Warming Soil In The Tropics Could Be A Bigger Problem Than We Thought

Researchers carrying experimental equipment on Barro Colorado Island, Panama. Courtesy of Geetha Iyer

Climate change is a clear case of “letting the genie out of the bottle.” Once begun, vicious circles can emerge that further disturb the planet’s climate and continue to accelerate the global temperature. One well-known climate feedback loop is melting ice. Climate change causes bright reflective ice in the Arctic and Antarctic to melt, leaving behind darker-colored land that absorbs more sunshine. In turn, this leads to further warming temperatures, more ice melting, and so on.

A new study has now unearthed how climate change could tip the balance of one more of these feedback loops: warming soils in the planet’s tropical regions could cause its microbial inhabitants to release carbon dioxide from storage. 

Researchers have long known that climate change could upset this cycle because warmer temperatures cause soil-dwelling bacteria and fungi to become increasingly active and respire more. However, the carbon in tropical soils – where microbes are even more diverse and plentiful – was previously thought to be less vulnerable to this process compared to other latitudes. The new research suggests this isn’t necessarily the case. 

Reported in the journal Nature this week, a real-world experiment led by scientists at the University of Edinburgh has revealed that carbon emissions of soil in the tropics could rise by 55 percent if the Earth's climate warms by 4°C. 

Researcher Ben Turner working in a soil pit on Barro Colorado Island. Courtesy of Sean Mattson

Vast quantities of carbon are stored in soil. This carbon, in the form of deadwood, roots, and decaying leaves, feeds the billions of bacteria and fungi that can live in a single teaspoon of soil. Carbon is then released into the atmosphere through the respiration of these trillions upon trillions of microorganisms. How much carbon is released, however, depends on the variety and quantity of microbes that live in the soil, as well as the mineral composition of the soil and the ground temperature. When scientists previously looked to see how much carbon is released by warming soils in the real world, they typically stuck to experiments in temperate regions simply because it's more practical.

To find out the effect of warming in real-world tropical soils, the researchers set up a large experiment in the much more challenging environment of tropical forests on Barro Colorado Island in the Panama Canal. They planted heating devices 1-meter (3.3-foot) into the soil that maintained the soil temperature by 4°C. Over the course of two years, they found that the artificially heated soils produced a high increase (up to 55 percent increase) in soil carbon dioxide emissions.

The researchers note that it’s unlikely the warmed soil will continue to pump out this increasing quantity of carbon as time goes on, although it’s currently unknown how long it might take. The long-term impact of soil warming on climate change is also not perfectly clear, but it's fairly evident there will be "consequences," the researchers say.

"Carbon held in tropical soils is more sensitive to warming than previously recognized," lead author Andrew Nottingham, a researcher at the University of Edinburgh's School of Geosciences, told AFP.

"Even a small increase in respiration from tropical forest soils could have a large effect on atmospheric CO2 concentrations, with consequences for global climate."


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