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Tropical soils may accelerate global warming

9 September 2020

New experiments challenge climate simulations that predict minimal carbon loss from soils in the tropics.

Hiking with equipment to heat soil.
Researchers hike through the forest on Barro Colorado Island, Panama, with heating structures to be buried in the soil. Credit: Geetha Iyer

Earth’s soils store about 1.5 trillion metric tons of carbon, but the carbon isn’t necessarily there to stay. It can be lost when microbes digest organic matter and respire carbon dioxide to the atmosphere. The decomposition process tends to accelerate as global temperatures increase. Studies in temperate and arctic forests have shown that high-latitude soils contribute positive feedback to climate change. Climate models predict smaller relative increases in carbon loss from tropical soils, even though they store about 30% of the world’s soil carbon. Now Andrew Nottingham (University of Edinburgh) and colleagues provide evidence that warmed tropical soil may release more stored carbon than expected. The findings suggest that tropical soils are less stable carbon stores in a warming world.

Tropical forest soil from Barro Colorado Island, Panama.
Tropical soil in plots like this one were analyzed in the experiment. Credit: Ben Turner

Models that relate microbial respiration response to temperature changes suggest that the response is less pronounced in tropical than in temperate soil. To see how that theory plays out in real-world conditions, Nottingham and his colleagues conducted an experiment in a tropical rainforest on Barro Colorado Island, Panama. By placing 1.2-m-long electrical rods into soil, arranged along the perimeters of 20 m2 plots, they increased the entire soil profile temperature by 4 °C for two years. They measured the CO2 output by placing weatherproof chambers equipped with monitoring instruments over the warmed plots and compared them with nearby ambient-temperature control plots. The measurements showed that the warmed soils emitted 55% more CO2 than the ambient plots. (For comparison, the emissions from heated soils increased by about 35% in a similar experiment in a temperate forest.) By ensuring that the sample plots encompassed areas free of plant roots and other nonmicrobial carbon sources, the researchers concluded that the higher measured CO2 release rate was a result of increased soil microbe respiration.

Differences in soil nutrients, moisture content, and microbial communities need to be considered when using the results to estimate effects at large scales and adapting climate models to more accurately represent global tropical soil contributions, the authors say. Human development, such as agricultural methods and changes in land use, should be carefully evaluated to reduce their effects on the vulnerable soils. (A. T. Nottingham et al., Nature 584, 234, 2020.)

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