Lower Greenhouse Gas Emissions from Drained Peatlands – Is It Possible?

A mineral soil cover combined with a rise in the water table can mitigate the loss of organic carbon in cultivated peatlands. At the same time, it would allow agricultural productivity to be maintained.

Water-saturated peatlands are an important store of organic carbon. They hold twenty percent of global soil organic carbon, despite covering only three percent of the earth’s surface. These soils are also an essential store of organic nitrogen.

Drained peatlands release greenhouse gases

Many of these soils are drained to make them suitable for growing crops, leading to soil subsidence and rapid mineralisation of the aerated peat. These processes release large amounts of greenhouse gases (GHGs). Drained peatlands are a major source of carbon dioxide (CO₂, 360–810 g C per square metre per year) and nitrous oxide (N₂O, 0.16–0.82 g N per square metre per year). In contrast, methane (CH₄) emissions are negligible. Rewetting and restoring peatlands have been identified as ways of reducing GHG emissions; however, this approach limits – or completely precludes – agricultural production.

One method that has been used to improve the workability of peatland soils and counteract soil subsidence is to apply a covering of mineral soil material recovered from construction sites. Experts at Agroscope and the University of Basel investigated whether a mineral soil cover can also significantly reduce GHG emissions.

Mineral soil cover does not reduce carbon losses

The study was conducted in the Rhine Valley in Switzerland on drained peatland with a 40 cm thick mineral soil cover and with no cover. CO₂ fluxes were measured over a four-year period using the Eddy covariance method, while N₂O and CH₄ fluxes were recorded over three years using an automatic chamber system. The drained peatlands showed substantial losses of soil organic carbon with high annual variations (175–786 g C per square metre per year for the uncovered and 125–826 g C per square metre per year for the covered peatland).  The soil material applied led to no significant reduction in soil organic carbon losses compared to the uncovered peatland. Thus the mineral soil cover has little potential to reduce soil organic carbon losses from drained peatland.

Mineral soil coverage cuts nitrous oxide emissions

However, soil coverage significantly reduced N₂O emissions (from 1.5 to 0.2 N per square metre per year). By reducing nitrous oxide emissions, the mineral soil covering improved the overall GHG budget (from 19.2±2.0 to 12.8±2.1 t CO₂ equivalent per hectare per year).

Raise water table to reduce carbon losses

The high annual variations in the loss of soil organic carbon were attributed to different groundwater table levels: the higher the water table, the lower the losses.

A high water table – ideally the entire peat body would be saturated – prevents rapid mineralisation of soil organic carbon. However, high water levels would make agricultural production virtually impossible. A mineral covering creates an aerated soil layer above the water-saturated peat and thus enables agricultural production to continue.

Thus a combination of mineral soil coverage and raising the water table could be an effective means of reducing GHG emissions from cultivated organic soils. Agricultural production would be maintained while at the same time reducing soil organic carbon losses and thus GHG emissions.