Biodiversity of nitrogen fixers facilitate tropical forest carbon recovery

Background/Question/Methods

Tropical forests are globally important sinks of atmospheric carbon, but their sustained function depends on how carbon uptake is influenced by the nitrogen cycle.  Symbiotic fixers are common, rich in species, and capable of fixing large quantities of N₂ in tropical forests, but we know little about whether and when fixation may influence the carbon sink.  We here ask whether carbon gain during secondary succession depends on nitrogen fixation, and, in turn, on the species diversity of fixers and their individual strategies of fixation.

We measured biomass recovery, fixation activity, and diversity of symbiotic fixers in a Panamanian rainforest chronosequence over several hundred years of recovery from pasture abandonment.  We focused on the nine species that make up the majority of basal area of potential nitrogen fixing trees, for a total of 272 intensively studied individuals across the chronosequence.  In addition, we considered both potential fixing and non-fixing species in 16 replicate plots (~0.1 ha) distributed across ages since disturbance.  For all plots we considered all individuals across all species and calculated biomass increment, community-level nitrogen demand, and nitrogen fixation with the goal of determining the extent to which biomass gain was supported by nitrogen fixation.

Results/Conclusions

We found that tropical fixers grew in forests of all stages of recovery, but that patterns of fixation depended on forest age since disturbance and species identity.  N₂ fixation peaked in forests of 12 years old, reached a minimum in 80-year-old forests, and had a resurgence of some activity in mature forests (300+ years).  Not all species of fixers behaved in similar ways: some specialized on young forests while others preferred mature forests; two species in particular contributed to the majority of fixation.  Fixation was not consistent with patterns of total soil nitrogen and phosphorus, which remained constant across forest ages, but met the nitrogen deficit experienced by the rapidly growing young forest.  We conclude that diverse fixers provides the nitrogen needed to support forest growth and carbon accumulation in recovering tropical forests, that species differences matter for ecosystem-level patterns of fixation, and that the presence of currently dormant potential fixers poise tropical forests for a rapid response to climate change.  These findings imply that more accurate predictions of terrestrial carbon uptake rest on the depiction of dynamic fixation in global biogeochemical models.