Effects of pH on nitrogen transformations in media-based aquaponics

•Aquaponics could tolerate a wide range of pH from 6.0 to 9.0.•Higher nitrogen utilization efficiency and N2O emission were achieved at pH 6.0.•Denitrification accounted for 75.2–78.5% of N2O emission from aquaponics.•Unfavorable conditions for denitrifiers led to higher N2O emission at pH 6.0.

To investigate the effects of pH on performance and nitrogen transformations in aquaponics, media-based aquaponics operated at pH 6.0, 7.5 and 9.0 were systematically examined and compared in this study. Results showed that nitrogen utilization efficiency (NUE) reached its maximum of 50.9% at pH 6.0, followed by 47.3% at pH 7.5 and 44.7% at pH 9.0. Concentrations of nitrogen compounds (i.e., TAN, NO2−-N and NO3−-N) in three pH systems were all under tolerable levels. pH had significant effect on N2O emission and N2O conversion ratio decreased from 2.0% to 0.6% when pH increased from 6.0 to 9.0, mainly because acid environment would inhibit denitrifiers and lead to higher N2O emission. 75.2–78.5% of N2O emission from aquaponics was attributed to denitrification. In general, aquaponics was suggested to maintain pH at 6.0 for high NUE, and further investigations on N2O mitigation strategy are needed.