Warming and drying soils are losing more carbon

Tropical forests play a crucial role in the global carbon cycle, accounting for over half of the world’s terrestrial carbon sink. However, climate change poses a significant threat to the carbon balance within these ecosystems.

A new study has revealed that the warming and drying of tropical forest soils could heighten the vulnerability of soil carbon. This is primarily due to the increased degradation of older carbon stores. 

The research was led by scientists from Lawrence Livermore National Laboratory (LLNL), in collaboration with researchers from Colorado State University and the Smithsonian Tropical Research Institute.

Carbon storage in tropical forests 

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

Tropical forests exchange more carbon dioxide (CO2) with the atmosphere than any other terrestrial biome, and they hold nearly a third of global soil carbon stocks. These ecosystems are characterized by a short carbon residence time, typically between 6 to 15 years. 

This short duration means that changes in carbon inputs or outputs, including CO2 emissions from soil, can have significant and relatively rapid effects on the carbon balance of tropical ecosystems, potentially exacerbating carbon-climate feedback loops.

Soil warming and reduced rainfall 

Projections for tropical regions suggest a future with warmer temperatures and reduced rainfall, leading to longer and more intense droughts, particularly in the Neotropics, which spans from southern Mexico through Central America and into northern South America, including the Amazon rainforest.

The research, which involved climate manipulation experiments in Panama’s tropical forests, demonstrated that both soil warming – by 4°C – and a reduction in rainfall – by 50% – increased the age of carbon released from the soil by 2 to 3 years, as reflected by higher carbon-14 levels in the emitted CO2.

Varying effects of warming and drying

Interestingly, the processes behind this carbon shift differed between warming and drying. Soil warming sped up the breakdown of older carbon by increasing CO2 emissions, as newer carbon was quickly depleted. 

In contrast, drying reduced the decomposition of fresh carbon inputs, leading to a decrease in overall soil CO2 emissions but a higher contribution of older carbon to the CO2 being released.

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

Previous studies in tropical forests primarily focused on the total amount of CO2 being released, which is important for understanding the overall carbon balance but falls short of explaining the underlying mechanisms. 

Age of carbon released by soil

By analyzing carbon-14 values, this study was able to determine the age of the carbon being metabolized and emitted as CO2, offering insights into these processes.

In this context, “new” or “young” carbon refers to carbon that has been fixed from the atmosphere in recent years, while “decadal aged” carbon contains higher levels of carbon-14 relative to the current atmosphere. Even older carbon, from centuries to millennia, is depleted in carbon-14.

The team studied the effects of warming and drying on the amount and age of carbon released as soil CO2 in two distinct lowland tropical forest areas in Panama, where experimental soil warming and drying were conducted. They measured the carbon-14 and carbon-13 isotopes of the CO2 emitted from the soil.

Soil warming releases older carbon 

Utilizing the Center for Accelerator Mass Spectrometry at LLNL, the team found that soil warming increased carbon-14 in CO2 emissions during the wet season, indicating a greater release of older carbon, including “bomb” carbon from nuclear testing in 1963, under both warm and wet conditions. 

The warming accelerated the breakdown of older carbon as microbial communities shifted to using older carbon after depleting fresh organic matter. 

Soil drying limits access to fresh carbon 

On the other hand, drying decreased the total release of CO2 from the soil but also increased the carbon-14 in the remaining emissions by limiting the availability of fresh carbon from sources like leaf litter and roots.

“This limitation of microbial access to fresh carbon explains the shift toward increased contributions of older carbon in total soil CO2 emissions with warming and drying,” said McFarlane.

“Our results suggest that climate change will increase the vulnerability of previously stored soil carbon in tropical forests by stimulating the decomposition and loss of old carbon.”

The study is published in the journal Nature.

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