Abstract
A major uncertainty in the land carbon cycle is whether symbiotic nitrogen fixation acts to enhance the tropical forest carbon sink. Nitrogen-fixing trees can supply vital quantities of the growth-limiting nutrient nitrogen, but the extent to which the resulting carbon–nitrogen feedback safeguards ecosystem carbon sequestration remains unclear. We combine (i) field observations from 112 plots spanning 300 years of succession in Panamanian tropical forests, and (ii) a new model that resolves nitrogen and light competition at the scale of individual trees. Fixation doubled carbon accumulation in early succession and enhanced total carbon in mature forests by ~10% (~12MgC ha−1) through two mechanisms: (i) a direct fixation effect on tree growth, and (ii) an indirect effect on the successional sequence of non-fixing trees. We estimate that including nitrogen-fixing trees in Neotropical reforestation projects could safeguard the sequestration of 6.7 Gt CO2 over the next 20 years. Our results highlight the connection between functional diversity of plant communities and the critical ecosystem service of carbon sequestration for mitigating climate change.
Highlights
A major uncertainty in the land carbon cycle is whether symbiotic nitrogen fixation acts to enhance the tropical forest carbon sink
We demonstrate that nitrogen fixation enhances the tropical carbon sink through a feedback with carbon accumulation, and that the strength of this effect depends on individual-scale competition between nitrogen fixing and non-fixing trees in the forest community
This plant functional type (PFT) was added to an existing set of three PFTs that have conventionally been used by ED2 to simulate tropical forests, including: (i) an early successional nonfixing PFT characterized by rapid growth, high mortality, low wood density, and high leaf nitrogen; (ii) a mid-successional nonfixing PFT, with medium growth, mortality, wood density, and leaf nitrogen; and (iii) a late-successional non-fixing PFT characterized by low growth and mortality, high wood density, and low leaf nitrogen (Methods; Table 1)
Summary
A major uncertainty in the land carbon cycle is whether symbiotic nitrogen fixation acts to enhance the tropical forest carbon sink. Nitrogen-fixing trees supplied ~50% of the nitrogen needed to support forest recovery in the first few decades of secondary succession in Panamanian forests[6] Whether this applies uniformly across tropical forests is an open question, and it is plausible that between-forest differences in populations of nitrogen-fixing trees contribute to the dramatically different rates by which tropical forests appear to recover from disturbance[7,8]. Of particular concern is whether models are structured in a way that captures the local-scale feedback between nitrogen fixation and the recovery of carbon pools following forest disturbance events[12,13,14] It is difficult, to directly measure the role of nitrogenfixing trees in the forest carbon cycle. By recycling fixed nitrogen, fixers can influence neighboring non-fixing trees, but this indirect effect is difficult to isolate and quantify in nature
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