Symbiotic nitrogen (N) fixers are unique among plants in their dual ability to take up N from the soil and access the vast atmospheric N pool through fixation. N fixation may provide a competitive advantage to these N fixers when soil N scarcity limits plant growth. However, the high energetic costs of N fixation could also create a competitive disadvantage for N fixers in N-rich ecosystems. Understanding the net effects of N fixation on the growth and competitive success of N fixers in different environmental conditions is critical to our theoretical predictions of ecosystem N dynamics. However, empirical data on how N fixation affects the growth and competitive success of N fixers are exceedingly rare. We grew active N-fixing and inactive N-fixing (N-fixing plant species without their symbiotic bacteria) tree seedlings in isolation and in direct competition with a common non-fixing tree species under gradients of light and soil N availability to assess how the ability to fix N and N fixation itself affects growth, competitive success, biomass allocation, and tissue chemistry.
Results/Conclusions
The ability to fix N did not convey a biomass advantage to active N fixers in any environmental treatment, even when N demonstrably limited growth. However, active N fixers were more resistant to competition effects on plant height than inactive N fixers in N-limited conditions. Active N fixers in N-limited conditions fixed less N in the presence of a competitor, likely increasing their competitive influence on neighbors when soil N is scarce. Moreover, N fixation did not represent a net structural cost to the plant, even in cases where N fixation more than exceeded plant N demand. These results suggest that the ability to fix N may be a key competitive strategy to avoiding competition when soil N is limiting in tropical forests, and that luxury N fixation may not represent a large cost to tropical N fixers under high-light conditions. Our results also suggest that allocation to the acquisition of other resources such as light or phosphorus might be important regulators of the competitive success of tropical N fixers. Further studies on the growth and competitive effects of the ability to fix N are needed to help refine our understanding of N fixers in community dynamics and ecosystem theory.