Co-Inoculation of Soybean with <i>Bradyrhizobium</i> and <i>Azospirillum</i> Promotes Early Nodulation

Abstract

Soybean inoculation with elite strains of Bradyrhizobium to improve nodulation, N2 fixation, and grain yield is well established worldwide. However, when grown in soils where N is deficient, soybean undergoes an initial phase of N starvation that may last up to 20 days after seedling germination due to the lack of synchronism between the phase when seed N reserves are exhausted and the moment when plants begin to benefit from the nitrogen fixed by the bacteria. Practices that promote early nodulation may play a key role in reducing the N starvation period. Azospirillum is a plant growth promoting rhizobacteria (PGPR) that can stimulate root hair formation and root growth, creating more sites for early root infection and nodule formation by N2-fixing Bradyrhizobium spp. In this study, the effects of co-inoculating soybeans with Bradyrhizobium spp. and Azospirillum brasilense on nodulation precocity and N2 fixation were evaluated under greenhouse and field conditions. Nodule number and dry weight, as well as plant and root dry weight and N accumulated in shoots at 15, 18, 21, 24 and 30 days after emergence (DAE) were evaluated in response to inoculation with Bradyrhizobium spp. alone or when co-inoculated with Azospirillum sp. In the greenhouse, co-inoculated plants nodulated precociously as indicated by a significant increase (p < 0.05) in nodule biomass observed at (include) 21 DAE. More pronounced effects of co-inoculation were observed in the field as early as 18 DAE, suggesting that the presence of Azospirillum helps plants to overcome environmental stresses.

Microbial inoculants: reviewing the past, discussing the present and previewing an outstanding future for the use of beneficial bacteria in agriculture

“…Even in areas with high population of compatible rhizobia for both crops (> 10 4 cells of compatible rhizobia/g soil), for the soybean crop single inoculation of Bradyrhizobium resulted in mean increases of 8.4% in grain yield compared with the naturalized population, whereas the co-inoculation with A. brasilense promoted an “upgrade” to 16.1%; for common beans, single inoculation with R. tropici increased yield by 8.3%, whereas the co-inoculation improved the yield by 19.6% (Hungria et al 2013) (Table 1). Since them, other benefits attributed to the co-inoculation of soybean with Bradyrhizobium and Azospirillum in Brazil are the promotion of early nodulation (Chibeba et al 2015), and increased tolerance to moderate water restriction (Cerezini et al 2016; Silva et al 2019).…”

“…Another technology with increasing application relies on the use of mixed inoculants, aiming to promote plant growth by combining distinct mechanisms of different microorganisms. Mixed inoculants can provide excellent results and show the great potential of being increasingly used by the farmers (Juge et al 2012; Hungria et al 2013, 2015; Chibeba et al 2015; Bulegon et al 2017; Ferri et al 2017).…”

Co-inoculation of Bradyrhizobium japonicum and Azospirillum brasilense on the physiological quality of soybean seeds

“…Gibberellins also produced by PGPBs are involved in the development of the embryo during the seed germination (Cheng et al, 2002). Co-inoculation with A. brasilense stimulates an earlier and more abundant nodulation in soybean (Chibeba et al, 2015), resulting in a more efficient BNF process, with greater N accumulation that results in the formation of denser and more protein-rich seeds.…”

Maize growth promotion by inoculation with Azospirillum brasilense and metabolites of Rhizobium tropici enriched on lipo-chitooligosaccharides (LCOs)

“…In Brazil, inoculants containing Azospirillum brasilense strains Ab-V5 and Ab-V6 were exponentially employed by farmers in the past 5 years for maize and wheat (Hungria et al 2010; Hungria 2011). More recently, co-inoculation of soybean with rhizobia and azospirilla has also been adopted as an agronomic practice by several farmers, with reported positive effects on nodulation precocity (Chibeba et al 2015) and increases in grain yield (Hungria et al 2013, 2015b); positive effects on common bean ( Phaseolus vulgaris L.) yield have also been reported (Hungria et al 2013). Among the benefits of inoculation with non-symbiotic PGPB, the contributions of biological nitrogen fixation (Ashraf et al 2011), production of phytohormones (Strzelczyk et al 1994), phosphate solubilization (Rodriguez et al 2004) and control of plant pathogens (Araujo et al 2005; Wang et al 2009) are commonly cited.…”