"Sodium intrigued us because plants by in large don't need it, while consumers do, and sodium has a geography – its availability typically drops the farther you are from the ocean," Mike Kaspari of the University of Oklahoma told environmentalresearchweb. "This suggested a disconnect between plant production and its subsequent consumption that is driven by the availability of sodium."
According to Kaspari, where an adequate supply of sodium is available, consumers should be able to "keep up" with photosynthesis, releasing the carbon as fast as it is fixed as plant tissue. But where sodium is in short supply, consumers should "cramp up" and slow down, while plants keep fixing carbon. "It sounded like an interesting hypothesis, and, frankly, we were surprised to find it hold up," he said.
Sodium tends to be readily available in terrestrial ecosystems near the coast because it's exported from the oceans in the form of oceanic aerosols that are deposited inland. "Any increase in the export, say due to the increased frequency of hurricanes, could, theoretically, decrease inland carbon stores by enhancing the activity of inland consumers," said Kaspari. So climate change could lead to a hitherto unexpected effect on tropical carbon storage.
Kaspari and colleagues from the University of Oklahoma, US, the Smithsonian Tropical Research Institute in Panama, the University of Arkansas, US, and the University of California Berkeley found that introducing sodium chloride solution to the leaf litter in an inland Amazon forest near Iquitos, Peru, enhanced mass loss by 41% and boosted decomposition of pure cellulose by up to 50%. Numbers of termites – a common decomposer – increased seven-fold, while ant numbers doubled. Ants are predators in the brown food web, a system of microbes and invertebrates that feed on detritus.
"The teeming Amazon jungles may teem a bit less than they otherwise would."
Because around 80% of global landmass lies more than 100 km inland, the researchers believe that carbon stocks and consumer activity may frequently be regulated by sodium limitation. While this study focused on tropical rainforest, they reckon that there could be a similar effect for inland boreal forests and fire-maintained grasslands. The use of road salt in cold climates in winter may also affect carbon storage.
"[Our work] suggests a major, previously unappreciated player in the action of the global carbon cycle," said Kaspari. "In the vast inland terrestrial ecosystems, sodium shortage should slow the activity of decomposers like termites and microbes, promoting the accumulation of carbon. For us, the notion that the activity of consumers in ecosystems may map onto a simple aspect of geography like distance inland, is simply intriguing. The teeming Amazon jungles may teem a bit less than they otherwise would."
Now the researchers have received funding for a pilot project that will fertilize large plots in Amazonian Ecuador with sodium solutions mimicking the concentrations found falling on coastal tropical forests. "If termite populations and decomposition rates of litter increase on these plots simply due to tweaking the sodium concentration, we will be more confident that our conclusions 'scale up'," said Kaspari.
The researchers reported their work in PNAS.