The summer air above New Hampshire’s White Mountains hums with cicadas and the low hiss of wind through sugar maple. Yet beneath the leaf-litter—no thicker than a folded newspaper—billions of microbes and tree roots are exhaling at rates unseen in living memory. What sounds like science-fantasy is, in fact, the sober read-out of 18 years’ worth of CO₂ measurements in the storied Hubbard Brook Experimental Forest. Think of a forest floor the size of Manhattan quietly doubling its breath in less than a decade and you have a sense of the scale at hand.
Increasing soil respiration in a northern hardwood forest indicates symptoms of a changing carbon cycle by Angela R. Possinger, Charles T. Driscoll, Mark B. Green, Timothy J. Fahey, Chris E. Johnson, Mary Margaret K. Koppers, Lisa D. Martel, Jennifer L. Morse, Pamela H. Templer, Angelina M. Uribe, Geoffrey F. Wilson, & Peter M. Groffman (2025) chronicles a startling metabolic acceleration in this 3,160-hectare research landscape.
For nearly two decades the team logged monthly CO₂ fluxes from PVC collars sunk ten centimeters into soils that grade from sugar-maple loam at 520 m to red-spruce podzols above 800 m. The story they tell has a hinge year: “soil respiration rates have notably increased since ~2015,” the authors report. In 2020, “cumulative summer respiration flux was approximately 90 % higher than the average summer flux over the 2002–2015 period.” Put another way, the forest floor—already holding about 2.6 kg of carbon per square metre, the mass of a half-gallon milk jug packed solid—now lost the carbon equivalent of a pocket-sized smartphone every summer square-metre, a jump that could empty one-fifth of that stock within a human lifetime.
Temperature? Not guilty. A 0.015 °C-per-year rise in July–August air would, at textbook Q₁₀ rates, lift respiration by well under 1 %. Even the warmest soil sensors predicted no more than a 12 % bump. Yet fluxes soared past 100 %. Likewise, a gentle half-unit rise in mineral-soil pH after decades of acid-rain recovery failed to correlate. “The increase in soil respiration cannot be explained directly by temperature or pH change alone,” the authors write.
So what switched on the subterranean furnace? The suspects are biological. In lab incubations the team saw that “heterotrophic microbial C mineralization and microbial biomass C have also increased rapidly since ~2015,” especially in the Oi–Oe litter—those crisp, root-laced layers where beech leaves meet the first twilight of decay. Microbial biomass C:N climbed too, hinting at a fungal surge. Deeper mineral horizons, meanwhile, showed falling biomass-specific respiration, a signature of microbes dining on easier calories.
The authors propose an eco-feedback fit for soft science fiction: with rising CO₂ and dwindling nitrogen, trees may be routing more sugars below-ground, greasing microbial engines to mine locked-up nutrients. “We suggest that these observations are consistent with a hypothetical increase in plant allocation of C belowground in response to changing climatic and soil conditions,” they note. One need only boost root-exudate flows by 10–20 %—about the weight of a postage stamp per square metre each summer—to match the observed flux.
Size matters. The additional CO₂ leaving these woods since 2015 equals roughly 15 g C m⁻² each summer—scale that across Hubbard Brook and you have four blue-whale masses of carbon wafting skyward every year. If replicated across temperate forests that store 2,680 Tg of floor carbon, the trend could erase the modest 4 Tg C the U.S. forest floor sequesters annually and tip a continental sink toward source.
Curiously, the metabolic kink spares no elevation, soil hydrology, or calcium-restored watershed. The breath quickens everywhere, an egalitarian pulse that undercuts easy attribution. As the authors caution, “Quantification of interactions among co-occurring global change factors … is needed to predict how the soil C reservoir will continue to respond.”
That future-facing work will require models where microbes are not passive digesters but agents whose efficiency, elemental stoichiometry, and alliances with roots reshape Earth-system feedbacks. It will also mean archiving leaf litter, dating carbon with radiocarbon, and perhaps—fantastic as it sounds—listening for isotopic whispers that trace exudates from canopy to clay.
In the meantime, the Hubbard Brook data offer a jolting reminder: ecosystems can pivot fast, and their subterranean stories may unfold out of sight but not out of consequence. As “the authors” conclude, “these observations underscore the need for further studies … to unravel the complex interactions between changing climate, atmospheric chemistry, and ecosystem processes that are driving major changes in the Earth system.” When a forest floor doubles its breath, the atmosphere is already listening.
Possinger, A. R., Driscoll, C. T., Green, M. B., Fahey, T. J., Johnson, C. E., Koppers, M. M. K., Martel, L. D., Morse, J. L., Templer, P. H., Uribe, A. M., Wilson, G. F., & Groffman, P. M. (2025). Increasing soil respiration in a northern hardwood forest indicates symptoms of a changing carbon cycle. Communications Earth & Environment, 6, 418. https://doi.org/10.1038/s43247-025-00999-x