Core Ideas Cowpea cropping after soybean is increasing in the Cerrado region of Brazil and can benefit from BNF.Soybean bradyrhizobia can abundantly nodulate cowpea but are often inefficient.Inoculated elite cowpea Bradyrhizobium strains increased grain yields.Nitrogen fixation inputs were increased by inoculation with selected cowpea bradyrhizobia.Nitrogen‐15 natural abundance showed inoculation increased N2 fixation by over 25 kg N ha–1 crop–1. Cowpea (Vigna unguiculata Walp.) is increasingly planted as a second season crop after soybean [Glycine max (L.) Merr.] in the Brazilian Cerrado region. Experiments were conducted at Sinop and Primavera do Leste in Mato Grosso State to investigate the impact of cowpea inoculation with recommended rhizobium strains on biological nitrogen fixation (BNF) and grain yield. Plants were either non‐inoculated, with or without 70 kg N fertilizer ha−, or inoculated with five rhizobium strains, separately or as a mixture. Nodulation and shoot dry matter (DM) were evaluated at 35 d after emergence (DAE). Dry matter and N accumulation were evaluated and BNF contributions quantified using 15N natural abundance at 50 DAE. Nodule number and dry weight were high regardless of inoculation treatment. However, several of the inoculation treatments at Sinop and Primavera do Leste promoted significant increases in total N at 50 DAE. At both sites, the 15N abundance results indicated that inoculation significantly increased BNF contributions from 36 to 64 kg ha− at Sinop and from 48 to 75 kg N ha− at Primavera. Grain yield was significantly increased from approximately 1200 to 1500 kg ha− by strains BR 3262 and INPA 03‐11B at the Primavera do Leste, and from approximately 850 to 1400 kg ha− by strain BR 3262 at Sinop. The results indicate inoculation of cowpea with elite rhizobium strains can increase BNF contributions and grain yields in the Brazilian Cerrado region, even when there are abundant populations of soybean rhizobium present in the soil.

The mung bean has a great potential under tropical conditions given its high content of grain protein. Additionally, its ability to benefit from biological nitrogen fixation (BNF) through association with native rhizobia inhabiting nodule microbiome provides most of the nitrogen independence on fertilizers. Soil microbial communities which are influenced by biogeographical factors and soil properties, represent a source of rhizobacteria capable of stimulating plant growth. The objective of this study is to support selection of beneficial bacteria that form positive interactions with mung bean plants cultivated in tropical soils, as part of a seed inoculation program for increasing grain yield based on the BNF and other mechanisms. Two mung bean genotypes (Camaleão and Esmeralda) were cultivated in 10 soil samples. Nodule microbiome was characterized by next-generation sequencing using Illumina MiSeq 16S rRNA. More than 99% of nodule sequences showed similarity with Bradyrhizobium genus, the only rhizobial present in nodules in our study. Higher bacterial diversity of soil samples collected in agribusiness areas (MW_MT-I, II or III) was associated with Esmeralda genotype, while an organic agroecosystem soil sample (SE_RJ-V) showed the highest bacterial diversity independent of genotype. Furthermore, OTUs close to Bradyrhizobium elkanii have dominated in all soil samples, except in the sample from the organic agroecosystem, where just B. japonicum was present. Bacterial community of mung bean nodules is mainly influenced by soil pH, K, Ca, and P. Besides a difference on nodule colonization by OTU sequences close to the Pseudomonas genus regarding the two genotypes was detected too. Although representing a small rate, around 0.1% of the total, Pseudomonas OTUs were only retrieved from nodules of Esmeralda genotype, suggesting a different trait regarding specificity between macro- and micro-symbionts. The microbiome analysis will guide the next steps in the development of an inoculant for mung bean aiming to promote plant growth and grain yield, composed either by an efficient Bradyrhizobium strain on its own or co-inoculated with a Pseudomonas strain. Considering the results achieved, the assessment of microbial ecology parameters is a potent coadjuvant capable to accelerate the inoculant development process and to improve the benefits to the crop by soil microorganisms.

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