Timing matters: Rhizobia strain rankings based on host biomass shift between early and late harvests

In the legume-rhizobia mutualism, symbiotic success changes with time, complicating early strain quality evaluations. We measured host biomass at three and six months after inoculation of bearded clover ( Trifolium barbigerum ) with 77 strains of Rhizobium leguminosarum . Across timepoints, strain performance rankings based on host biomass varied: some top strains at three months later declined, whereas initially low-ranking strains ultimately surpassed them. This suggests biological tradeoffs in the timing of nitrogen fixation and the allocation of the host resources. Our results highlight that symbiont function can vary over time and that single timepoint data collection risks inaccurately identifying long-term beneficial strains.