Comparisons of nitrogen and phosphorus mass balance for tomato-, basil-, and lettuce-based aquaponic and hydroponic systems

Abstract

Aquaponics is known to be one of the most efficient agriculture production systems by far; however, limited quantitative data are available on the level of efficiency, particularly in comparison to its counterpart, hydroponics. This study compared aquaponics with hydroponics to assess nitrogen (N) and phosphorus (P) distributions, N- and P-use efficiency (NUE and PUE), and potential environmental impacts. Cherry tomato, basil, and lettuce were cultured in recirculating tilapia (Oreochromis niloticus)-based aquaponic or stand-alone hydroponic systems over a 3-month period. N and P mass balances were developed by using N and P concentrations in fish feed, solid waste, wastewater, fish biomass, and plant biomass for aquaponic systems, and chemical fertilizer, wastewater, and plant biomass for hydroponic systems, which were to estimate N and P loss from each system via denitrification and P precipitations, respectively. In aquaponics, 59–70% of total N input was estimated to be lost via denitrification (45–50%), wastewater (14–20%), and solid waste (0.7–2%) and 30–41% was assimilated into biomass (21–24% fish; 9–17% plants). In hydroponics, 76–87% was estimated to be lost via denitrification (69–78%) and wastewater (7–15%), and only 14–24% was assimilated into biomass. Similarly, 38–54% of total P input in aquaponics was lost via wastewater (22–28%), precipitations (8–25%), and solid waste (2–7%) and 46–62% was assimilated into biomass (35–45% fish; 11–25% plants). In hydroponics, 79–89% of total P input was lost via wastewater (52–74%) and precipitations (15–27%), and only 11–21% was assimilated into plant biomass. Plant species had a significant impact on N and P distribution, and tomato was most effective in removing N from wastewater and reducing denitrification via higher biomass production compared to basil and lettuce. NUE was lower for aquaponics than for hydroponics when only edible plant parts were taken into consideration, but 71–114% higher when both fish and plant biomass were considered. Meanwhile, PUE was higher for aquaponics than for hydroponics even only when edible parts were considered and 336–369% higher when the biomass of both crops was considered. Our N and P mass balance analyses demonstrated for the first time that aquaponics is more efficient than hydroponics. Greater N and P losses in hydroponics were mainly due to higher N and P inputs, causing 2 times higher N loss via gas emission and up to 3 times higher P loss via inorganic P in wastewater compared to aquaponics. Therefore, reducing nutrient inputs is critical in improving NUE and PUE for both aquaponics and hydroponics, which should be combined with proper crop choice, operation conditions, and management practices to further improve the efficiency of the systems.