Seasonal isotopic and isotopomeric signatures of nitrous oxide produced microbially in a eutrophic estuary

Anthropogenic input of excess nutrients stimulates massive nitrous oxide (N2O) production in estuaries with distinct seasonal variations. Here, nitrogen isotopic and isotopomeric signatures were utilized to investigate the seasonal dynamics of N2O production and nitrification at the middle reach of the eutrophic Pearl River Estuary in the south of China. Elevated N2O production primarily via ammonia oxidation (> 1 nM-N d−1) occurred from April to November, along with increased temperature and decreased dissolved oxygen concentration. This consistently oxygenated water column showed active denitrification, contributing 20–40 % to N2O production. The water column microbial N2O production generally constituted a minor fraction (10–15 %) of the estuarine water-air interface efflux, suggesting that upstream transport and tidal dilution regulated the dissolved N2O inventory in the middle reach of the estuary. Nitrification (up to 3000 nM-N d−1) played a critical role in bioavailable nitrogen conversion and N2O production, albeit with N2O yields below 0.05 %.