Bottom-up rather than top-down processes regulate the abundance and activity of nitrogen fixing plants in two Connecticut old-field ecosystems
The maintenance of nitrogen limitation in terrestrial ecosystems remains a central paradox in biogeochemistry. Although plants that form a symbiotic association with nitrogen fixing bacteria should be at a competitive advantage over non-fixing plant species in N limited environments, N2 fixing plants are uncommon in most mid- to high-latitude ecosystems. Theory and observation suggest that preferential grazing on N-rich tissues by herbivores, resource limitations to growth, reproduction and N2 fixation, and temperature limitations to the activity of the N2 fixing enzyme nitrogenase, explain the rarity of N2 fixing plants. These ideas, however, have never been confronted by multifactor experiments in the field. In a 3 year field experiment, we found that the abundance, growth, reproductive output and fraction of plant-N derived from N2 fixation in temperate, old-field ecosystems was constrained by the availability of phosphorus (P). Although the availability of light was crucial to the performance of old-field N2 fixing plants, the largest gains in biomass and the rate of N2 fixation were observed in the plots fertilized with P. By contrast, herbivory had no effect on the abundance, biomass and activity of N2 fixing plants and inconsistent effects on foliar nitrogen concentrations (opposing directions, depending upon year), suggesting that herbivores do not affect the ecology of N2 fixing plants in old field ecosystems, at least not over the course of 3 years. Together with a recent study demonstrating that C limitation explains the absence of N2 fixing trees in temperate forests our analysis suggests that stand replacing disturbances shift the limitation on the abundance and activity of N2 fixing plants from P early in secondary succession to light later in succession, as the forest canopy closes and incident light levels decline precipitously.
Biogeochemistry | Earth Sciences
Finzi, Adrien C., and Vikki L. Rodgers. 2009. “Bottom-up rather than top-down processes regulate the abundance and activity of nitrogen fixing plants in two Connecticut old-field ecosystems.” Biogeochemistry 95, no. 2-3: 309-321.