Mutualistic interactions can break down despite their ancient history, especially in the face of environmental disruptions caused by anthropogenic change. The legume-rhizobia mutualism is an ecologically and economically important system where the legume host exchanges fixed carbon for fixed nitrogen (N) from the rhizobia. The key to its evolutionary stability is the ability for host legumes to sanction or differentially allocate more resources to more beneficial rhizobial strains. However, global change can affect the plant N demand and the environmental (non-rhizobial) N supply in ways that may undermine the relative benefits of highly-beneficial symbiotic partners and weaken sanctions.
We conducted two studies – one greenhouse and one growth chamber – to test the effects of plant N demand and environmental N supply on the fitness of high- and low-fixing rhizobial strains. Our studies were conducted on Phaseolus vulgaris (common bean) hosts. In the greenhouse study, we exposed plants to elevated and ambient CO2conditions inside transparent chambers to manipulate plant N demand. We also manipulated the environmental N supply by watering the plants with 0 or 5 mM of nitrate. Plants were singly-inoculated with either a high- or low-fixing strain. In the growth chamber study, plants were either singly-inoculated with one of the two strains or had mixed inoculation. We, again, manipulated the environmental N supply by watering the plants with 0, 1, or 5 mM of nitrate. We harvested the plants at time of flowering and measured shoot/root dry weights, counted nodule numbers, and estimated nodule wet weights as proxies of rhizobial fitness.
The greenhouse study revealed that plants only accumulate significantly more biomass under elevated CO2 when additional nitrate is supplied. However, increasing plant N demand, by elevating CO2, disproportionately increased the average nodule weights of low-fixing strains more than for high-fixing strains. Increasing environmental N supply significantly decreased the average nodule weights of high-fixing strains, but had little effect for low-fixing strains. In the dual-inoculation conditions in the growth chamber study, the relative fitness (average weight per nodule) of the high-fixing strain to the low-fixing strain decreased with increasing environmental N supply. We conclude that relative sanctions of low-fixing strains are dependent on the environmental N supply and plant N demand. Adding nitrogen to the environment not only reduces plant dependency on the symbiosis, but undermines relative selection of highly-beneficial rhizobial strains and the stability of the mutualism.