Groundwater level effects on greenhouse gas emissions from undisturbed peat cores

Peat soils store a large part of the global soil carbon stock, which can potentially be lost when they are drained and taken into cultivation, resulting in CO₂ emission and land subsidence. Groundwater level (GWL) management has been proposed to mitigate peat oxidation, but may lead to increased emissions of nitrous oxide (N₂O) and methane (CH₄).

The aim of this experiment was to study trade-offs between greenhouse gas emissions from peat soils as a function of GWL. We incubated 1 m deep, 24 cm diameter undisturbed bare soil cores, after removal of the grass layer, from three contrasting Dutch grassland peat sites for 370 days at 16 °C. The cores were subjected to drying-wetting cycles, with the GWL varying between near the soil surface to 160 cm below the surface. We measured gas fluxes of CO₂, N₂O and CH₄ from the soil surface, extracted pore water for DOC and mineral nitrogen analysis, and measured soil hydraulic and shrinkage characteristics.

Emissions of CO₂ increased after lowering the GWL, but showed different GWL-response curves during rewetting of the soil. On average, highest CO₂ emissions of 1.5 g C·m⁻² day⁻¹ were found at a GWL of 80 cm below the surface. However, the 0 cm GWL was the only treatment with significantly lower CO₂ emissions than other GWLs. Cumulative CO₂ emissions differed significantly between sampling sites. Emissions of N₂O showed a different response, peaking at GWL heights above −20 cm, particularly after a recent GWL rise. Though not significantly different, the highest N₂O emissions were measured at the 0 cm GWL treatment. We confirmed this pattern for N₂O in un-replicated soil cores with grass sward, although emission values were lower in these cores due to the root uptake of mineral nitrogen. CH₄ emissions or −uptake remained low under any GWL. We conclude that raising the GWL is a successful strategy to reduce CO₂ emissions from peat oxidation. However, raising the GWL close to the soil surface could lead to N₂O emissions that negate any gains in terms of global warming potential. Our results suggest that raising the GWL in peat grasslands to −20 cm creates such a risk. A constant GWL at the surface (0 cm) would be preferential for mitigating both CO₂ and N₂O emissions, although such conditions don’t allow for agricultural grass production (mowing or grazing).