Competition between introduced Bradyrhizobium japonicum strains and indigenous bradyrhizobia in Minnesota organic farming systems

Influence of nitrogen fertilization, seed inoculation and the synergistic effect of these treatments on soybean yields under conditions in south-western Poland

Soybean, belonging to legumes, has a specific ability to biological nitrogen fixation, which can be reinforced by seeds inoculation. However, support with a starter dose of mineral nitrogen fertilizer may be necessary to achieve high seed yields. A four-year field experiment was conducted to determine the effect of mineral N fertilization (0, 30, 60 kg ha-1), seed inoculation with two commercial inoculants and combinations of these treatments on yield components and yielding of soybean in conditions of south-western part of Poland. The synergistic effect of mineral fertilization at dose 30 kg ha-1 and inoculation on soybean productivity was the most beneficial. Similar effects were observed when 60 kg N ha-1 was applied both separately and with inoculation. However, due to the environmental impact of mineral fertilizers and to promote plants to biological nitrogen fixation (BNF), it is advisable to use lower doses of N fertilizer (at 30 kg ha-1) and inoculate soybean seeds in agro- climatic conditions of south-western Poland. Therefore, based on this study we recommend to apply starter dose of N and inoculation.

Genetic and Morphological Diversity of Indigenous Bradyrhizobium Nodulating Soybean in Organic and Conventional Family Farming Systems

Organic farming systems are gaining popularity as agronomically and environmentally sound soil management strategies with potential to enhance soil microbial diversity and fertility, environmental quality and sustainable crop production. This work aimed at understanding the effect of organic and conventional farming on the diversity of soybean nodulating bradyrhizobia species. Field trapping of indigenous soybean Bradyrhizobium was done by planting promiscuous soybeans varieties SB16 and SC squire as well as non-promiscuous Gazelle in three organic and three conventional farms in Tharaka-Nithi County of Kenya. After 45 days of growth, 108 nodule isolates were obtained from the soybean nodules and placed into 13 groups based on their morphological characteristics. Genetic diversity was done by polymerase chain reaction (PCR) targeting 16S rDNA gene using universal primers P5-R and P3-F and sequencing was carried out using the same primer. High morphological and genetic diversity of the nodule isolates was observed in organic farms as opposed to conventional farms. There was little or no genetic differentiation between the nodule isolates from the different farms with the highest molecular variation (91.12%) being partitioned within populations as opposed to among populations (8.88%). All the isolates were identified as bradyrhizobia with close evolutionary ties with Bradyrhizobium japonicum and Bradyrhizobium yuanminense. Organic farming systems favor the proliferation of bradyrhizobia species and therefore a suitable environmentally friendly alternative for enhancing soybean production.

Environmental Factors Associated With Nitrogen Fixation Prediction in Soybean

Biological nitrogen (N)-fixation is the most important source of N for soybean [Glycine max (L.) Merr.], with considerable implications for sustainable intensification. Therefore, this study aimed to investigate the relevance of environmental factors driving N-fixation and to develop predictive models defining the role of N-fixation for improved productivity and increased seed protein concentration. Using the elastic net regularization of multiple linear regression, we analyzed 40 environmental factors related to weather, soil, and crop management. We selected the most important factors associated with the relative abundance of ureides (RAU) as an indicator of the fraction of N derived from N-fixation. The most relevant RAU predictors were N fertilization, atmospheric vapor pressure deficit (VPD) and precipitation during early reproductive growth (R1-R4 stages), sowing date, drought stress during seed filling (R5-R6), soil cation exchange capacity (CEC), and soil sulfate concentration before sowing. Soybean N-fixation ranged from 60 to 98% across locations and years (n = 95). The predictive model for RAU showed relative mean square error (RRMSE) of 4.5% and an R² value of 0.69, estimated via cross-validation. In addition, we built similar predictive models of yield and seed protein to assess the association of RAU and these plant traits. The variable RAU was selected as a covariable for the models predicting yield and seed protein, but with a small magnitude relative to the sowing date for yield or soil sulfate for protein. The early-reproductive period VPD affected all independent variables, namely RAU, yield, and seed protein. The elastic net algorithm successfully depicted some otherwise challenging empirical relationships to assess with bivariate associations in observational data. This approach provides inference about environmental variables while predicting N-fixation. The outcomes of this study will provide a foundation for improving the understanding of N-fixation within the context of sustainable intensification of soybean production.

Co-inoculation of two symbiotically efficient Bradyrhizobium strains improves cowpea development better than a single bacterium application

The co-inoculation of Bradyrhizobium with other non-bradyrhizobial strains was already assessed on cowpea, but the co-inoculation of two Bradyrhizobium strains was not tested up to now. This study aimed to evaluate the cowpea growth, N accumulation, and Bradyrhizobium competitiveness of the elite strain B. pachyrhizi BR 3262 when co-inoculated with other efficient Bradyrhizobium from the Brazilian semiarid region. Three potted-plant experiments were carried out. In the first assay, 35 efficient Bradyrhizobium isolates obtained from the semiarid region of Brazil were co-inoculated with the elite strains B. pachyrhizi BR 3262. The experiment was conducted in gnotobiotic conditions. The plant growth, nodulation, N nutritional variables, and nodular occupation were assessed. Under gnotobiotic and non-sterile soil conditions, ten selected bacteria plus the elite strain B. yuanmingense BR 3267 were used at the second and third experiments, respectively. The cowpea was inoculated with the 11 bacteria individually or co-inoculated with BR 3262. The plant growth and N nutritional variables were assessed. A double-layer medium spot method experiment was conducted to evaluate the interaction among the co-inoculated strains in standard and diluted YMA media. The co-inoculation treatments showed the best efficiency when compared to the treatments inoculated solely with BR 3262. This strain occupied a low amount of cowpea nodules ranging from 5 to 67.5%. The treatments with lower BR 3262 nodule occupancy showed the best results for the shoot nitrogen accumulation. The culture experiment showed that four bacteria inhibited the growth of BR 3262. In contrast, seven strains from the soils of Brazilian semiarid region were benefited by the previous inoculation of this strain. In the second and third experiments, the results indicated that all 11 co-inoculated treatments were more efficient than the single inoculation, proofing the best performance of the dual inoculation of Bradyrhizobium on cowpea.