Global warming may trigger rapid release of soil carbon

Earth’s soils are a powerful carbon reservoir. In fact, soils contain more than twice as much carbon as the atmosphere holds. As temperatures climb and weather patterns shift, the storage or release of soil carbon becomes a critical piece of the climate puzzle.

Scientists have long focused on the melting permafrost, where rising temperatures thaw frozen soil and unleash carbon into the air. But in subtropical and tropical regions, where vast stores of organic carbon also sit, the key driver of carbon release has been less clear.

Some research has pointed to shifts in rainfall as the main culprit. Other studies identified temperature as the bigger influence.

Dr. Vera Meyer, a scientist at the MARUM Center for Marine Environmental Sciences, is the lead author of a recent investigation.

“Microbes that break down organic matter are generally more active under warm and humid conditions, so the carbon content in tropical soils responds very quickly to climatic changes. Some studies report a main influence of changing hydroclimatic conditions, while in others temperature plays the main role,” said Dr. Meyer.

Studying changes in soil carbon

To sort this out, the research team decided not to analyze soil directly. Instead, they studied what the Nile River had washed out to sea over thousands of years.

The Nile drains a massive area that stretches across subtropical and tropical parts of northeast Africa. As it flows, it picks up organic material from the land and eventually deposits it into the Mediterranean.

To study how soil carbon changed over time, the researchers turned to a sediment core collected from the seafloor near the Nile Delta. This record holds layers of material built up over thousands of years and offers a timeline that stretches back to the last ice age.

“The age of the organic material delivered by the Nile essentially depends on two factors: how long it was in the soils, and how long it took to be transported in the river. The advantage of our approach is that long time scales can be investigated, in this case the last 18,000 years since the last ice age,” said Enno Schefuß, also from MARUM.

Carbon release from soils

The results surprised the team. It turned out that changes in rainfall and runoff didn’t significantly affect how old the soil carbon was. Temperature, on the other hand, had a big impact.

As the Earth warmed after the last ice age, the age of carbon in the Nile sediments dropped sharply. This meant that warmer conditions had accelerated the breakdown of soil organic matter.

Microbes had gotten to work more quickly, releasing carbon dioxide in the process. This spike in decomposition was far stronger than what current carbon cycle models had predicted.

“The fact that the models underestimate carbon release from soils so strongly shows us that we need to revise the sensitivity of soil carbon in our models,” said study co-author Dr. Peter Köhler from AWI Bremerhaven.

Major factor in climate predictions

Most people don’t think of soil as something that affects the air we breathe, but it does. Soils are one of the planet’s largest natural carbon sinks, storing huge amounts of organic matter.

When this material breaks down, carbon dioxide is released – and that has direct consequences for the climate.

In subtropical and tropical regions, this breakdown can happen faster. Warm temperatures and moisture help microbes become more active, speeding up the process. That means these soils can quickly go from storing carbon to releasing it.

Scientists have started to pay closer attention to this shift. How soils respond to temperature changes is turning out to be a major factor in climate predictions. Yet many climate models still underestimate just how sensitive these soils are.

The study of Nile River sediments helps fill in some of these gaps. By tracking how soil carbon changed over thousands of years, researchers are getting a clearer picture of just how much influence soils can have on our future.

A warning for the future

This isn’t just a story about ancient climates. The same temperature-driven feedback that happened thousands of years ago could be repeating itself today – with serious consequences.

As the planet continues to heat up, tropical and subtropical soils may once again start releasing more carbon into the air. If our models are underestimating this effect, we may also be underestimating how fast climate change could accelerate.

The research was supported by the Cluster of Excellence “Ocean Floor – Earth’s Uncharted Interface,” based at MARUM at the University of Bremen.

The team is working to better understand how carbon travels through the ocean floor system, from land to sea – and what that means for our future.

The full study was published in the journal Nature Communications.

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