A review of indirect N2O emission factors from artificial agricultural waters
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–N: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.