A framework for scaling symbiotic nitrogen fixation using the most widespread nitrogen fixer in eastern deciduous forests of the United States
1. Carbon uptake by the terrestrial biosphere depends on supplies of new nitrogen (N) from symbiotic N fixation, but we lack a framework for scaling fixation accurately and for predicting its response to global change.
2. We scaled symbiotic N fixation from individual N fixers (i.e. plants that host N-fixing bacteria), by quantifying three key parameters—the abundance of N fixers, whether they are fixing N and their N fixation rates. We apply this framework to black locust, a widespread N-fixing tree in temperate forests of the eastern United States, and harness long-term data from southern Appalachian forests to scale fixation from trees to the landscape and over succession.
3. Symbiotic N fixation at the landscape scale peaked in the first decade following forest disturbance, and then declined. This pattern was due to the declining density and declining fixation rates of individual black locust trees over succession. Independent of forest succession, and coincident with chronic atmospheric N deposition, we have evidence suggesting that nodule biomass produced by black locust trees has declined by 83% over the last three decades. This difference in nodule biomass translates to a maximum fixation rate of 11 kg N ha−1 year−1 and a landscape average of 1.5 kg N ha−1 year−1 in contemporary forests.
4. Synthesis. We find key controls on symbiotic N fixation by black locust over space and time, suggesting lower fixation rates in eastern deciduous forests than previous estimates. Our scaling framework can be applied to other N fixers to aid predictions of symbiotic N fixation and ecosystem response to global change.