Abstract
Drained peatlands support highly profitable agriculture, but also represent a globally important source of greenhouse gas (GHG) emissions. Grasslands can typically be maintained at higher water levels than croplands, so conversion of cropland to grassland represents a potential CO2 mitigation strategy that allows for continued agricultural production. However, the presence of high water levels and livestock on grasslands risks generating high emissions of N2O, particularly associated with livestock urine patches. In the present study, a controlled mesocosm experiment was carried out to quantify the interactive impacts of groundwater level (10 cm, 30 cm and 50 cm water table depth, WTD) and sheep urine deposition on GHG emissions from peat soils. Our results showed that N2O emissions were significantly higher at 30 cm for both urine-treated and control mesocosms, due to the conditions favouring the interplay of nitrification and incomplete denitrification. The urine N2O emission factor was 0.25±0.17% at the 30 cm WTD and 0.20±0.07% at 50 cm WTD, lower than typical values for grasslands. No significant difference was observed in ecosystem respiration or methane flux between 30 cm and 50 cm WTDs. Overall, we conclude that strategies to raise water levels in drained peatlands through conversion of cropland to grassland need to account for the potential impacts of N2O emissions when seeking to minimise overall GHG emissions. Shifting from cropland to grassland management on peatlands for climate change mitigation also requires consideration of the effects of livestock methane emissions, and displaced emissions resulting from increased land demand for crop production elsewhere.
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