LAUSR

Abstract Nitrogen plays critically important role in dual biomasses aquaponic production. This study examined the fate of nitrogen in an aquaponics at different plant-fish (P/F) biomass ratios. A floating raft hydroponic bed with water celery (Oenanthe javanica) as plant species coupled with an aquaculture tank reared with Koi carp (Cyprinus carpio) was evaluated using nitrogen mass balance techniques at P/F biomass ratio of 0.06–0.95 and feed conversion ratios (FCR) maintained at 1.73–1.85 at a constant hydraulic retention time of 24 h. The FCRs in the study were significantly lower than FCR (1.96) in the control (aquaculture only). Nitrogen use efficiency (NUE) in the aquaponics increased from 29.3 ± 3.5 to 39.1 ± 4.2% as the P/F biomass ratio increased from 0.06 to 0.95, respectively. Furthermore, the increase in P/F biomass ratio to 0.95 exhibited the highest nitrogen uptake rate by plants (~56 mg/day in the hydroponic bed) which resulted in a levelling-off of the nitrate concentration at 62.5 ± 4.2 mg/L in the aquaculture tank. The increase in P/F biomass ratio also lowered the nitrous oxide (N2O) emissions by 17% from the aquaponics compared to control. Overall, this study presents evidence that an aquaponics with higher P/F biomass ratio emits less N2O and has higher nutrient recovery with perpetual fish production in a zero water exchange-aquaponic system.