Soil Warming and Drying Both Increase the Age of Soil Carbon Dioxide Emissions

The Science

Tropical forests account for over half of the global terrestrial carbon sink, but climate change threatens to alter the carbon balance of these ecosystems. A research team identified changes in soil carbon sources contributing to soil surface carbon dioxide (CO₂) emissions in Panamanian tropical forests subjected to either soil warming or drying. Researchers found both warming and drying increased the average age of carbon in soil CO₂ emissions by 2 to 3 years but for different reasons. Warming accelerated decomposition of older carbon as increased CO₂ emissions depleted newer carbon. Drying suppressed decomposition of newer carbon inputs and decreased soil CO₂ emissions.

The Impact

Tropical forests exchange more CO₂ with the atmosphere than any other terrestrial biome, store nearly one-third of global soil carbon stocks, and have the shortest mean residence time for carbon, as short as 6 to 15 years. Climate projections suggest a future that will be both warmer and drier for much of the tropics with increasing drought intensity and dry season length for the Neotropics. These findings imply both warming and drying will exacerbate soil carbon losses in tropical forests, which could have large and relatively rapid consequences for tropical ecosystem carbon balance and carbon-climate feedbacks.

Summary

The team measured soil CO₂ flux rates and collected CO₂ emitted from the soil surface at two climate manipulation experiments in Panamanian tropical forests. The Soil Warming Experiment in Lowland Tropical Rainforest (SWELTR) experiment uses whole-profile in situ heating of soil by 4°C to achieve soil warming. The Panama Rainforest Changes with Experimental Drying (PARCHED) experiment uses throughfall exclusion structures to remove 50% of precipitation inputs to achieve soil drying. Samples were collected in 2019 during contrasting seasons to assess the impact of experimental treatments on CO₂ flux and the carbon source contributing to soil CO₂ emissions.

Researchers observed increased carbon-14 in CO₂ released by soil with experimental warming and drying corresponding to an increase in the average age of the carbon by the equivalent of approximately 2 to 3 years. Importantly, the mechanisms underlying this shift differed between warming and drying. Warming accelerated decomposition of older carbon, while increased CO₂ emissions depleted newer carbon. Drying suppressed decomposition of newer carbon inputs and decreased soil CO₂ emissions, thereby increasing contributions of older carbon to CO₂ release. These results suggest both climate warming and drying will increase the vulnerability of previously stored soil carbon in tropical forests by stimulating the decomposition and loss of old carbon.

Principal Investigator

Karis McFarlane
Lawrence Livermore National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

Support was received from the Biological and Environmental Research program in the U.S. Department of Energy Office of Science through Early Career Research awards SCW1572 and DE-SC0015898. The study was further supported by a United Kingdom Natural Environment Research Council Grant NE/T012226.

References