3.4. N2O emissions from aquaponics
Monitoring of N2O emissions from aquaponics were conducted
during phase II and III after stable NO2
concentrations were
reached. Fig. 3 shows the diurnal variation of dissolved N2O concentrations
in tomato and pak choi aquaponics. The time-weighted
N2O-N emissions from aquaponics are listed in Table 4.
About 1.5% and 1.9% of nitrogen input were emitted to the
atmosphere as N2O in tomato- and pak choi-based aquaponics,
respectively. Previous research showed that N2O conversion ratio
of intensive aquaculture system was about 1.3% (Hu et al., 2013).
Higher N2O conversion ratios were observed, mainly because of
higher fish stocking density in the present study. In our previous
research on intensive aquaculture systems, fish stocking density
was around 20 kg/m3, while in present study fish stocking density
of 30 kg/m3 was used and this value eventually reached over
50 kg/m3 at the end of the experiment (Hu et al., 2013). Higher fish
density led to more TAN excretion, which explained the higher
TAN concentrations in the present aquaponics. It is believed that
nitrification plays an important role in N2O emissions from fresh
water system (Ni et al., 2013). High TAN concentration could
increase the nitrification-driven N2O emission. Overall, the present
results showed that aquaponics could be an important source of
anthropogenic N2O emission, and further study is needed to verify