Sodium shortage as a constraint on the carbon cycle in an inland tropical rainforest
- Departments of aZoology and
- eGeography, Graduate Program in Ecology and Evolutionary Biology, University of Oklahoma, Norman, OK 73019;
- bSmithsonian Tropical Research Institute, Apartado 2072, Balboa Panama;
- cDepartment of Biology, University of Arkansas, Little Rock, AR 72204; and
- dDepartment of Integrative Biology, University of California, Berkeley, CA 94720
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Edited by Gordon H. Orians, University of Washington, Seattle, WA, and approved September 29, 2009 (received for review June 9, 2009)
Abstract
Sodium (Na) is uncommon in plants but essential to the metabolism of plant consumers, both decomposers and herbivores. One consequence, previously unexplored, is that as Na supplies decrease (e.g., from coastal to inland forests), ecosystem carbon should accumulate as detritus. Here, we show that adding NaCl solution to the leaf litter of an inland Amazon forest enhanced mass loss by 41%, decreased lignin concentrations by 7%, and enhanced decomposition of pure cellulose by up to 50%, compared with stream water alone. These effects emerged after 13–18 days. Termites, a common decomposer, increased 7-fold on +NaCl plots, suggesting an agent for the litter loss. Ants, a common predator, increased 2-fold, suggesting that NaCl effects cascade upward through the food web. Sodium, not chloride, was likely the driver of these patterns for two reasons: two compounds of Na (NaCl and NaPO4) resulted in equivalent cellulose loss, and ants in choice experiments underused Cl (as KCl, MgCl2, and CaCl2) relative to NaCl and three other Na compounds (NaNO3, Na3PO4, and Na2SO4). We provide experimental evidence that Na shortage slows the carbon cycle. Because 80% of global landmass lies >100 km inland, carbon stocks and consumer activity may frequently be regulated via Na limitation.
Footnotes
- 1To whom correspondence should be addressed. E-mail: mkaspari{at}ou.edu
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Author contributions: M.K., S.P.Y., R.D., and N.A.C. designed research; M.K., S.P.Y., and N.A.C. performed research; M.K. and M.Y. analyzed data; and M.K. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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↵* The litter chemistry of two +NaCl plots was unknown as termites had eaten all of the litter.
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↵† Murphy SF, Stallard RF, The Third Interagency Conference on Research in the Watersheds, September 8–11, 2008, Estes Park, CO.
