期刊
JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES
卷 120, 期 4, 页码 661-672出版社
AMER GEOPHYSICAL UNION
DOI: 10.1002/2014JG002842
关键词
nitrate; dual stable isotopes; nitrogen cycle; the Pearl River Estuary
资金
- National Natural Science Foundation of China [41061160498, 41306102, 41276072]
- China Postdoctoral Science Foundation [2012 M521629]
Nitrate (NO3-) concentrations and the isotopic composition (N-15 and O-18) of water samples from the Pearl River Estuary (PRE), south China, were measured to constrain N sources and processing in springtime. Nitrate concentrations decreased at higher salinities, and dual isotopic values were correlated with 1/[NO3-], both of which suggest mixing of eutrophic freshwater and oligotrophic seawater. However, N-15 and O-18 values did not closely follow the expected mixing lines. At low salinities (0-3.0), some samples exhibited high N-15 and low O-18 values compared with those of the riverine end-member, indicating that sewage is a significant source of nitrate (up to 19.0% calculated from N-15). At salinities of >3.0, N-15 and O-18 values are linearly correlated with ln integral (the fraction of NO3- remaining in the system), which is attributable to isotopic Rayleigh fractionation during phytoplankton uptake of nitrate. However, the linear relationship between N-15 and O-18 is different between the west and east PRE, with a slope of 1.18 in the west and 1.59 in the east. This difference most likely resulted from varying degrees of nitrification due to different water velocities and residence times in the two areas. Our data therefore indicate that dual nitrate isotopic signatures are a valuable way to constrain the sources and behavior of nitrate in river-dominated estuaries.
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