Abstract
BackgroundThe co-inoculation of soybean with Bradyrhizobium and other plant growth-promoting rhizobacteria (PGPR) is considered a promising technology. However, there has been little quantitative analysis of the effects of this technique on yield variables. In this context, the present study aiming to provide a quantification of the effects of the co-inoculation of Bradyrhizobium and PGPR on the soybean crop using a meta-analysis approach.MethodsA total of 42 published articles were examined, all of which considered the effects of co-inoculation of PGPR and Bradyrhizobium on the number of nodules, nodule biomass, root biomass, shoot biomass, shoot nitrogen content, and grain yield of soybean. We also determined whether the genus of the PGPR used as co-inoculant, as well as the experimental conditions, altered the effect size of the PGPR.ResultsThe co-inoculation technology resulted in a significant increase in nodule number (11.40%), nodule biomass (6.47%), root biomass (12.84%), and shoot biomass (6.53%). Despite these positive results, no significant increase was observed in shoot nitrogen content and grain yield. The response of the co-inoculation varied according to the PGPR genus used as co-inoculant, as well as with the experimental conditions. In general, the genera Azospirillum, Bacillus, and Pseudomonas were more effective than Serratia. Overall, the observed increments were more pronounced under pot than that of field conditions. Collectively, this study summarize that co-inoculation improves plant development and increases nodulation, which may be important in overcoming nutritional limitations and potential stresses during the plant growth cycle, even though significant increases in grain yield have not been evidenced by this data meta-analysis.
Highlights
The soybean crop (Glycine max (L.) Merrill) is one of the main commodities in the world, mainly for its high protein and oil contents, favoring its use in several areas of the agroindustry (Hart, 2017; Nguyen, 2018)
The Bradyrhizobium-soybean symbiosis is considered one of the most important natural relations exploited by the agricultural activity, since these bacteria can lead to grain yield increase and, eliminate or reduce the dependence on inorganic N fertilizers in crop cultivation (Chang, Lee & Hungria, 2015; Hungria, Marco & Ricardo, 2015; Collino et al, 2015)
The final article number was reduced to 42 based on the following criteria: (i) articles written in English, Spanish, or Portuguese; (ii) studies that presented a measure of variance: coefficient of variation (CV), mean square residual (MSR), standard error of the mean (SE), or standard deviation of the mean (SD); (iii) studies showing the number of nodules, nodule biomass, shoot biomass, root biomass, shoot N content, and/or grain yield traits; and (iv) studies comparing inoculated treatments (Bradyrhizobium) × co-inoculated (Bradyrhizobium + plant growth-promoting rhizobacteria (PGPR))
Summary
The soybean crop (Glycine max (L.) Merrill) is one of the main commodities in the world, mainly for its high protein and oil contents, favoring its use in several areas of the agroindustry (Hart, 2017; Nguyen, 2018). In BNF, the soybean establishes a symbiotic relationship with rhizobia, providing photoassimilates in exchange for biologically active N (Hungria, Menna & Delamuta, 2015; Gresshoff, 2018) These microorganisms usually inhabit the plant root system, where they colonize and grow endophytically, producing the enzyme complex nitrogenase, which allows them to convert atmospheric nitrogen (N2) to ammonia and its further incorporation into biomolecules in several forms of organic N (Hungria et al, 2006; Oldroyd, 2013; Hungria, Nogueira & Araujo, 2013). A total of 42 published articles were examined, all of which considered the effects of co-inoculation of PGPR and Bradyrhizobium on the number of nodules, nodule biomass, root biomass, shoot biomass, shoot nitrogen content, and grain yield of soybean. The co-inoculation technology resulted in a significant increase in nodule number (11.40%), nodule biomass (6.47%), root biomass (12.84%), and shoot biomass (6.53%) Despite these positive results, no significant increase was observed in shoot nitrogen content and grain yield. This study summarize that co-inoculation improves plant development and increases nodulation, which may be important in overcoming nutritional limitations and potential stresses during the plant growth cycle, even though significant increases in grain yield have not been evidenced by this data meta-analysis
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