Global warming could unlock carbon in tropical soil

Humble dirt could pack an unexpected climate punch, according to a study published in the journal Nature. An experiment that heated soil underneath a tropical rainforest to mimic temperatures expected in the coming decades found that hotter soils released 55 per cent more planet-warming carbon dioxide than did nearby unwarmed areas.

If the results apply throughout the tropics, much of the carbon stored underground could be released as the planet heats up. "The loss rate is huge," said Dr 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 covers much of our planet's land stores vast amounts of carbon - more, in total, than in all plants and the atmosphere combined.

That carbon feeds hordes of bacteria and fungi, which build some of it into more microbes while respiring the rest into the atmosphere as carbon dioxide. Many of these microbes grow more active at warmer temperatures, increasing digestion and respiration rates.

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

In an attempt to forecast the future, ecologists began, in the early 1990s, building apparatuses to artificially heat soils. Such experiments in temperate and boreal forests have shown that carbon-rich soils almost always belch carbon dioxide when warmed.

In 2016, a group of researchers estimated that, by 2050, soils 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 study left out the perpetually warm, mega-biodiverse tropics, where a third of all soil carbon resides. Figuring out the fate of this carbon would require grappling with the many pitfalls of doing research 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.

For understanding soils' contributions to climate change, the tropics "is a really important region" that "really hasn't been studied", said Dr Margaret Torn, an ecologist at Lawrence Berkeley Lab in California, who was not involved in the study.

In 2014, Dr Nottingham, then a postdoctoral fellow at the University of Edinburgh, travelled to Barro Colorado Island, a human-created 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 1.2m. For protection from the elements and ravenous insects, he shielded the wires inside metal structures shaped like freakishly large spiders. Measurements were logged inside weatherproof boxes.

"Our experiment was basically me as a postdoc making things out of a DIY shop," Dr Nottingham said.

The team encountered a number of hiccups, including poor electrical connections that blew up and cost the researchers nearly a year and much of their budget to repair.

Starting in November 2016, the electrical resistance of the wires began warming the soil by almost 6 deg F, within the range of how much the tropics are projected to warm by century's end according to current climate models.

Other equipment measured the carbon dioxide coming out of both the experimental plots and nearby plots that were not artificially warmed, as well as microbial activity in the plots.

An experiment warming soil in El Yunque National Forest in Puerto Rico had started two months earlier but was pummelled by back-to-back Category 5 hurricanes in September 2017; the study team did not turn the power back on for a year.

The results from Dr Nottingham's team are sobering: Over two years, warmed soils spewed out 55 per cent more carbon than control plots.

"This is a very large response," said Dr Torn, who runs a similar warming experiment in a California forest that reported a roughly 35 per cent increase in carbon emissions after two years. "It's one of the largest I've heard of."

If the entire tropics were to behave similarly, the researchers estimate that 65 billion metric tons of carbon would enter the atmosphere by 2100 - more than six times the annual emissions from all human-related sources.

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

And while there is agreement that climate models need to treat soil more realistically, how best to do that is unclear. The new study strikes a blow against simple theories predicting that tropical soils will respond weakly to warming, said Dr 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 a handle on the problem, she said, modellers will need information about how microbes respond to variations in soil moisture and nutrients in addition to temperature.

By warming only the soil, the Barro Colorado Island experiment did not capture how plants would fare under warmer conditions, said Dr Tana Wood, a US Forest Service ecologist who is leading the Puerto Rico experiment.

If plants were to photosynthesise more, for example, they could take up some of the carbon dioxide that soils release, making the overall impact on the climate less severe.

"This is only telling half the carbon story," she said. (Her team is warming both the soil and the air with infrared heaters and measuring how plants and microbes respond.)

Dr Torn said she was eager to see more than two years' worth of data, which could reveal whether the carbon dioxide spike is prolonged or short-lived. "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 another five years.

But even a short two-year period has shown how critical it is to find ways to keep ecosystems intact, he said. "It makes you realise how fortunate we've been up until this point to have a relatively stable climate."

NYTIMES