Nitrogen transformations in aquaponics with different edible plant species, i.e., tomato (Lycopersicon
esculentum) and pak choi (Brassica campestris L. subsp. chinensis) were systematically examined and
compared. Results showed that nitrogen utilization efficiencies (NUE) of tomato- and pak choi-based
aquaponic systems were 41.3% and 34.4%, respectively. The abundance of nitrifying bacteria in
tomato-based aquaponics was 4.2-folds higher than that in pak choi-based aquaponics, primarily due
to its higher root surface area. In addition, tomato-based aquaponics had better water quality than that
of pak choi-based aquaponics. About 1.5–1.9% of nitrogen input were emitted to atmosphere as nitrous
oxide (N2O) in tomato- and pak choi-based aquaponic systems, respectively, suggesting that aquaponics
is a potential anthropogenic source of N2O emission. Overall, this is the first intensive study that examined
the role plant species played in aquaponics, which could provide new strategy in designing and
operating an aquaponic system.