Date of Award

Spring 2006

Document Type

Restricted Thesis

Terms of Use

© 2006 Aaron L. Strong. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts



First Advisor

José-Luis Machado


The nitrogen (N) input from non-symbiotic heterotrophic N fixing bacteria in the leaf litter is often overlooked in ecosystem N budgets. As a result, the ecological controls of the rate of nitrogen fixation by non-symbiotic nitrogen-fixing bacteria living on decaying leaf litter are poorly understood. The nitrogenase enzyme activity of free-living bacteria in a long-term leaf litter manipulation experiment in a lowland tropical rain forest in Panama was measured. Bacteria growing in plots with artificially doubled standing litter fixed nitrogen, on a per gram of litter basis, at a marginally higher rate than bacteria on control litter plots, indicating that physical or chemical changes associated with increased litter mass encourage N-fixation. There was, however, substantial micro- geographic variation indicating the potential for extremely localized "hot-spots" of N-fixation. In addition, C:N ratio and leaf chemistry may directly influence heterotrophic N-fixation. To investigate this, we measured nitrogenase enzyme activity of bacteria living on leaf litter of four common tree species: Oenocarpus panamanus, Aspidosperma cruenta, Tetragastris panamensis, and Prioria copaifera, N-fixation by bacteria growing on A. cruenta leaves was an order of magnitude higher than other species. A. cruenta is an evergreen canopy tree with unique alkaloid compounds and extremely long-lived leaves. Elemental analysis of A. cruenta leaves revealed that high C:N ratio, and, specifically, low N levels were driving rates of N-fixation on this species of leaf. These results suggest that traditional bulk assays of mixed litter may miss "hotspots" driven by particular combinations of leaf traits, underestimating fixation at the stand level.