Project #9

Nitrogen Fixation
Alexander Barron

Nitrogen fixation, the ability to convert atmospheric nitrogen to a form usable by plants, is a key functional trait of many members of the species-rich Legume family (which includes soybean, common beans, and alfalfa). It allows legumes to flourish in many nitrogen limited habitats like grasslands and other fire-prone ecosystems. However, legumes are also abundant in most neotropical rainforests where nitrogen
is not thought to limit production. Why should legumes be so abundant in an ecosystem where nitrogen fixation is energetically more expensive than nitrogen uptake? And how is nitrogen fixation related to the relatively high rates of nitrogen transformation and nitrogen loss found in many tropical forest soils? My project is focused on understanding these questions by examining the links between nitrogen fixation at the
plant and the ecosystem scales.

My project is based at Barro Colorado Island (BCI), a fantastic field station run by Smithsonian Tropical Research in Lago Gatun in Panama. The scientists have been conducting research on BCI for over 80 years which provide a wealth of expertise, data, facilities. I spend significant amounts of time living on BCI surveying nodules on legume roots and measuring fixation in the field. In addition, I am measuring rates of fixation by microbes living in leaf litter and by epiphytes in the canopy (which I access with single rope climbing techniques or the canopy crane located at Ft. Sherman). A long term nutrient fertilization experiment on the Gigante peninsula (run by Drs. Joe Wright, Joe Yavitt and Kyle Harms) will allow me to examine how nutrient availability influences fixation rate. I am also collecting data to try to link nitrogen fixation to patterns of plant nutrient use and other aspects of plant strategy.

Alex climbing an Anacardium (Wild Cashew) for epiphyte samples
(photo by Charlotte Jander).

Root nodules of Lonchocarpus (Gallito). The pink color is a the
compound leghaemoglobin, a chemical produced by the tree to
prevent oxygen from damaging the nitrogen fixing bacteria living
in its roots (photo by Greg Dimijian).