Effect of seasonal variation on nitrogen transformations in aquaponics of northern China

• Seasonal variation has little effect on NUE and N2O conversion ratio of aquaponics.
• Seasonal variation could affect the equilibrium between nitrification and plant growth.
• Higher production yield was achieved in summer.

Aquaponics is considered to be a promising production system in the future, due to its high nitrogen utilization efficiency (NUE). In this study, lab-scale aquaponic systems with native species, common carp (Cyprinus carpio) and pakchoi (Brassica chinensis), were achieved successfully in northern China. And nitrogen transformations in aquaponics during summer and autumn were thoroughly investigated. High fish and plant yields were observed in summer because of suitable environment. However, seasonal variation has minimal effect on NUE, which was 43.8 ± 5.9% and 44.6 ± 3.7% in summer and autumn, respectively. Similar phenomenon occurred for N2O. N2O emission flux of aquaponics was 508.2 ± 80.6 ug m−2 h−1 in summer, and decreased to 218.5 ± 30.6 ug m−2 h−1 in autumn, whereas the N2O conversion ratio changed from 1.6 ± 0.2% to 1.3 ± 0.1%, correspondingly. This was mainly because of the decrease of nitrogen input at lower temperature. TAN concentration was lower in summer (0.5 mg/l) than autumn (1.0 mg/l), indicating faster nitrification occurred at higher temperature. And NO3−-N concentrations showed an obvious increase and decrease period in summer but only a slow increase in autumn, which was ascribed to the effect of seasonal variation on the balance between nitrification, denitrification and plant growth. Results of this study could provide theoretical supports of aquaponics’ seasonal performance for aquaponics’ application in northern China.