Soil enzyme activity shows little adaptation to long-term geothermal warming

In the rolling geothermal fields of New Zealand’s North Island, where soil temperatures shift dramatically over distances of just a few meters, scientists have found one of nature’s most instructive natural laboratories for studying how soil life responds to heat. A new study published in the journal Biogeochemistry suggests that one of the most important steps in the soil carbon and nutrient cycle—the breakdown of organic matter by extracellular enzymes—may be far more resistant to thermal adaptation than researchers had assumed. The finding, based on more than two decades of natural warming along a geothermal gradient, complicates predictions about how soils will behave as the planet warms and raises new questions about whether different stages of decomposition might respond to climate change in fundamentally different ways.

The research was led by Erica J. Prentice of Te Aka Mātuatua School of Science at the University of Waikato, together with Vickery L. Arcus and Louis A. Schipper of the same institution, and Charlotte J. Alster of the Department of Soil and Physical Sciences at Lincoln University. Working at a field site on a golf course at Arikikapakapa, near the geothermal heartland of Rotorua, the team took advantage of a remarkable natural experiment: soils that have been sitting at different mean annual temperatures for more than twenty years, not because of a laboratory manipulation, but because of the steady, spatially variable heat rising from the Earth below. This kind of long-term geothermal gradient is highly prized by soil scientists precisely because it allows them to ask whether microbial communities adapt to sustained warming in ways that short-term laboratory experiments simply cannot capture.

The focus of the study was extracellular enzyme activity, a critical but often overlooked stage in the decomposition cascade. Soil microbes cannot directly swallow the large, complex molecules that make up dead plant and microbial material. Instead, they invest energy in secreting enzymes into the surrounding soil, where those proteins cleave polymers into small, soluble units that can be transported into microbial cells. Three enzyme classes were measured in the new work: β-glucosidases, which break down cellulose-derived sugars; β-N-acetylglucosaminidases, which target chitin-like nitrogen-containing compounds; and phosphatases, which liberate phosphate from organic molecules. Because these enzymes perform the substrate-acquisition step of decomposition, their temperature sensitivity effectively sets the pace at which nutrients and carbon become available to the rest of the soil food web.

To characterize the thermal responses, the researchers incubated soils collected from across the gradient and measured enzyme activities across a broad range of assay temperatures, extending up to 60 degrees Celsius. Rather than fitting the data to a simple bell curve, the team quantified key parameters of the thermal performance profile: the activation energy, which describes how steeply activity rises with temperature at low temperatures; the curvature of the response; and both the minimum and optimum temperatures of activity. These parameters matter because thermal adaptation theory makes specific predictions about them. If microbial communities adapt to warmer conditions, their enzymes should, in principle, shift their thermal optima upward, and their activity curves should adjust in ways that reflect a physiology tuned to the prevailing temperature regime.

What the team actually found was striking. Across the entire long-term gradient, enzyme thermal responses showed increasing rates of activity all the way up to the highest characterized temperature of 60 degrees Celsius, regardless of the mean annual soil temperature at which the communities had been living. The parameters that theory says should shift under adaptation—activation energy, curvature, the optimum and minimum temperatures of activity—showed minimal change across the gradient. In other words, enzymes collected from soils that have been warm for decades behaved, thermally, much like enzymes from cooler soils only meters away. There was no measurable upward shift in thermal optima, no flattening of the temperature response, no evidence that decades of sustained warmth had sculpted the catalytic machinery of decomposition into a heat-tuned form.

The surprise deepens when these results are set against earlier work at the same site. Previous measurements of soil respiration and microbial growth along the same geothermal gradient had told a very different story. Respiration and growth, the processes by which microbes convert acquired substrates into carbon dioxide, energy, and new biomass, showed optimum temperatures of activity around 30 to 45 degrees Celsius, well below the assay ceiling used for the enzymes. More importantly, those processes displayed clear, measurable rates of thermal adaptation across the gradient: microbial communities in warmer soils had shifted their respiratory and growth responses in ways consistent with long-term acclimatization. The new enzyme data therefore reveal a fundamental divergence within a single soil ecosystem between the thermal behavior of substrate acquisition and the thermal behavior of substrate utilization.

