What Is Peat?

Peat is a soil material formed from partially decomposed plant remains that accumulate where waterlogging keeps the ground anoxic and slows decay below the rate of plant production. Under those conditions organic matter builds up rather than mineralizing, producing a deposit that can be many meters thick and that is by definition dominated by organic carbon rather than by mineral grains. Soil classification systems treat peat-dominated profiles as a distinct order, the Histosols, defined by organic carbon content and horizon thickness rather than by texture. Accumulation is slow, typically on the order of half a millimeter to a millimeter per year, so a meter of peat may represent a thousand years or more of deposition.

Peatlands are the landforms built from this material. They divide broadly into bogs, which are ombrotrophic and receive water and nutrients only from precipitation, and fens, which are minerotrophic and fed by groundwater or surface flow. Bogs are acidic and usually dominated by Sphagnum mosses; fens are less acidic and support sedges and reeds. Tropical peat swamp forests in Southeast Asia, the Congo Basin, and Amazonia accumulate woody peat under a different vegetation regime but by the same waterlogging mechanism.

Composition, Structure, and Classification

The degree of decomposition governs most of peat's physical behavior and is described in the field by the von Post scale, a ten-point index based on the color and consistency of material squeezed by hand. Fibric peat retains recognizable plant structure, hemic peat is partly decomposed, and sapric peat is amorphous. Bulk density is very low, often below 0.2 grams per cubic centimeter, while water content can exceed 90 percent by volume. Those properties make peat highly compressible and give it poor bearing capacity, a persistent problem for road, rail, and wind turbine foundations built across peatland, where excavation, preloading, or piling is usually required. The same low density and high calorific value when dried made peat a domestic and industrial fuel across Ireland, Finland, Russia, and the Baltic states, and its phenolic smoke remains part of the malting process for certain Scotch whiskies.

Carbon Storage and Degradation

Peatlands occupy only about 3 to 4 percent of global land area yet hold roughly a third of the world's soil carbon. The Global Peatlands Assessment puts the stock at around 600 gigatons, more carbon than is held in all the world's forest biomass. That store depends entirely on the water table staying near the surface. Drainage for agriculture, forestry, and plantation development admits oxygen, accelerates microbial decomposition, and converts a sink into a substantial source: degraded peatlands contribute a disproportionate share of global emissions relative to their area, along with land subsidence and vulnerability to smoldering peat fires that produce regional haze. Restoration through rewetting is the principal response, and studies of rewetted boreal sites examine how quickly carbon uptake resumes and how the balance between carbon dioxide and methane fluxes shifts as the water table recovers.

Mapping and Monitoring

Peatland extent, depth, and condition are difficult to measure at scale, which has made remote sensing central to the field. Optical and synthetic aperture radar imagery classify surface vegetation and detect drainage networks; interferometric SAR measures the centimeter-scale subsidence that signals ongoing oxidation; and airborne lidar resolves surface topography beneath forest canopy. Ground penetrating radar and coring calibrate depth models, and eddy covariance flux towers provide the carbon exchange measurements those models are validated against.

Applications

Peat and peatland science support work in a range of fields, including:

• Carbon accounting and greenhouse gas inventory reporting
• Wetland restoration and conservation planning
• Geotechnical design for infrastructure on organic soils
• Horticultural growing media and substrate research
• Paleoecology and archaeological preservation studies
• Water resource and flood regulation management