Currently, inorganic nitrogen fertilizer becomes a serious threat to the environment and human health. Thus, finding alternate source of nitrogen is a viable option in assuring sustainable agricultural system. Biological nitrogen fixation is a critical and key process in sustainable agricultural systems in tropical soils, which are frequently deficient in N and susceptible to leaching of plant nutrients. This process transforms atmospheric nitrogen to ammonia, nitrate and nitrogen dioxide. Several key abiotic and biotic factors limit legume productivity and biological nitrogen fixation in World agriculture, especially in sub-Saharan Africa. Within the soil, rhizobia frequently encounter various stresses that affect their growth, initial steps of symbiosis and capability of nitrogen fixation. Biotic and abiotic stresses impose a major threat to agriculture and symbiotic nitrogen fixation is dependent on host cultivar and rhizobia, but as well may be limited by pedoclimatic factors. The most common factors affecting biological nitrogen fixation and symbiosis activity in western parts of Ethiopia are soil acidity, quality of inoculants and low soil fertility. In most cases, the microsymbiont is the more affected partner, with plants growing on mineral N usually less sensitive to these stresses. Thus, it can be concluded that, particularly in a western part of Ethiopia, many studies should be focused on acidity related constraints on biological nitrogen fixation, screening of acid tolerant inoculants and low soil fertility improvements to enhance biological nitrogen fixation in smallholder farming system.

The objectives of this study were to identify high yielding and stable medium maturity soybean varieties across environment and examine the influence of genotype × environment interaction (GEI) on grain yield of soybean varieties in western Oromia. Seven early soybean varieties were evaluated at five locations (Bako, Gute, Billo, Chewaka and Uke) using randomized complete block design (RCBD) with three replications for two consecutive years (2016 and 2017). Combined analysis of variance showed that grain yield was significantly (P<0.01) affected by environments, genotypes and GEI. The environment, genotype and genotype by environment interaction accounted for 57.4, 20.9 and 19.8% variations, respectively. The first two principal components (IPCA 1 and IPCA 2) were used to create a two-dimensional genotype and genotype by environment interaction (GGE) biplot and explained 68.9 and 15.6% of the total sums of squares of GEI, respectively. According to the average environment coordination (AEC) views of the GGE-biplot, soybean variety Didhessa and Hawassa-04 were identified as the most stable and high yielding varieties. In addition, Didhessa and Hawassa-04 also showed better stability performance according to AMMI stability value (ASV), genotypic selection index (GSI), Wricke's ecovalence and cultivar superiority measure among the evaluated varieties whereas variety Davis and AFGAT were identified as the least stable and low yielding variety. Therefore, among medium maturing soybean varieties, Didhessa, Hawassa-04 and Cheri were recommended for further production in most soybean growing areas of western Oromia.