This divergence is not merely a technical curiosity. It strikes at the heart of how scientists model soil carbon feedbacks under climate change. Most Earth system models treat decomposition as a single temperature-sensitive process, implicitly assuming that the different steps—enzyme-catalyzed depolymerization, microbial uptake, respiration, and growth—respond to temperature in a coordinated way. If, instead, the acquisition step remains poised for ever-increasing rates up to 60 degrees Celsius while the utilization step peaks and adapts at much lower temperatures, then sustained soil warming could decouple the two. Enzymes might liberate carbon and nutrients from organic matter faster than microbes can process them, or faster than the microbial biomass can be maintained, with unpredictable consequences for nutrient bioavailability, carbon storage, and the balance of greenhouse gas fluxes from soils.

The authors point out that understanding these processes has long been hampered by the sheer complexity and variability of soil systems, and by inconsistencies in the methodologies used to quantify microbial thermal adaptation. Different studies measure different endpoints—respiration, growth, enzyme activity, community composition—over different timescales, and short-term assays can be confounded by the immediate effects of changing temperature on existing enzymes rather than on the organisms that produce them. The geothermal gradient approach sidesteps one of the central difficulties: by sampling soils that have experienced different temperatures for over twenty years in situ, the researchers could look for adaptation that reflects genuine, long-term evolutionary and community-assembly processes rather than transient physiological adjustment.

The study also speaks to a long-running debate in thermal biology about the extent to which enzyme thermal properties constrain the organisms that depend on them. One influential framework holds that the temperature dependence of microbial growth is ultimately dictated by the thermodynamics of the enzymes themselves, so that adaptation of growth should be mirrored by adaptation of key enzymes. The new results challenge that expectation in soil systems: growth and respiration adapted, but the extracellular enzymes upstream of them apparently did not. Whether this reflects the fact that the enzymes were assayed outside their cellular context, that different microbial taxa with different enzyme variants dominate different temperature zones, or that enzyme production rather than enzyme catalysis is what adapts, remains an open question that the authors’ findings are likely to stimulate considerable follow-up work on.

The practical implications for future soil warming are significant. Soils worldwide hold vast reserves of organic carbon, and the rate at which microbes decompose that material is a key uncertainty in climate projections. If extracellular enzymes maintain rising activity with warming, up to surprisingly high temperatures, while the microbial processes that consume the released substrates slow or adapt downward, the resulting imbalance could alter both the timing and the form of carbon loss from soils. Nutrients such as nitrogen and phosphorus, whose availability depends on enzymatic release, might be liberated at rates that outstrip biological demand, potentially increasing leaching losses or shifting competition among soil organisms. Conversely, the apparent robustness of enzyme thermal profiles might, in some scenarios, buffer decomposition against temperature change in ways not captured by current models.

The study is also a reminder of the value of unusual natural settings for fundamental science. The Arikikapakapa site, better known to golfers than to most climate scientists, provided precisely the combination of long-term temperature differences, shared parent material, vegetation, and climate history that controlled experiments struggle to achieve. The authors acknowledge the groundskeeping team at the golf course for their continued support and access, and Seager Ray for sample collection. The work was supported by the Marsden Fund of New Zealand under grant number 19-UOW-035, with open access funding enabled and organized by CAUL and its member institutions.

As the authors note in their published abstract, the divergence between the thermal response of extracellular enzyme activity and that of respiration and growth “raises questions around future nutrient bioavailability and utilisation if these two processes are decoupled at elevated temperatures under future soil warming.” Those questions are now likely to move to the center of soil biogeochemistry research. If the enzymes that unlock soil organic matter are largely indifferent, in their thermal tuning, to two decades of warming, while the microbes that depend on them continue to adapt, then the assumption that soil carbon cycling responds to temperature as a unified process may need serious revision. For a planet whose soils are steadily warming, that revision could matter a great deal.