Global warming could unleash carbon from tropical soils

Humble dirt could pack an unexpected climate punch, according to a new study published Wednesday in the journal Nature. An experiment that heated soil under a tropical rainforest to temperatures expected in the coming decades found that warmer soil released 55 percent more planet-warming carbon dioxide than near unheated areas.

If the results were true in the entire tropics, many of the carbon stored under coal could be released as the planet warms.

“The loss rate is enormous,” said Andrew Nottingham, an ecologist at the University of Edinburgh who led the study. “It’s a bad news story.”

The thin skin of soil that occupies much of the land of our planet absorbs large amounts of carbon – more, in total, than in all plants and the atmosphere combined. That carbon feeds on hordes of bacteria and fungi, some of which build up in more microbes, while the rest in the atmosphere breathes like carbon dioxide. Many of these microbes grow actively at warmer temperatures, increasing their digestion and respiration rates.

The finding “is another example of why we need to worry more” about how fast the world is warming, said Eric Davidson, an environmental scientist at the University of Maryland College of Environmental Science in Frostburg who was not involved in the research.

In an attempt to predict the future, ecologists began in the early 1990s to build devices on soils on an artificial basis. Such experiments in temperate and boreal forests have shown that carbonaceous soils almost always emit carbon dioxide when heated. In 2016, a group of researchers estimated that soil, by 2050, could release so much of the planet-warming gas that it would be like adding the carbon emissions of a new country the size of the United States.

But that research left out the ever-hot, mega-biodiverse tropics, where a third of all soil carbon lives. Calculating the fate of this carbon would need to deal with the many pitfalls to investigate in the tropics: humidity, storms and a multitude of hungry animals that can take a toll on research equipment – chewing through electrical wires or protective coverings, for example – and on researchers themselves.

To understand the contribution of soil to climate change, the tropics are “a really important region” that “really has not been studied,” said Margaret Torn, an ecologist at Lawrence Berkeley Lab in California, who was not involved. at the study.

In 2014, Dr. Nottingham, then a postdoctoral fellow at the University of Edinburgh, moved to Barro Colorado Island, a man-made island in the Panama Canal area that is home to the Smithsonian Tropical Research Institute. He buried electrical wires in five circular plots to a depth of nearly four feet. For protection against the elements and ravenous insects, he protected the wires in metal structures that were shaped like freakishly large spiders. Measurements were logged in value boxes.

“Our experiment was basically me as a postdoc making things out of a DIY store,” said Dr. Nottingham.

The team encountered a number of hurdles, including poor electrical connections that blew the researchers away and cost nearly a year and much of their budget to repair. In early November 2016, the electrical resistance of the wires began to warm the soil by almost 6 degrees Fahrenheit, within the range of how much the tropics are expected to reach by the end of the century according to current climate models. Other equipment measures the carbon dioxide emitted from both experimental plots and nearby plots that were not artificially heated or microbial activity in the plots.

A warming soil experiment in El Yunque National Forest in Puerto Rico had turned on two months earlier, but was pummeled in September 2017 by back-to-back Category 5 hurricanes; the student team did not re-enter power for a year.

The results of the team of Dr. Nottingham are sober: Over two years, warm soils emit 55 percent more carbon than control plots. “This is a very big reaction,” said Dr. Torn, who is conducting a similar warming experiment in a California forest that reported a roughly 35 percent increase in carbon emissions after two years. “It’s one of the biggest I’ve heard.”

If the entire tropics behaved in the same way, researchers estimate that 65 billion metric tons of carbon would enter the atmosphere by 2100 – more than six times the annual emissions of all human-related resources.

Scaling the results to account for the entire tropics, however, is complicated. The soils on Barro Colorado Island are richer in nutrients than many others, such as those of much of the great Amazon rainforest, noted Dr. Davidson. This could make it easier for Panamanian microbes to increase their activity. Microbial communities in African and Asian lands are very different from those in America, added Drs. Torn.

And while it has been agreed that climate models need to treat soil more realistically, it is best to do so vaguely. The new study strikes a blow against simple theories that predict that tropical soils will respond poorly to warming, said Kathe Todd-Brown, a soil scientist at the University of Florida in Gainesville who was not part of the research team. But to really get to the problem, she said, modellers will need information on how microbes respond to variations in soil meal and nutrients in addition to temperature.

By simply heating the soil, the Barro Colorado Island experiment did not determine how plants would ride under warmer conditions, said Tana Wood, an ecologist from the U.S. Forest Service who is leading the experiment in Puerto Rico. If plants were more photosynthesizing, they could absorb some of the carbon dioxide released by the soil, making the overall impact on the climate less severe. “This accounts for only half of the carbon footprint,” she said. (Their team heats both soil and air with infrared heaters and measures how plants and microbes react.)

Dr Torn said she was apprehensive about seeing data from more than two years, which could reveal if the carbon dioxide spike is long or of short life. “In the life of a tropical forest, that’s a very short time,” she noted.

Dr Nottingham has funding to keep the Panama project running for at least five years. But even two years has shown how critical it is to find ways to keep ecosystems intact, he said. “It makes you realize how happy we are up to this point to have a relatively stable climate.”