Efficacy of mitigation measures for reducing greenhouse gas emissions from intensively cultivated peatlands

Drained and cultivated fen peats represent some of the world's most productive soils, however, they are susceptible to degradation and typically exhibit high rates of greenhouse gas (GHG) emission. We hypothesised that GHG losses from these soils could be reduced by manipulating water table depth, tillage regime, crop residue application or horticultural fleece cover. Using intact soil columns from a horticultural peatland, emissions of CO2, N2O and CH4 were monitored over a six-month period, using a closed-chamber method. Concurrent measurements of soil properties allowed identification of the key controls on GHG emissions. Raising the water table to the soil surface provided the strongest reduction in global warming potential (GWP 100; 25 ± 6 kg CO2-e ha−1 d−1), compared to a free-draining control (80 ± 1 kg CO2-e ha−1 d−1), but this effect was partially negated by an emission pulse when the water table was subsequently lowered. The highest emissions occurred when the water table was maintained 15 cm below the surface (168 ± 11 kg CO2-e ha−1 d−1), as this stimulated N2O loss. Placement of horticultural fleece over the soil surface during spring had no significant effect on GWP 100, but prolonged fleece application exacerbated GHG emissions. Leaving lettuce crop residues on the surface increased soil GWP 100 (105 ± 4 kg CO2-e ha−1 d−1) in comparison to when residues were incorporated into the soil (85 ± 4 kg CO2-e ha−1 d−1), however, there was no evidence that this promoted positive priming of native soil organic matter (SOM). For maximum abatement potential, mitigation measures should be applied during the growing season, when GHG emissions are greatest. Our results also suggest that introduction of zero- or minimum-till practices may not reduce GHG emissions. Maintaining a high water table was the only option that reliably reduced GHG emissions, however, this option is impractical to implement within current horticultural systems. We conclude that alternative strategies or a major change in land use (e.g., conversion from horticulture/arable to wetland) should be explored as a means of preserving these soils for future generations.