How is N fixation regulated in fire-adapted ecosystems?
The capacity for nitrogen (N) fixing organisms to up-regulate fixation in N-poor environments allows ecosystems to overcome N limitation. However, we poorly understand the ecological controls on N fixation in ecosystems, particularly those characterized by frequent fire. Longleaf pine savannas are fire-dependent ecosystems that offer an ideal context to investigate how disturbance and chronic N loss affect N-fixing organisms. Here, we hypothesized that increased fire frequency stimulates leguminous fixation and suppresses free-living fixation. To test these hypotheses, we quantified the activity of free-living fixers (O horizon mass and acetylene reduction assays) and legume abundance and diversity in 23 1 ha plots of sandhill longleaf pine forests distributed along a fire frequency gradient (fire return intervals of 2, 3, and 15+ years) at Fort Benning (GA) and Eglin Air Force Base (FL).
Increased fire frequency resulted in substantial increases in legume abundance and concomitant declines in free-living fixation. Mean legume abundance in savannas with 2-year fire return intervals was 46% and 362% greater than those with 3 and 15+ year fire return intervals, respectively. These results suggest that increasing fire frequency triggers N limitation and promotes legume abundance. We also observed strong species-specific trends in the legume community with fire frequency. Low-capacity N-fixers, such as Rhychosia cytisoides, dominated legume communities at high fire return intervals (62%), but were scarce in savannas that burned frequently (10%). Conversely, high-capacity N-fixers, such as Tephrosia virginia and Galactia regularis, reached peak abundances in frequently burned savannas (19% and 37%) but were notably absent in savannas that burned infrequently. Free-living fixation increased linearly with years since fire (r2=.71), but fixation rates were low. These results suggest that free-living fixers facilitate small inputs of N in savannas that experience low N losses. In contrast, the legume community is highly responsive to fire frequency, suggesting that leguminous N fixation is the primary mechanism replenishing N losses in longleaf pine savannas.