Some of the planet’s most important carbon vaults aren’t vast forests or deep oceans — they’re hidden in chilly mountain soil, rich with the charred remains of ancient fires.
New research reveals that the high-altitude tropical forests of the Colombian Andes store significantly more carbon from wildfires than lower-elevation rainforests. These fire leftovers, called pyrogenic carbon or PyC, may linger in soil for hundreds to thousands of years, making them a surprisingly stable and underappreciated part of the Earth’s carbon cycle.
“These are the first data of this type for the northern Andes,” says Ted Feldpausch, an ecosystem scientist at the University of Exeter and one of the study’s senior authors. “The substantially larger fire-derived soil carbon found in the high Andean forests represents an important persistent carbon reservoir.”
Scientists collected and analyzed soil samples from 36 forest plots across Colombia — from the hot, humid Amazon Basin to the misty ridges of the High Andes. While all sites had some fire-derived carbon, the highest-elevation forests stored up to 10 times more PyC than the lowland rainforests.
That’s despite those cooler regions being drier and more fire-prone — or perhaps because of it. Fires that once scorched these mountains may have left a lasting imprint, depositing slow-decaying carbon that stuck around while more easily decomposed organic matter vanished over time.
The study, led by Carmen R. Montes-Pulido of Universidad Nacional Abierta y a Distancia, also found that the amount of PyC in the soil was linked to climate and soil chemistry. Cooler temperatures, lower rainfall, and more clay-rich soils all helped preserve this ancient carbon.
“The findings highlight the larger-than-expected contribution of PyC to the total carbon pool in Andean forest soils,” Montes-Pulido says. “It’s an important and stable form of carbon, particularly under projected global warming scenarios.”
Pyrogenic carbon forms when plant material burns but doesn’t completely combust — leaving behind blackened charcoal-like remnants. Unlike other forms of soil carbon, which cycle in and out of ecosystems over years or decades, PyC breaks down slowly, sticking around for centuries.
That makes it a key — and often overlooked — player in global climate models, which typically don’t account for the uneven distribution of fire carbon across ecosystems.
The team’s results also raise bigger questions. Have humans — through centuries of land use, agriculture, and controlled burning — played a role in shaping PyC stocks in the Andes? Or have natural fires driven most of the accumulation? Either way, the legacy of fire appears to be etched into the soil itself.
Understanding how much PyC exists — and how long it stays locked in place — could help refine carbon accounting in tropical mountain regions, which are often hotspots of biodiversity and under pressure from climate change.
As temperatures rise and fire patterns shift, researchers say it’s critical to better understand how fire and soil interact, especially in vulnerable ecosystems perched on tropical mountaintops.
“Our study highlights the long-term legacy effects of wildfires on Andean forest soils,” Feldpausch says. “And it underscores just how much history may be hidden in the ground beneath our feet.”
The study has been published in Global Change Biology.
