Tropical forests face increased soil carbon loss due to climate change

Tropical forests account for more than 50% of the global terrestrial carbon sink, but climate change threatens to alter the carbon balance of these ecosystems.

New research by scientists at Lawrence Livermore National Laboratory (LLNL) and colleagues at Colorado State University and the Smithsonian Tropical Research Institute has found that warming and drying of tropical forest soils can increase soil carbon vulnerability by increasing the degradation of older carbon. The research is published in Nature.

“These results imply that warming and drying, by accelerating the loss of carbon from older soil or reducing the incorporation of new carbon inputs, will intensify soil carbon losses and negatively impact carbon storage in tropical forests under climate change,” said LLNL scientist Karis McFarlane, lead author of the study.

Tropical forests exchange more CO₂ with the atmosphere than any other terrestrial biome and store nearly one-third of the world’s soil carbon stocks. Tropical terrestrial ecosystems also have the shortest average carbon residence time on Earth, as short as 6 to 15 years, meaning that any changes in carbon inputs or outputs (including CO₂ (emitted by the soil) could have significant and relatively rapid consequences on the carbon balance of tropical ecosystems and on carbon-climate feedbacks.

Climate projections suggest a future that is both warmer and drier for much of the tropics, with increasing drought intensity and increasing dry season length for the Neotropics (a region extending from southern Mexico through Central and northern South America, including the vast Amazon rainforest).

The research, conducted during climate manipulation experiments in the rainforests of Panama, shows that warming the entire soil profile in situ by 4°C and excluding 50% of precipitation increased carbon-14 in CO₂ released from the soil, increasing the average carbon age by the equivalent of about 2 to 3 years.

It is important to note that the mechanisms underlying this change differ between warming and drying. Warming accelerated the decomposition of ancient carbon as CO increased₂ Emissions depleted newer carbon. Drying suppressed decomposition of new carbon inputs and decreased soil CO.₂ emissions, thereby increasing the contributions of older carbon to CO₂ release.

“Field and laboratory experiments suggest that global warming will stimulate a net loss of carbon from global soils to the atmosphere, but how climate warming and drying will interact to influence the carbon balance in forests and other ecosystems is less clear,” McFarlane said.

Most previous work on tropical forests only considered total CO.₂ Flux rates, which are important for determining the overall carbon budget of tropical forests, are, however, limited in their ability to uncover the mechanisms behind the observed changes. These mechanisms can be revealed by carbon-14 values, which indicate the average age of carbon sources metabolized and released as CO₂.

“New” or “young” carbon has been fixed in the atmosphere in recent years, while older “decadal-age” carbon is enriched in carbon-14 relative to the current atmosphere. Even older “centennial-age” or “millennial-age” carbon is depleted in carbon-14 relative to the current atmosphere.

In the current study, the team determined how warming and drought impact the amount and age of carbon released as CO2 in the soil.₂ in two separate areas of lowland tropical forests in Panama that are undergoing experimental soil warming or drying. They measured the carbon-14 and carbon-13 isotopes of CO2 respired by the soil₂.

Using LLNL’s Accelerator Mass Spectrometry Facility, McFarlane and his team found that soil warming increased the carbon-14 content of respired CO.₂ during the rainy season, indicating a greater release of “bomb” carbon (circa 1963, from underground nuclear tests) under warm, humid conditions. Specifically, warming stimulated the decomposition of older soil carbon by increasing the overall soil CO concentration₂ release, causing a microbial shift in resource use from fresh organic matter to older soil carbon. In contrast, drying reduced total soil CO₂ release, but also increase of carbon-14 from respired CO₂ by limiting the supply of fresh carbon (from leaf litter or roots) to decomposers.

“This limitation of microbial access to fresh carbon explains the shift towards increased contributions of older carbon to total soil CO.₂ “CO2 emissions linked to global warming and drought are increasing,” McFarlane said. “Our results suggest that climate change will increase the vulnerability of carbon stored in tropical forest soils by stimulating the decomposition and loss of ancient carbon.”