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The development of intensive recirculating aquaculture systems (RAS) with low water exchange has accelerated in recent years as a result of environmental, economic and other concerns. In these systems, fish are commonly grown at high density, 50 to 150 kg/m 3 , using high-protein (30%–60%) feeds. Typically, the RAS consists of a solid treatment and a nitrification unit; in more advanced RAS, there is an additional denitrification step. Nitrous oxide (N 2 O), a byproduct during nitrification and denitrification processes, is a potent greenhouse gas that destroys the ozone layer. The aim of this study was to measure and assess N 2 O emissions from a near-zero discharge land-based saline RAS. N 2 O flux was monitored from the RAS's fish tank, and moving-bed nitrification and activated-sludge (with intrinsic C source) denitrification reactors. N 2 O emission potential was also analyzed in the laboratory. N 2 O flux from the denitrification reactors ranged between 6.5 and 48 mg/day, equivalent to 1.27 ± 1.01% of the removed nitrate-N. Direct analysis from the fish tank and nitrification reactors could not be performed due to high aeration, which diluted the N 2 O concentration to below detection limits. Thus, its potential emission was estimated in the laboratory: from the fishponds, it was negligible; from the nitrification reactor, it ranged between 0.4 and 2.8% of the total ammonia-N oxidized. The potential N 2 O emission from the denitrification reactor was 3.72 ± 2.75% of the reduced nitrate-N, within the range found in the direct measurement. Overall, N 2 O emission during N transformation in a RAS was evaluated to be 885 mg/kg feed or 1.36 g/kg fish production, accounting for 1.23% of total N application. Consequently, it is estimated that N 2 O emission from aquaculture currently accounts for 2.4% of the total agricultural N 2 O emission, but will decrease to 1.7% by 2030. [ABSTRACT FROM AUTHOR]

This work reviews the concentrations, the dynamics and the emissions of nitrous oxide (N 2 O) in groundwater. N 2 O is an important greenhouse gas (GHG) and the primary stratospheric ozone depleting substance. The major anthropogenic source that contributes to N 2 O generation in aquifers is agriculture because the use of fertilizers has led to the widespread groundwater contamination by inorganic nitrogen (N) (mainly nitrate, NO 3 − ). Once in the aquifer, this inorganic N is transported and affected by several geochemical processes that produce and consume N 2 O. An inventory of dissolved N 2 O concentrations is presented and the highest concentration is about 18.000 times higher than air-equilibrated water (up to 4004 μg N L −1 ). The accumulation of N 2 O in groundwater is mainly due to denitrification and to lesser extent to nitrification. Their occurrence depend on the geochemical (e.g., NO 3 − , dissolved oxygen, ammonium and dissolved organic carbon) as well as hydrogeological parameters (e.g., groundwater table fluctuations and aquifer permeability). The coupled understanding of both parameters is necessary to gain insight on the dynamics and the emissions of N 2 O in groundwater. Overall, groundwater indirect N 2 O emissions seem to be a minor component of N 2 O emissions to the atmosphere. Further research might be devoted to evaluate the groundwater contribution to the indirect emissions of N 2 O because this will help to better constraint the N 2 O global budget and, consequently, the N budget. [ABSTRACT FROM AUTHOR]