main-plot factor, TSP rates as the sub-plot factor while the AMF isolates constituted the sub-sub plot factor. In a separate experiment, an early maturing maize variety (Omankwa) was planted following the minor cropping season of 2017 to evaluate the residual effects of the imposed treatments on root length colonization, N and P uptake, shoot biomass and grain yields. The greenhouse study revealed that R. irregularis and G. mosseae improved P uptake, plant height, stem girth and shoot biomass yield of soybean relative to G. etunicatum and G. fasciculatum. In the field experiment, inoculation with the selected AMF isolates combined with 15 kg P ha-1 resulted in the highest soybean root length colonization, P use efficiency, shoot and grain P uptake, plant height, shoot biomass and grain yields. Furthermore, AMF + 15 kg P ha-1 reduced soil microbial biomass P by stimulating P release by soil microbes and thus increased soil available P. Again, G. mosseae showed higher performance in soybean root length colonization and P uptake, which reflected in the better growth and yield improvement than R. irregularis. The economic analysis indicated that inoculation using the selected mycorrhizal isolates combined with 15 kg P ha-1 had the highest net benefit and marginal rates of return and is therefore economically more profitable for soybean production. Glomus mosseae, underfield conditions persisted even after two cropping years. Maize root length colonization was higher in plots previously inoculated with AMF, which reflected in highest maize shoot N and P uptake, grain P uptake, shoot biomass and grain yields relative to the control. This study established that, the appropriate management of AMF inoculation is a potential to reducing the use of inorganic P fertilizers.
A thesis submitted to the Department of Crop and Soil Sciences, Faculty of Agriculture, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana, in partial fulfilment of the requirements for the award of degree of Doctor of Philosophy in Soil Science.