Hydrogeological Controls on Regional-Scale Indirect Nitrous Oxide Emission Factors for Rivers

Indirect nitrous oxide (N 2 O) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF 5r ) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF 5r estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, N 2 O and nitrate (NO 3 – ) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median N 2 O–N concentration (3.0 μg L –1 ), EF 5r (0.00036), and N 2 O–N flux (10.8 kg ha –1 a –1 ). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median N 2 O–N concentration (0.8 μg L –1 ), EF 5r (0.00016), and N 2 O–N flux (2.6 kg ha –1 a –1 ), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high N 2 O yield from high permeability chalk contrasting with significantly lower median N 2 O–N concentration (0.7 μg L –1 ), EF 5r (0.00020), and N 2 O–N flux (2.0 kg ha –1 a –1 ) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF 5r can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale N 2 O emission estimates.