Successional dynamics of nitrogen fixation and forest growth in regenerating Costa Rican rainforests.

Regenerating tropical forests have an immense capacity to capture carbon and harbor biodiversity. The recuperation of the nitrogen cycle following disturbance can fuel biomass regeneration, but few studies have evaluated the successional dynamics of nitrogen and nitrogen inputs in tropical forests. We assessed symbiotic and asymbiotic nitrogen fixation, soil inorganic nitrogen concentrations, and tree growth in a well-studied series of five tropical forest plots ranging from 19 yr in age to old-growth forests. Wet-season soil inorganic nitrogen concentrations were high in all plots, peaking in the 29-yr-old plot. Inputs from symbiotic nitrogen fixation declined through succession, while asymbiotic nitrogen fixation peaked in the 37-yr-old plot. Consequently, the dominant nitrogen fixation input switched from symbiotic fixation in the younger plots to asymbiotic fixation in the older plots. Tree growth was highest in the youngest plots and declined through succession. Interestingly, symbiotic nitrogen fixation was negatively correlated with the basal area of nitrogen-fixing trees across our study plots, highlighting the danger in using nitrogen-fixing trees as a proxy for rates of symbiotic nitrogen fixation. Our results demonstrate that the nitrogen cycle has largely recuperated by 19 yr following disturbance, allowing for rapid biomass regeneration at our site. This work provides important insight into the sources and dynamics of nitrogen that support growth and carbon capture in regenerating Neotropical forests.