Long-term nitrogen addition alters the ecology and evolution of legume-rhizobium resource mutualisms

Background/Question/Methods

Increased resource availability due to nitrogen deposition is predicted to alter the ecological and evolutionary outcomes of resource mutualisms. Both theory and empirical work suggest that high resource availability shifts the ecological outcome of resource mutualism towards parasitism.  Several heuristic theories also predict that high nutrient availability will influence the evolution of resource mutualists, by selecting for more “aggressive”, poorer quality mutualists. Thus, nitrogen deposition may alter both the population dynamics and the evolution of soil mutualist populations in ways that affect the outcome of resource mutualisms.  In this study, we use an existing long-term nitrogen fertilization experiment to investigate how nitrogen deposition influences the ecological and evolutionary outcomes of the classic legume-rhizobium resource mutualism. Specifically, we investigate whether nitrogen deposition causes mutualism breakdown, either via ecological effects on rhizobium population dynamics or evolutionary changes in rhizobium populations.  We isolated rhizobium strains from N-addition vs. control treatments to test for genetic divergence (evolutionary change) between rhizobium populations that have evolved for the past twenty years in high nitrogen vs. control environments.

Results/Conclusions

We found that Trifolium plants inoculated with soils collected from N-addition plots produced less than half the biomass of plants inoculated with soils collected from control plots (F_1,10 = 70.44, P < 0.0001).  In addition, plants inoculated with soils from the control plots produced 5-10 fold more nodules than plants inoculated with soils from the N-addition plots (F_1,10 = 41.62, P < 0.0001).  Both the plant biomass and nodule production results were consistent across the three Trifolium species tested. These nodule production and growth differences could be due to a change in rhizobium density or evolutionary change in the soil rhizobium populations. In a second experiment, in which we inoculated plants with constant cell densities of pure rhizobium cultures, we found that plants inoculated with strains isolated from N-addition plots were smaller (produced fewer leaves) than plants inoculated with strains isolated form control plots (F_1,10 = 5.33, P < 0.044).  These results indicate that long-term N-addition treatments have resulted in the evolution of less mutualistic rhizobia. This study provides an empirical test of resource mutualism theory and investigates both the ecological and evolutionary outcomes of N-deposition for a common resource mutualism.