Nutrient management regime affects water quality, crop growth, and nitrogen use efficiency of aquaponic systems

Sustainable nutrient management is of critical importance to achieve high crop yield and quality and to improve nutrient use efficiency in agricultural production systems but has not been fully established for aquaponics. The objective of this study was to determine the effects of feeding regime on water quality, crop performance and yield, and nitrogen (N) use efficiency in recirculating aquaponic systems. The same amount of total N (120 g) was applied to aquaponics with different feeding regimes: aquaponic increasing feeding (AIF; the standard feeding regime), uniform feeding (AUF), and intermediate feeding (AMF), for one-month production of six vegetable and herb species. Crops grown in AIF and AUF showed contrasting results in yield and SPAD value (chlorophyll content), and therefore were further evaluated for nutrient profile in aquaponic solution and crop growth and performance compared to those in hydroponics (HYD), using eight leafy vegetable (Chinese cabbage, Mizuna, Swiss chard, lettuce, pac choi), herb (basil, chia), and fruity vegetable (cherry tomato) species. AUF improved water quality by reducing average concentrations of harmful compotmds (i.e., NO2-N and Na) compared to AIF and crop growth and yield similar to those of HYD. Particularly, AUF tended to increase concentrations of mineral nutrients (i.e., NO3-N, PO4-P, Ca, and Mg) in aquaponic solution during the first week after transplanting, while decreasing the concentrations of harmful compounds in comparison to AIF. Regardless of feeding regime, aquaponics reduced fine root growth in leafy vegetables and herbs, compared to hydroponics. Overall, vegetables and herbs grown in AUF had a greater photosynthetic rate (Pn) from the first to the second week after transplanting and throughout production period and showed higher SPAD value and leaf total N content to the level similar to or slightly lower than HYD. Consequently, AUF increased N use efficiency (NUE) of the system by 30% and up to 600% compared to those in AIF and HYD, respectively. In conclusion, aquaponic crop production and N use efficiency can be increased by uniform feeding regime as it improves water quality and nutrient availability for better seedling establishment, consequently enhancing quality and/or yield of vegetables and herbs in aquaponics.