Aquacultural source of nitrous oxide revealed by nitrogen isotopes

The rapid expansion of coastal aquaculture has led to an increase in the coverage of aquaculture ponds, where intense feed-derived nitrogen is causing significant emissions of nitrous oxide (N₂O). Multiple N₂O production pathways and the relative importance of water column vs. sedimentary production in aquaculture ponds remain uncertain. Clarifying these pathways is vital for sustainable aquaculture development. Using ¹⁵N-labeled dissolved inorganic nitrogen, the pathways and rates of N₂O production in subtropical aquaculture ponds located in south China, cultivating whiteleg shrimp, Japanese seabass, and giant river prawn, were successfully characterized. Total N₂O production rates ranged from 6 to 70 µmol-N m^-2 d^-1, with the shrimp pond exhibiting the highest total N₂O production rates, followed by ponds for seabass and prawn. These differences are primarily due to varying feed amounts causing differences in dissolved nutrients in water column and sediment. Particularly, nutrient and organic matter accumulation at the surface sediment stimulated N₂O production. The oxygenated sediment on a centimeter scale could produce substantially more N₂O compared to the water column above on a meter scale. Partial denitrification, i.e., nitrate and nitrite reduction to N₂O, was more important (> 60 %) for N₂O production in aquaculture ponds. The availability of nitrite is likely a major factor driving partial denitrification for both sedimentary and water column N₂O production.