Soil Greenhouse Gas Fluxes, Environmental Controls, and the Partitioning of N₂O Sources in UK Natural and Seminatural Land Use Types

Natural and seminatural terrestrial ecosystems (unmanaged peatlands and forests and extensive and intensive grasslands) have been under-represented in the UK greenhouse gas (GHG) inventory. Mechanistic studies of GHG fluxes and their controls can improve the prediction of the currently uncertain GHG annual emission estimates. The source apportionment of N₂O emissions can further inform management plans for GHG mitigation. We have measured in situ GHG fluxes monthly in two replicated UK catchments and evaluated their environmental controlling factors. An adapted ¹⁵N-gas flux method with low addition of ¹⁵N tracer (0.03-0.5 kg ¹⁵N ha^-1) was used to quantify the relative contribution of denitrification to net N₂O production. Total N₂O fluxes were 40 times higher in the intensive grasslands than in the peatlands (range: -1.32 to 312.3 μg N m^-2 h^-1). The contribution of denitrification to net N₂O emission varied across the land use types and ranged from 9 to 60%. Soil moisture was the key parameter regulating the partitioning of N₂O sources (r² = 0.46). Total N₂O fluxes were explained by a simple model (r² = 0.83) including parameters such as total dissolved nitrogen, organic carbon, and water content. A parsimonious model with the soil moisture content as a single scalar parameter explained 84% of methane flux variability across land uses. The assumption that 1% of the atmospherically deposited N on natural ecosystems is emitted as N₂O could be overestimated or underestimated (0.3-1.6%). The use of land use-specific N₂O emission factors and further information on N₂O source partitioning should help constrain this uncertainty.