A review of indirect N₂O emission factors from artificial agricultural waters

Abstract

Nitrous oxide (N₂O) produced from dissolved nitrogen (N) compounds in agricultural runoff water must be accounted for when reporting N₂O budgets from agricultural industries. Constructed ('artificial') water bodies within the farm landscape are the first aquatic systems that receive field N losses, yet emission accounting for these systems remains under-represented in Intergovernmental Panel on Climate Change (IPCC) emission factor (EF) guidelines and global N₂O budgets. Here, we examine the role of artificial waters as indirect sources of agricultural N₂O emissions, identify research gaps, and explore the challenge of predicting these emissions using default EFs. Data from 52 studies reporting dissolved N₂O, nitrate (NO₃), and EFs were synthesised from the literature and classified into four water groups; subsurface drains, surface drains, irrigation canals, and farm dams. N₂O concentration varied significantly between artificial waters while NO₃ did not, suggesting functional differences in the way artificial waters respond to anthropogenic N loading. EFs for the N₂O-NO₃-N concentration ratio were highly skewed and varied up to three orders of magnitude, ranged 0.005%-2.6%, 0.02%-4.4%, 0.03%-1.33%, and 0.04%-0.46% in subsurface drains, surface drains, irrigation canals, and farm dams, respectively. N₂O displayed a non-linear relationship with NO₃, where EF decreased exponentially with increasing NO₃, demonstrating the inappropriateness of the stationary EF model. We show that the current IPCC EF model tends to overestimate N₂O production in response to NO₃ loading across most artificial waters, particularly for farm dams. Given their widespread existence, there is a need to: (a) constrain their global abundance and distribution; (b) include artificial waters in the global N₂O budget, and (c) expand the study of N processing in artificial waters across a geographically diverse area to develop our biogeochemical understanding to the level that has been achieved for rivers and lakes.