Effects of subsurface irrigation on greenhouse gas emissions from intensively managed grassland on bog peat soil

Artificial drainage is prerequisite for conventional agricultural use of peatlands, but causes high emissions of greenhouse gases (GHG), mainly carbon dioxide (CO₂). Furthermore, grassland renewal is regularly practiced to maintain the high fodder quality required for dairy farming. Raising water levels is necessary to reduce CO₂ emissions, but whether a partial raise of water levels by subsurface irrigation (SI) is a sustainable mitigation measure is a matter of intense debate.

In this study, we evaluated the effects of subsurface irrigation on GHG exchange by comparing an experimental intervention site (INT) with SI and a deeply drained reference site (REF) for six years. Both sites are intensively used grasslands on deep bog peat with the same management history. In the first year of the experiment, grassland renewal was conducted at INT, followed by the raise of the water levels. At both sites, CO₂ (eddy covariance) as well as nitrous (N₂O) and methane (CH₄) (manually employed chambers) were measured.

SI effectively raised and stabilized mean annual water levels (-0.25 ± 0.05 m) in comparison to the REF site (-0.68 ± 0.14 m). However, a high spatial variability was observed at INT, causing parts of the site being too wet for management with regular machinery.

The initial grassland renewal resulted in very slow re-growth of grass and, in combination with the raised water levels, to extremely high N₂O emissions. N₂O emissions declined during the course of the study, but remained higher than at the REF site. CO₂ emissions at the INT site were lower than at the REF site, particularly during the second year with a strong development of a new sward. Towards the end of the study period, CO₂ emissions from both sites became more similar. Overall, CO₂ emissions of the INT site were 41% of those of the REF site, but total GHG emissions were 126%. Furthermore, Juncus effusus (soft rush) became more frequent at the INT site, which deteriorates fodder quality and would necessitate, again, grassland renewal. We conclude that at this bog site, SI is not an adequate solution to mitigate GHG emissions while maintaining production.