Influence of different types of Strains of Bradyrhizobium japonicum Inoculation and Phosphorus Fertilizer Rates on Nitrogen Uptake Efficiency and N<span class="sub">2</span> Fixation of Soybean (Glycine max L. (Merrill)) in Ethiopia

Influence of different types of Strains of Bradyrhizobium japonicum Inoculation and Phosphorus Fertilizer Rates on Nitrogen Uptake Efficiency and N2 Fixation of Soybean (Glycine max L. (Merrill)) in Ethiopia

Department of Plant Sciences, College of Agriculture & Veterinary Sciences, Ambo University, Ethiopia, P.O. Box -19

Soybean production in western Ethiopia particularly at Bako has remained low, partly due to soil nutrient depletion and degradation, which is considered serious threats to agricultural productivity and food security. This study was conducted to assess the effect of Bradyrhizobium japonicum strains and phosphorus rates on nitrogen uptake efficiency, N2 fixation of soybean, seed yield, yield components, yield and quality to enhance soybean production and yeild. Treatments included factorial combinations of four Bradyrhizobium japonicum strains with un-inoculated, (Soybean murodk, Soybean 12 and Soybean MAR-1495) and four phosphorus levels (0,23, 46 and 69 kg P2O5 ha-1) which were laid out in Randomized Complete Block Design (RCBD) with three replications. Interaction of Phosphorus and Bradyrhizobium inoculation, revealed that application of 69 kg P2O5 ha-1 with Soybean 12 inoculation significantly resulted enhanced leaf area (64.2 cm2), and above ground biomass (8241 kg ha-1), whereas Soybean MAR-1495 strain inoculation produced highest harvest index (48.77% plant1). Among all treatments, combined application of Soybean 12 along with 46 kg of P2O5 ha-1 gave better performance of soybean with highest seed yield. While, phosphorus rates at 46 and 69 kg P2O5 ha-1 with Soybean 12 inoculation resulted in maximum leaf area index of 0.157 plant1. Hence, 46 kg P2O5 ha-1 phosphorus rate with Soybean 12 inoculation was found to be economical and superior in yield.

Days to 50% flowering and physiological maturity, leaf area, leaf area index, above ground biomass yield, seed yield and harvest index of soybean were increased with combined application of B. japonicum and phosphorus rates.

Combined application of soybean 12 plus 46 kg of P2O5 ha-1 gave better performance in soybean, with maximum leaf area and seed yield.

In the International trade market, soybean ranks number one among the major oil crops with an average protein contents of 40% on dry matter basis, and it is second only to groundnut in terms of oil content 20% (Collombet 2013) and carbohydrates content (20 - 26%) in addition to some nutrients and vitamins among the food legumes (Ahmad et al.2010). The oil obtained from the crop is mostly unsaturated fatty acids which is free from cholesterol. Dugje et al.(2009) reported that soybean is richer in protein than any of common vegetable or legume food sources in Africa. It is used as a good source of unsaturated fatty acids, minerals (Ca and P) and vitamins A, B, C and D (Alam et al.2009). Soybean is a crop that can play major role as protein source for resource poor farmers of Ethiopia who cannot afford animal products. Besides, it can also be used as oil crop, animal feed, poultry meal, for soil fertility improvement and more importantly as income for the country (NSRL, 2007).

In Oromia region in Ethiopia, 9,611.04 ha land was cultivated with production of 22,300.63 tons with the productivity of 2.32 tons ha1; which is low as compared to world average of 2.6 tons ha-1 (Akpalu et at2014). In west Shewa Zone, the crop has good potential particularly at Bako area, however its productivity is low (1.9 tons ha1). Availability of phosphorus in the soil influences the efficiency of Rhizobium that fixes atmospheric nitrogen in association with nodulating legumes as it is directly involved in biological nitrogen fixation via Brady rhizobium japonicum. This has attracted considerable research attention world-wide due to its economic viability for resource poor farmers and environmental friendliness. In fact, soybean is estimated to fix 80% of its nitrogen needs (Smaling et at2008). Soybean N2-fixation is beneficial as it provide necessary N to the plant from the atmosphere which otherwise would proceed from soil and manure (Chianu et at2011). The fixation of soybean as much as 300 kg of N ha -1 in addition to the release in the soil, of 20 - 30 kg N ha-1 for the following crop had been estimated (Hungria et al.2006). To improve soybean yield, biological nitrogen fixation, contribution to soil fertility restoration, inoculation with efficient strains of Bradyrhizobia has already been tested in several countries (Tairo and Ndakidemi 2014). Objectives of the study was to assess the effect of different strains of Bradyrhizobium japonicum and to determine optimal phosphorus rates on growth, nodulation, uptake of nitrogen and phosphorus fertilizers, yield components and yield of soybean.

Field experiment was carried out in Bako Agricultural Research Center (BARC) research station, Oromia National Regional State, during 2018 main cropping season. High yielding soybean variety Jalele-AGS-217 was used in the study. Crop specific carrier based B. japonicum strains namely, Soybean murodk, Soybeanl2 and Soybean MAR-1495 were utilized. Triple superphosphate (TSP) consisting 46% P2O 5 was applied in row basis as per the treatment and mixed with soil just at the time of planting. Treatments consisted two factors namely four levels of phosphorus fertilizer (0, 23, 46 and 69 P2O5 kg ha1), and three different types of Bradyrhizobium japonicum strains inoculants and control (Un-inoculated, Soybean murodk, Soybean 12 and Soybean MAR-1495). Treatments were arranged in 4 4 in factorial combination in randomized complete block design (RCBD) replicated thrice.

The main effect of B. japonicum inoculation and phosphorus rates, and their interaction effects had highly significant (P 0.01) influenced on soybean root length (Table 1). The longest root of soybean crops (29.45 cm plant1) was obtained from application of 69 kg P2O5 ha-1 plus Soybean 12 strain that statistically at par with seeds inoculated with similar strain with 46 kg P2O5 ha-1 (28.92 cm plant-1) and seeds inoculated with Soybean MAR-1495 strain combined with 46 and 69 kg P2O5 ha-1 (27. 77 and 27.92 cm plant1) respectively (Table 1). The lowest root length (13.13 cm plant1) was recorded on control treatment (Table 1). However, when phosphorus fertilizer at rates of 46 and 69 kg P2O 5ha-1 either with all strains of B. japonicum or without all strains of B. japonicum inoculation on all eight treatment parameters of root length was statistically at par each other. Thus, both rates 46 and 69 kg of P2O5 ha-1 with Soybean 12 strain increased soybean root length by 120 and 124 % respectively compared to un-treated control treatment. The presence of Phosphorus in the combined application of phosphorus with B. japonicum strains inoculation might be improved the length of soybean tap roots. In line with the study Tahir et al.(2009) stated that the combination of Rhizobium inoculation, 25 kg N ha-1 and 90 kg P2O5 ha-1 increased soybean root length by 96% over the control treatment. In general application of reduced level of nitrogen as starter fertilizer at planting together with application of phosphorus fertilizer for root growth and development, and B. japonicum strains inoculation for nodulation collectively enhanced plant growth and development.

Table 1: Interaction effects of B. japonicum strains and phosphorus fertilizer rates on root length (cm plant-1) of soybean