Fast track to environmentally adapted rhizobia for growing soybean at northern latitudes using citizen science.

Soybean serves as a crucial source of plant-based protein for human diets. Recently, there is a growing incentive to extend the range of this crop to more northern latitudes, in order to enable profitable soybean production in Europe. To reach economic yields, soybean requires inoculation with symbiotic, diazotrophic rhizobial bacteria. However, the performance of commercial inocula is often variable under local conditions. Here, we present the citizen science project "Soy in 1,000 Gardens", a large-scale trapping experiment for isolating local soybean-nodulating rhizobia in Flanders, Belgium. We identified two locally isolated Bradyrhizobium strains performing at least as well as commercial strain B. diazoefficiens G49 in local field trials. Additionally, we found that nutrient content, microbial alpha diversity, and the presence of arbuscular mycorrhizal fungi in the soil were correlated with nodulation. Finally, we report a correlation between low bacterial alpha diversity and red nodule interior, and identified Tardiphaga as a dominant colonizer of red nodules.