Large variations in indirect N₂O emission factors (EF₅) from coastal aquaculture systems in China from plot to...

Aquaculture ponds are important anthropogenic sources of nitrous oxide (N₂O). Direct N₂O emissions arising from feed application to ponds have been widely investigated, but indirect emissions from N₂O production from residual feeds in pond water are much less understood and characterized to refine the IPCC emission factor. In this study, we determined the concentrations and spatiotemporal variations of dissolved N₂O and NO₃^--N in situ in three aquaculture ponds at the Min River Estuary in southeastern China during the culture period over two years, and calculated the indirect N₂O emission factor (EF₅) for aquaculture ponds using the N₂O-N/NO₃^--N mass ratio methodology. Our results indicated that the EF₅ values in the ponds over the culture period ranged between 0.0007 and 0.0543, with a clear seasonal pattern which closely followed that of the DOC:NO₃^-N ratio. We also observed significant spatial variations in EF₅ among the three ponds, which could be attributed to the difference in feed conversion rate. In addition, we assessed the EF₅ values from aquaculture ponds in five regions of the Chinese coastline across the latitudinal gradient from the tropical to the temperate zones. The average EF₅ value from aquaculture ponds across the five coastal regions was 0.0093±0.0024, which was approximately 3.7 times of the IPCC default value for rivers and estuaries (0.0025). Moreover, the EF₅ values demonstrated considerable spatial variations across these coastal regions with a coefficient of variation of 59%, which were largely related to the difference in water salinity. Our findings filled a key knowledge gap about the indirect N₂O emission factor from aquaculture ponds, and provided field evidence for the refinement of EF₅ value currently adopted by IPCC in the national greenhouse gas inventory.