Program

  The Jiulong River Estuary has been receiving increasing amount of riverine nitrate during last decades due to the intensified anthropogenic activities in the Jiulong River watershed.  However, the sources and biogeochemical cycling of nitrate in the Jiulong River Estuary still remain ambiguous.  The water samples in the Jiulong River Estuary were collected bimonthly along a salinity gradient from February to December, 2014 and the δ¹⁵N and δ¹⁸O values of nitrate were analyzed using the azide method coupling to IR-MS to constrain the sources and biogeochemical cycling of nitrate.  Overall, the average values of δ¹⁵N and δ¹⁸O of nitrate in the Jiulong River Estuary were 10.4±3.2‰ and 5.6±2.3‰, respectively, suggesting the major contribution of the industrial and domestic sewage and livestock wastes.  Nitrate generally followed a conservative binary mixing behavior except on April and June when removal of nitrate in low salinity stations was observed.  In contrast, both δ¹⁵N and δ¹⁸O of nitrate deviated significantly from the binary mixing lines during all sampling cruises, indicating the apparent transformation of nitrate in the Jiulong River Estuary.  The deviation of δ¹⁵N and δ¹⁸O of nitrate from the modelled conservative mixing lines (Δδ¹⁵N and Δδ¹⁸O) is then estimated as their relative isotopic fractionation during the estuarine mixing.  A significant linear correaltion between Δδ¹⁵N and Δδ¹⁸O is observed although the slope varied from 1.07 during dry season to 0.74 during wet season.  The slope close to one (1.07) during dry season indicates the biological assimilation as the major process determining the alteration of isotopic compositions of nitrate as supported by the high chrolophyll a concentration and dissolved oxygen content.  Meanwhile, the slope of 0.74  during wet season suggests the nitrification as the major process controlling the variation of isotopic compositions of nitrate in the Jiulong River Estuary.  Nevertheless, our results indicate that nitrogen cycles in the subtropical estuary is more complicated than expected as suggested by the spatial and seasonal variations of dual nitrate isotopic compositions.