The nitrogen isotopic composition of dissolved nitrate in the Yangtze River (Changjiang) estuary, China

The nitrogen isotopic composition of dissolved nitrate (δ15N-NO3−) in surface water of the Yangtze River estuary was determined in four seasons of 2006. δ15N-NO3− ranged from 0.4‰ to 6.5‰ and varied with seasons and geographic regions, reflecting the dynamics of nitrogen cycling in the estuarine ecosystem. δ15N-NO3− was markedly lower in February than in other seasons and exhibited conservative mixing, which was probably attributed to the NO3− being sourced from the atmospheric deposition and agricultural fertilizer. In the upper estuary, the influence of riverine inputs was important during all surveys. In the turbidity maximum zone, nitrification was found with nitrate depleted in 15N in May, whereas denitrification resulting in heavy δ15N-NO3− played an important role in August. More enriched δ15N-NO3− values coinciding with losses of nitrate concentrations based on the conservative mixing model were found in the adjacent marine area in May, and may reflect obvious phytoplankton assimilation of dissolved nitrate. In this manner, δ15N-NO3− may be a sensitive indicator of nitrogen sources and biogeochemical processing existing in this estuary in conjunction with the variations of dissolved nitrate and other environmental factors.