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 (NO). Multiple NO 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 N-labeled dissolved inorganic nitrogen, the pathways and rates of NO production in subtropical aquaculture ponds located in south China, cultivating whiteleg shrimp, Japanese seabass, and giant river prawn, were successfully characterized. Total NO production rates ranged from 6 to 70 µmol-N m d, with the shrimp pond exhibiting the highest total NO 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 NO production. The oxygenated sediment on a centimeter scale could produce substantially more NO compared to the water column above on a meter scale. Partial denitrification, i.e., nitrate and nitrite reduction to NO, was more important (> 60 %) for NO production in aquaculture ponds. The availability of nitrite is likely a major factor driving partial denitrification for both sedimentary and water column NO production.