The review zeroes in on soil heterotrophic respiration to carbon dioxide, the process by which microbes exhale CO2 as they digest peat. It is the main loss pathway in the peatland carbon cycle, and it is the term models must get right if they are to simulate whether tropical peatlands keep accumulating carbon or start bleeding it into the atmosphere. The problem, the authors argue, is a double bind: scientists lack consensus on which environmental drivers matter most, and empirical data from the tropics are so scarce that model development has stalled. None of the models in the latest Coupled Model Intercomparison Project phase included peatlands at all, and even the land surface schemes that have since added peat modules — JULES, ORCHIDEE and CLM5 — were built around northern peatlands, leaving tropical soil carbon cycling essentially unrepresented.

Moisture emerges as the principal control. The relationship between respiration and water content follows a bell-shaped curve: respiration collapses when peat is bone dry because microbes cannot transport solutes and slip into dormancy, and it collapses again when peat is fully saturated because oxygen becomes limiting. Between those extremes lies an optimum moisture level where microbial activity peaks. The review found that this optimum varies enormously — from twenty-five to ninety-five percent water-filled pore space across compiled studies — and that degraded, drained peatlands show significantly lower optima than intact ones, consistent with the loss of porosity that accompanies drainage. For intact peatlands globally, the mean optimum sits at sixty-eight percent saturation.

The second critical parameter on the moisture curve is what the authors call the moisture resting point: the fraction of maximum respiration that persists under saturated conditions. Here the review delivers its sharpest quantitative insight. When saturation coincides with true anoxia — waterlogged conditions flushed free of oxygen — respiration drops to a median of just 0.105 of the optimum value, with a range of 0.005 to 0.290. That ratio, the authors stress, is the single most important number for tropical peat decomposition, because it is the reason peat accumulates in the first place. Intriguingly, the analysis showed that anoxia alone, whether or not the sample was water-saturated, produced the same dramatic suppression, while saturation without oxygen limitation only slightly reduced respiration. Water, in other words, kills respiration mainly by shutting off oxygen.

Temperature comes next, and the review confronts one of the most contested questions in soil carbon science: how sensitive is peat decomposition to warming? The standard metric, Q10, expresses the proportional increase in respiration for every ten degrees Celsius of warming. The rule of thumb holds that Q10 is roughly two — respiration doubles with a ten-degree jump. The authors compiled 873 peatland Q10 estimates from forty-four incubation studies worldwide and found a median of 2.04, with ninety percent of values between 1.16 and 3.62. Tropical estimates, though far fewer in number, were not significantly different from boreal and temperate ones. Given the noise from methodological differences, incubation lengths and moisture treatments, the authors conclude that a constant Q10 of about two remains the most parsimonious assumption for models — a defensible simplification, not a settled truth.

The review does not let Q10 off without criticism. The metric has no theoretical foundation, assumes an exponential response that only holds over limited temperature ranges, and is itself temperature-dependent. The authors advocate a shift toward the Arrhenius function, which is grounded in enzyme kinetics and applies across the full range where enzymes function. Existing incubation data could be reanalyzed this way if researchers reported raw data and activation energies — a change that would require no new experiments, only better reporting.

Substrate decomposability — how easily the peat’s organic matter can be mineralized — is the third major driver, and here the review exposes a field in disarray. Studies use wildly different proxies: peat depth, age, chemistry, the lignin-to-cellulose index, labile carbon fractions. Most observe that respiration declines with increasing recalcitrance, but the absence of a unified metric makes quantitative comparison across the tropics impossible. Some studies even found no significant relationship between lignin abundance or the lignin-cellulose index and CO2 emissions in tropical peat. Adding to the complexity, labile carbon inputs from litter and root exudates can trigger a priming effect, stimulating microbes to decompose even recalcitrant material faster.

Nutrients, by contrast, appear to matter less. Nitrogen and phosphorus addition experiments in tropical peatlands produced conflicting results, but all available studies agree that nutrient limitation is a weaker control on decomposition than moisture, temperature and substrate quality. The review also examined the home-field advantage hypothesis — the idea that decomposer communities specialize on local litter and decompose it faster at home — and found mixed evidence in the tropics, with one Panamanian study reporting a clear advantage and another finding only weak support.

Interactions between drivers remain the murkiest territory. Does warming sensitivity rise or fall with moisture? Some studies report higher Q10 under wetter, anoxic conditions; others report the opposite. The compiled database showed that anoxic conditions led to significantly higher temperature sensitivity in tropical peat, but no significant relationship between Q10 and moisture content emerged overall. Kinetic theory predicts that recalcitrant organic matter should be more temperature-sensitive because its decomposition demands higher activation energies, and the review found weak support for this under unsaturated oxic conditions — but no evidence for peat under saturated or anoxic conditions.

From this synthesis, the authors distill three priorities for advancing tropical peatland carbon modelling: narrowing the constraint on the optimum moisture range for respiration in tropical peat, investigating how moisture and temperature sensitivity interact, and identifying the most widely applicable metric for peat decomposability that would enable quantitative comparison across the tropics. Until those gaps close, the review argues, simple assumptions — anoxia with saturation, a constant Q10 near two — remain the justifiable foundation for models. But the stakes could hardly be higher: as warming, drainage, fires and land-use change amplify across the tropics, the fate of a quarter of the planet’s soil carbon hangs on how well these equations capture the breath of the peat.

Subject of Research: Drivers of soil heterotrophic respiration in tropical peatlands

Article Title: Drivers of soil heterotrophic respiration in tropical peatlands: a review to inform peat carbon accumulation modelling

Article References: Dehaen, E. M., Burke, E. J., Chadburn, S. E., Kaduk, J., Sitch, S., Smith, N. D., & Gallego-Sala, A. V. (2025). Drivers of soil heterotrophic respiration in tropical peatlands: a review to inform peat carbon accumulation modelling. Frontiers in Geochemistry, 3, Article 1492386. https://doi.org/10.3389/fgeoc.2025.1492386

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2025.1492386

Keywords: Drivers, soil, heterotrophic, respiration, tropical, peatlands, review, inform, peat, carbon, accumulation, modelling