Nitrogen (N) is typically one of the most limiting resources in terrestrial ecosystems and some plants (e.g., legumes, Fabaceae) overcome this limitation through mutualistic associations with nitrogen fixing bacteria (e.g., rhizobia). A widespread threat to this common symbiosis is deposition of atmospheric N, which may disrupt the ecology and evolution of resource exchange. In many legume-rhizobia systems, plants form fewer and smaller nodules under high N conditions. And in long term experiments, adding N to natural systems has resulted in the evolution of less cooperative rhizobia. While in both of these scenarios legumes may benefit from N deposition, we hypothesized that such resources are more likely to be usurped by non-N-fixing competitors, which may result in negative fitness impacts on legumes.
Amphicarpa bracteata is an annual legume native to the Eastern US, commonly co-occurring with another annual, Impatiens capensis (Balsaminaceae), which does not associate with nitrogen fixing bacteria. Both species frequently compete along streams and rivulets in temperate forests where both light and water are abundant. In a factorial greenhouse experiment, we manipulated competition, nitrogen availability, and rhizobia to test the relative and interactive fitness effects of N deposition and rhizobial symbiosis competitive outcomes.
Results/Conclusions:
We detected a three-way interaction between competition, fertilization and inoculation on A. bracteata biomass and a two way interactive effect of competition and fertilization on A. bracteata seed number. Fertilization marginally increased A. bracteata growth and reproduction when alone, but decreased growth and reproduction under competition (~60% reductions).
A separate experiment demonstrated the effects of fertilization on A. bracteata-rhizobia mutualisms without competition. Application of N to pots containing A. bracteata and rhizobia reduced nodule number and nodule diameter. Specifically, nodules were on average 25% as numerous, and 30% as large with the addition of 23 ppm N in the watering solution as when no N was added. However, even with decreased colonization by Bradyrhizobia, fertilized plants grew on average 110% larger than non-fertilized plants. Taken together, these results indicate that while fertilization alone is not detrimental to legume growth, the interactive effects of N deposition and competition are likely to severely threaten legume persistence in competitive communities.