Effect of subsurface irrigation on greenhouse gas emissions and yield on a boreal agricultural drained peatland

Drained agricultural peatlands are a major source of greenhouse gas (GHG) emissions in Finland and globally, and increasing pressure is being placed on their management in the context of climate change mitigation targets. Drainage, fertilization, and other management practices accelerate the decomposition of soil organic matter, leading to the release of carbon dioxide (CO₂) and nitrous oxide (N₂O) into the atmosphere. Raising the groundwater table through subsurface irrigation or ditch blocking has been proposed as a mitigation measure for intensively managed peatlands. However, field-based evidence remains limited, particularly from northern regions, and results vary across studies. Key concerns include whether the water table depth can be maintained sufficiently shallow during the growing season considering the needs of crop and management, and whether reductions in CO₂ emissions may be offset by enhanced methane (CH₄) and N₂O emissions under wetter soil conditions.

To address these uncertainties, a field-scale subsurface irrigation system was established at the NorPeat research facility in Ruukki, Finland (64.68°N, 25.11°E), operated by the Natural Resources Institute Finland (Luke). The field is a 26-ha cultivated peatland under a grass intensive crop rotation for beef cattle feed production and is divided into eight drainage blocks (2.5-3.9 ha each). Peat depth at the site ranges from 20 to 80 cm, with sulfidic material occurring below one meter depth. Since 2022, a water storage reservoir (9000 m³) has been connected to the subsurface drainage system, allowing block-specific subsurface irrigation. During grass cultivation years, the target for groundwater table has been at approximately 30 cm depth.

Greenhouse gas fluxes have been measured year-round since 2019 using chamber, snow-gradient, and eddy covariance methods, complemented by floating chamber measurements in open ditches. In addition, other environmental variables have been monitored intensively, with continuous measurements of soil moisture and water table depth.

In the presentation, we will show whether GHG emissions can be reduced by subsurface irrigation under field-scale management conditions, whether the mitigation effect depends on peat depth, and how the irrigation affects crop yields.