Sodium shortage as a constraint on the carbon cycle in an inland tropical rainforest

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 NaP[O.sub.4]) resulted in equivalent cellulose loss, and ants in choice experiments underused Cl (as KCl, Mg[Cl.sub.2], and Ca[Cl.sub.2]) relative to NaCl and three other Na compounds (NaN[O.sub.3], [Na.sub.3]P[O.sub.4], and [Na.sub.2]S[O.sub.4]). 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. biogeochemistry | biogeography | decomposition | fungi | termites www.pnas.org/cgi/doi/10.1073/pnas.0906448106