Prolonged soybean absence in the field selects for rhizobia that accumulate more polyhydroxybutyrate during symbiosis

During symbiosis, C that rhizobia respire to power N fixation can be stored as polyhydroxybutyrate (PHB), shown to support rhizobia survival under laboratory starvation. We collected soil in 2015 from four replicate plots per treatment in two long-term experiments at Waseca, MN. Treatments differed in the intervals between soybean [Glycine max (L.) Merr.] hosts. We measured PHB accumulation in eight nodules per plant from four soybean (cv. MN0095) trap plants per soil sample. Trap plants were arranged in a greenhouse, common-garden experiment, and PHB accumulation was measured using flow cytometry. Treatments sampled after long intervals without soybean (greater than 2 years) showed a greater relative abundance of high-PHB strains. Treatments sampled after the first year of soybean following 5 years of a non-host crop showed a decreased relative abundance of high-PHB strains, compared to treatments sampled after long intervals without soybean. The latter result is consistent with the hypothesis (not tested directly here) that some high-PHB strains were “sanctioned” by plants as less beneficial. Our results suggest that rhizobia strains with the tendency to allocate more C to N fixation at the expense of PHB accumulation may be less likely to persist where soybean is grown infrequently or where soil conditions make PHB particularly valuable. However, with typical 2-year rotations in Minnesota, differences in PHB storage are unlikely to have a major effect on the relative survival of strains.