Amazon's Low Salt Content Keeps Carbon Emissions at Bay

Christine Dell'Amore
National Geographic News
November 4, 2009
Supposedly the most robust of the world's rain forests, the Amazon jungle suffers from "chronic malnutrition" due to a lack of salt, according to the lead scientist behind a new study.

And that might not be a bad thing, because the carbon build-up spurred by lack of salt in some forests may be keeping our atmosphere cooler.

Decomposers—life-forms that munch on dead plants—don't get enough of the vital mineral, which deep in the rain forest comes primarily from mammal urine.

That lack of salt keeps decomposer numbers in check, while plants, which don't need salt, flourish, piling up carbon on the forest floor when they die.

"The tropics is a place for happy plants and less happy" decomposers, said study leader Michael Kaspari, a zoologist at the University of Oklahoma.

When researchers sprinkled a salt solution in the Peruvian Amazon, plant-eaters such as termites and bacteria sprung to life and quickly devoured the detritus.

"We were terribly surprised to find how fast a little bit of sodium in the ecosystem starts [the process of] breaking down accumulated litter," Kaspari said.

Recycling dead matter keeps forests more productive and moves carbon more quickly through the environment—one of the most crucial cycles of life. But decomposition releases carbon dioxide, a potent greenhouse gas that contributes to global warming.

To better predict how much greenhouse gas might be released in the future, scientists are struggling to find any unknown carbon stores. "I suspect," Kaspari said, "some of that missing carbon may be lying around in tropical rain forests."

Just Add Salt

Salt hunger is innate in most animals, and previous studies have shown that landlocked animals and insects particularly crave salt.

In additon to mammal urine, hurricanes bring salt to inland environments—though rarely—by blowing seawater thousands of kilometers from the coasts.

For their experiment, Kaspari and colleagues added measured amounts of water to pairs of plots in an old-growth forest near Iquitos (map), Peru, about 1,242 miles (2,000 kilometers) from the ocean.

Every other day the team added salt and stream water to 35 of the plots, while another 35 received just stream water.

For each salted plot, the team used roughly the amount of sodium that's in a splash of monkey urine—apparently a common type of precipitation in the Amazon.

"If you work in a tropical rain forest long enough, you'll be buzz-bombed by a monkey," Kaspari added.

After 18 days "some of the scariest-looking termites I've ever seen" began swarming in the salted plots, increasing sevenfold in number, Kaspari said.

Ants—a common termite predator—increased twofold in number, Kaspari said.

Overall the leaf litter in the salty plots began disappearing 41 perecent faster than before, according to the study, which appears this week in the journal Proceedings of the National Academy of Sciences.

The new study reveals that salt can not only speed up decomposition, it can encourage population explosions in species higher up the food chain.

"What is new about it—and is actually pretty cool—is we had very little idea of how [salt] propagates in the ecosystem," said Carlos Martínez del Rio, a physiological ecologist at the Univeristy of Wyoming who was not involved in the research. "It's like a salt cascade."

The research was funded in part by a grant from the National Geographic Society's Committee for Research and Exploration. (The National Geographic Society owns National Geographic News.)

Salt in the Wound?

If hurricanes ramp up due to warming temperatures, as some scientists have suggested, inland ecosystems could get saltier.

Such a sodium jolt could in turn worsen climate change, Kaspari said.

"What happens if suddenly the Amazon gets all the sodium it wants?" Kaspari said.

"All this litter starts breaking down, and there's a heck of a lot more [of the greenhouse gas] CO2 in the atmosphere."

In that sense, Kaspari said, "the Amazon forest animals' loss is our gain."

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