The Influence of Bradyrhizobium on Nodulation, Nitrogen Fixation and Yield of Fungicide Treated Soybean, Cowpea and Lablab Seeds.

Land degradation and loss of soil fertility remain an important global concern because of the adverse effects on agricultural production, food security and the environment. Nitrogen is one of the most important nutrients to plants yet declining soil fertility causes limitations on its availability in crop production around the world. The lack of adequate nitrogen in most soils limits farmers’ goals of increasing yield per unit area and thus need for research on alternative nitrogen sources. In addition, more research is needed to develop a combination of fungicides and seed inoculation that will not negatively affect nodulation and yield.

A field study was conducted for 2 years to determine the existence of interactions between fungicide and bradyrhizobium, ascertain the abundance and effectiveness of indigenous rhizobia in the soil and how they affect the potential for improving biological nitrogen fixation through inoculation. The research also determined what percentage of nodules formed on the roots were actually fixing nitrogen to the soil.  

The plants were uprooted at R₄ just before pod filling. Results revealed that nitrogen fixation by nodules peaked and then rapidly decreases when pod filling begun. All treatments were not significantly different from each other. However, soybean plots that received both bradyrhizobia and fungicide had higher mean values of yield per hectare compared to other treatments. Soybean plots that received only fungicide but no rhizobia had the lowest mean yield. High yield in plots with no rhizobia would be an indication of presence of already existing competitive indigenous rhizobia strains in the soil. Soybeans were the most efficient at nitrogen fixation followed by CA46 cowpeas.