Results/Conclusions: Similar results were observed across both experiments. In low nitrogen, high light environments, rhizobia significantly increased plant biomass by over 62%. In contrast, when plants were light-limited, the costs of mutualism increased: rhizobia did not increase above-ground biomass and significantly reduced belowground biomass by 46%. Similarly, fertilization reduced the fitness benefits of association with rhizobia, and rhizobia only significantly increased plant biomass under low nitrogen conditions. Thus, consistent with theoretical predications, the legume-rhizobium symbiosis ranges from mutualism to comensalism to parasitism depending on the availability and costs of synthesizing the traded resources. Rhizobium fitness also differs across plants grown in different abiotic conditions, however, and these impacts on rhizobia may limit how far this symbiosis shifts towards parasitism--plants produced fewer nodules when grown in conditions where the benefits conferred by rhizobia to their plant hosts were reduced. In this situation, plants may reduce investment in mutualists in parallel with the decrease in benefits of mutualism, thereby minimizing costs and limiting the extent of negative fitness effects for plant hosts.