Nutrient pollution often increases nutrient availability and plant productivity, while also reducing plant diversity and altering interspecific interactions. Resource mutualisms are particularly vulnerable to changes in nutrient availability and can be disrupted when resources become scarce or overabundant. Long term fertilization has been shown to induce rapid evolution of less mutualistic microbial partners. This can result in a disadvantage for plant hosts and might lead to the net outcome of the interaction to be more parasitic than mutualistic. Here we test whether long term history of fertilization followed by cessation of nutrient enrichment will result in persistent changes in mutualistic interactions. We collected soil and arbuscular mycorrhizal (AMF) spores from experimental plots in a Minnesota grassland that have received: no nutrients (controls), nutrient addition (nitrogen and phosphorus or phosphorus) for 30 years, or nutrient addition for 10 years followed by 20 years of no nutrient addition. We conducted a greenhouse experiment where we grew the two most dominant grass species in the experiment (Schizachyrium scoparium, which dominates the unfertilized plant communities, and Elymus repens, which dominates the fertilized plant communities) with live soil inoculum, mycorrhizal spore inoculum or sterilized soil (control) from field plots that had different histories of fertilization.
Inoculating with live soil increased aboveground plant biomass (P < 0.001) when compared to plant growth on sterile soil. Similarly, plants inoculated with AMF experienced a marginally significant increase in aboveground biomass production (P = 0.08). S. scoparium had significantly lower biomass when grown with AMF from plots that had no history of fertilization (P = 0.04). We found a marginally significant interaction between cessation and nutrients added for S. scoparium biomass production (P = 0.054); plants inoculated with AMF from plots that had received phosphorus fertilization and cessation had lower biomass than those inoculated with AMF from plots that had received both phosphorus and nitrogen fertilization and cessation. We found an opposite pattern for E. repens, which showed a marginally significant increase in biomass when grown with AMF coming from plots with no history of fertilization (P = 0.07), but no difference in biomass when grown with AMF from fertilized plots, regardless of the history of cessation. Our results indicate that chronic nutrient enrichment could have lasting legacies in the outcome of resource mutualisms, as we observed in the persistent lower biomass of E. repens, and could indicate that less cooperative mutualists persist decades after cessation of nutrient addition.