Regenerating tropical forests are a significant carbon sink, but their regrowth is likely to be constrained by limiting nutrients, such as nitrogen. Nitrogen-fixing trees, which are able to convert atmospheric nitrogen gas to plant-available forms of nitrogen, are the main pathway of overcoming nitrogen limitation during forest succession. Although nitrogen enrichment via nitrogen-fixing trees can promote producer growth and carbon sequestration thereby mitigating climate change, it also has the potential to exacerbate climate change by stimulating soil nitrous oxide emissions. Understanding the degree to which the atmospheric cooling effects of carbon sequestration are offset by the atmospheric warming effects of soil nitrous oxide emissions remains an unresolved challenge. Nitrogen cycle dynamics are further complicated by low soil nitrogen levels (due to previous land use) and projected increases in atmospheric nitrogen deposition (due to intensifying emissions of nitrogen-based gases).
We developed and analyzed a simple ecosystem model to predict how nitrogen-fixing trees influence the capacity of the regenerating forest carbon sink. We examine the influence of the nitrogen fixation strategy of the nitrogen-fixing trees (their ability to down-regulate nitrogen fixation), initial soil nitrogen levels, and nitrogen deposition rate on the net atmospheric effect of the regenerating forest carbon sink.
The nitrogen fixation strategy of the nitrogen-fixing trees and nitrogen deposition rate significantly influence the net atmospheric effect of a forest ecosystem, whereas initial soil nitrogen level does not. Both nitrogen fixation and nitrogen deposition can alleviate a forest ecosystem from nitrogen limitation.
Under lower nitrogen deposition rates, nitrogen-fixing trees increase the atmospheric cooling effect of forest ecosystems (as compared to non-fixing trees). Under higher nitrogen deposition rates, nitrogen-fixing trees that can completely down-regulate nitrogen fixation have no significant effect on the atmospheric cooling effect of forest ecosystems, whereas nitrogen-fixing trees that sustain nitrogen fixation decrease the net atmospheric cooling effect of forest ecosystems (as compared to non-fixing trees). Under extremely high nitrogen deposition rates, forests ecosystems display a net atmospheric warming effect, and are thus emitting more nitrous oxide than they are sequestering carbon dioxide.
Nitrogen-fixing trees, which are projected to increase in abundance due to climate change, thus have a significant influence on the net atmospheric effects of forest carbon sinks and warrant further investigation for the management of reforestation and for estimates of the capacity of the regenerating forest carbon sink.