Tropical forests usually stand as protectors. Their trees capture carbon, cool the planet, and support countless species. For decades, scientists assumed these forests would resist warming better than others.
A new study shows the opposite. The soils of the tropics could release vast amounts of carbon dioxide as the Earth heats up.
The research comes from the U.S. Forest Service, with Chapman University as a senior collaborator. The results suggest that tropical soils are not passive players. They may speed up global warming instead of slowing it.
Soil warming boosts carbon
The experiment took place in Puerto Rico’s Luquillo Experimental Forest. Using infrared heaters, scientists raised soil temperatures by 4°C (7.2°F) and monitored respiration every half hour for an entire year.
The findings stunned the team. Warmed soils released 42 to 204 percent more carbon dioxide than unheated plots.
“This research shows that as the planet warms, tropical soils may begin to amplify that warming,” said Dr. Christine Sierra O’Connell of Chapman University.
“If these patterns persist across time and regions, we may be drastically underestimating the extent to which tropical forests will lose carbon and accelerate climate change.”
Soil microbes drive warming
The expectation was clear: roots would fuel most of the extra carbon loss. Yet the opposite happened. Root biomass actually declined, dropping by more than 30 percent in warmed plots.
At the same time, microbial biomass grew by more than 50 percent. The boost in microbial activity drove the surge in carbon emissions, while root respiration faded into the background.
This shift matters. Microbes decompose organic matter relentlessly, and under higher temperatures they seem to push soil carbon into the atmosphere at record levels. This mechanism explains why the Puerto Rican results outpaced similar warming experiments elsewhere in the tropics.
Water adds complexity
Heat was not the only factor in play. Moisture shaped the outcomes too. On lower slopes, warming dried soils, reducing aeration and limiting respiration after heavy rains.
Mid-slope soils also dried but still produced higher emissions. Strangely, the upper slopes became wetter under warming, and there the soil pumped out extraordinary amounts of carbon.
These contrasts show that even within a single forest, warming does not act uniformly. Topography and water balance interact with heat, producing both “hotspots” and “hot moments” of soil carbon release. Predicting future emissions without accounting for these details will likely miss the scale of the threat.
A new climate state
The experiment also tested sensitivity, measured by Q10, which tracks how respiration changes with temperature.
Surprisingly, Q10 dropped by more than 70 percent in warmed plots. Respiration no longer followed predictable rises with each degree. Instead, microbes adapted to sustain high emission rates regardless of further warming.
This means tropical soils may not just increase output with heat. They may lock into a new functional state where carbon release remains high even without additional temperature jumps. Such shifts challenge Earth system models that rely on fixed assumptions.
Global warning signs
“We are witnessing a troubling shift,” O’Connell explained. “The very systems we rely on to stabilize the climate may now be pushing us in the opposite direction.”
The numbers support her warning. On one slope, the extra carbon released each year matched the productivity of entire temperate forests.
Considering the vast area covered by tropical forests, the potential global impact is enormous. If soils across the tropics respond this way, warming could accelerate far beyond current projections.
Warming challenges soil role
Earlier theories held that tropical soils would show limited sensitivity to warming. Those ideas stemmed from lab incubations and elevation studies rather than real-world tests.
The Puerto Rico experiment proves otherwise. By running the first full warming trial in a tropical rainforest, the researchers showed how soil microbes, roots, and moisture interact under heat.
The lesson is sobering. Tropical soils may not resist change. Instead, they may shift quickly, releasing carbon in ways both powerful and unpredictable.
Tropics may flip sink to source
The TRACE project highlights one site, but the message applies widely. Tropical forests cover more than 30 ecological life zones. Each will likely respond differently, but all carry the risk of tipping from sink to source. Scientists argue that only direct experiments across varied forests can reveal the full picture.
Climate action often focuses on stopping deforestation and cutting aboveground emissions. This study shows another front that cannot be ignored. The soil beneath the forest is alive, active, and capable of reshaping the climate’s future.
If the processes seen in Puerto Rico spread across the tropics, global warming could accelerate in ways humanity has yet to prepare for.
The study is published in the journal Nature Communications.
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