Indirect nitrous oxide emissions: Revised emission factors

Indirect nitrous oxide (N2O) emissions arising from ground and surface waters are thought to be significant, but considerable uncertainty surrounds the emission factors for dissolved nitrate in aquifers, drainage ditches (EF5-g), rivers (EF5-r) and estuaries (EF5-e). Through measurements of nitrate and N2O concentrations in aquifers and field drains, the existing IPCC default emission factor EF5-g has been thrown into doubt. Here, we present a synthesis of the work of two independent UK-based research groups working as part of the Global Nitrogen Enrichment (GANE) initiative. We put forward a revised emission factor for EF5-g based on these findings. We suggest the downward revision of the indirect N2O emission factor EF5-g from 0.015 to 0.002, and a consequent decrease in EF5 from 0.025 to 0.012. Such radical downward revision would halve current estimates of N2O emissions associated with N leaching and runoff from agriculture. In the UK, estimated annual N2O emissions via this route would be cut from around 14 Gg N y−1 to less than 7 Gg y−1. Globally, the EF5 emission estimate would come down from 1.6 Tg N y−1 to less than 0.8 Tg N y−1.