Dissolved Nitrous Oxide and Hydroxylamine in the Northern South China Sea in Summer

Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas and a strong ozone‐depleting substance. Ocean is an important natural source of atmospheric N 2 O, which has attracted increasing attention recently as to understand the biogeochemical cycles of nitrogen in marine systems. Nitrification is one of the major N 2 O production pathways during which N 2 O is produced as a side product and hydroxylamine (NH 2 OH) is produced as an obligate intermediate. However, few studies have reported the NH 2 OH variability in natural seawater and its relationship between N 2 O remains ambiguous. In summer 2022, spatial distribution and influencing factors of N 2 O and NH 2 OH in the northern South China Sea (SCS) were investigated. Dissolved N 2 O varied from 5.79 to 34.80 nmol L −1 in water column and generally increased with water depth above 800 m. N 2 O was correlated with dissolved oxygen and nitrate, inferring that N 2 O was mainly produced via nitrification. NH 2 OH varied from below detection limit to 3.91 nmol L −1 . The adoption of multiple standard additions can largely reduce the bias in NH 2 OH measurement. Structural equation modeling showed that NH 2 OH has a potential effect favoring excessive N 2 O. Surface N 2 O saturation ranged between 102% and 278%, indicating that the northern SCS was a net source of atmospheric N 2 O in summer.