Review PaperA meta-analysis of the effectiveness of diverse rhizobia inoculants on soybean traits under field conditions

- Field soybean is nodulated by diverse Bradyrhizobium and Sinorhizobium strains.
- Soybean inoculants vary in their efficacy for nodule number, yield and grain-N.
- Greatest increases in nodulation occur when compatible, indigenous soil rhizobia are low.
- Moderate pH is critical for inoculants to improve nodulation.
- Soil, environment, and management practices affect rhizobia survival in soil.

Soybean is the most widely grown legume in the world. A beneficial feature of soybean is its ability to associate with rhizobia bacteria in its root nodules to fix atmospheric nitrogen through symbiotic nitrogen (N) fixation. Here, we provide a meta-analysis of 28 peer-reviewed studies examining the effectiveness of diverse rhizobia inoculants under field conditions, with the goal of understanding the underlying factors that affect inoculant success or failure. The data demonstrate that a diversity of Bradyrhizobium and Sinorhizobium species/isolates can be effective inoculants, including some indigenous strains. These inoculants varied in their efficacy for nodule number (−28 to +178 nodules), grain yield (−34% to +109%), and grain-N yield (−6% to +176%) compared to uninoculated controls. The greatest increase in nodule numbers occurred when background nodulation by indigenous soil rhizobia was absent or extremely low. Some studies demonstrated that indigenous rhizobia strains may be better adapted to local environmental stress conditions compared to introduced rhizobia, suggesting native rhizobia may have potential for local commercialization. There was a positive but moderate correlation between inoculant-mediated increases in nodulation and grain yield/grain-N. Moderate pH conditions were critical for inoculants to improve nodulation. Inoculant success was affected by the soybean genotype and soybean x rhizobia strain interactions, inoculant titre, formulation, and application method. Potential additional explanations from the literature for the failure/success of inoculants pertain to the persistence of rhizobia (inoculants) in soil, since seed-coated rhizobia colonize roots through the rhizosphere, and furthermore stress may favor indigenous, locally-adapted competitors. Rhizobia survival in soil is strain dependent and affected by soil organic matter, nutrients, pH, salinity, agricultural practices (e.g. organic, no till, rotations, application of pesticides) as well as temperature and drought. We conclude by proposing new studies to fill current research gaps.