EAGER: Does Na availability regulate tropical decomposers?

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Much of Earth's carbon cycle can be described briefly thus: plants scrub carbon dioxide from the atmosphere; animals, fungi, and bacteria consume plants and release carbon dioxide back into the atmosphere. This proposal posits that sodium (the Na in NaCl, or table salt) plays a key, under-appreciated role in regulating atmospheric carbon dioxide. Most plants don't need sodium, but decomposer animals and fungi do (and suffer in low-sodium ecosystems like a sports team deprived of Gatorade). Low-sodium ecosystems should thus tend to store carbon. Ecosystems farther inland receive less and less ocean source sodium in their rainfall. Decomposers of earth's vast continental interior should thus be especially sodium deprived, less active, and less likely to release carbon back into the atmosphere. We will test this hypothesis in two ways. First, for three months, 5 x 5 m patches in an Ecuador forest 2500 km inland will receive one of two treatments. Half will be regularly watered from a nearby river. The other half will get river water charged with NaCl (about the amount found in a year's worth of coastal rainfall). This experiment will test if more of a forest's leaf and woody litter decomposes with added sodium. Second, small-scale experiments will compare how ecosystem responses vary geographically between two forests: one, our site in Ecuador, the other in Panama, only 25 km from the ocean. A common sodium supplement-urine-will be the experimental focus. Four treatments--synthetic urine, urine sans NaCl, NaCl, and water--will be added to small patches of forest for three weeks. If the above hypothesis is true, the NaCl component of urine should increase decomposition more in the Ecuador forest. Both experiments will monitor sodium effects up the food web on decomposer arthropods and their predators, which comprise about half the biodiversity in a tropical forest.Most of Earth's terrestrial ecosystems are far enough from the ocean to experience sodium limitation. The work funded by this proposal thus has the potential to significantly increase understanding of the Earth's climate system. If the amount and distribution of rainfall changes with global warming, as predicted by current models, this may mean changes in sodium deposition, which in turn could increase or decrease atmospheric carbon dioxide. The biodiversity part of this study will collect thousands of specimens (and, almost certainly, species not yet described) in this poorly understood part of the world. A graduate student will be trained in a multidisciplinary endeavor linking earth science, evolutionary biology, and ecosystem ecology. She in turn will mentor an undergraduate from the University of Oklahoma, who will join us in the field and experience biodiversity science first hand.