Dual isotopic evidence for nitrate sources and active biological transformation in the Northern South China Sea in summer

Fajin Chen; Xin Zhou; Qibin Lao; Shuangling Wang; Guangzhe Jin; Chunqing Chen; Qingmei Zhu

doi:10.1371/journal.pone.0209287

Dual isotopic evidence for nitrate sources and active biological transformation in the Northern South China Sea in summer

PLoS One. 2019 Jan 2;14(1):e0209287. doi: 10.1371/journal.pone.0209287. eCollection 2019.

Authors

Fajin Chen^{1

2}, Xin Zhou^{1

2}, Qibin Lao^{1

2

3}, Shuangling Wang^{1

2}, Guangzhe Jin^{1

2}, Chunqing Chen^{1

2}, Qingmei Zhu^{1

2}

Affiliations

¹ Guangdong Province Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang, China.
² College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang, China.
³ Marine Environmental Monitoring Centre of Beihai, State Oceanic Administration, Beihai, China.

Abstract

Nitrate (NO3-) concentrations and their dual isotopic compositions (δ15N-NO3- and δ18O-NO3-) were measured to constrain N sources and their cyclic processes in summer using samples from the water column of the northern South China Sea (NSCS). Our data revealed that higher NO3- concentrations and δ15N-NO3- values were observed in the upper waters of the coastal areas near the Pearl River Estuary (PRE). The Bayesian stable isotope mixing model was used to calculated the proportion of nitrate sources, the results indicated that the nitrate in the upper waters of the coastal areas near PRE were mainly influenced by manure and sewage (63%), atmospheric deposition (19%), soil organic nitrogen (12%) and reduced N fertilizer (6%). For the upper waters of the outer areas, low NO3- concentrations and δ15N-NO3- values, but high δ18O-NO3- values, reflected that NO3- was mainly influenced by Kuroshio water intrusion (60%), atmospheric deposition (32%) and nitrogen fixation/nitrification (8%). Complex processes were found in bottom waters. Nitrification and phytoplankton assimilation may be responsible for the higher nitrate concentrations and δ15N-NO3- values. Our study, therefore, utilizes the nitrate dual isotope to help illustrate the spatial variations in nitrate sources and complex nitrogen cycles in the NSCS.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Bayes Theorem
China
Environmental Monitoring
Fertilizers
Manure
Nitrates / analysis*
Nitrates / chemistry
Nitrification
Nitrogen Cycle
Nitrogen Fixation
Nitrogen Isotopes / analysis
Oxygen Isotopes / analysis
Pacific Ocean
Rivers / chemistry
Sewage
Soil
Water Pollutants, Chemical / analysis*
Water Quality

Substances

Fertilizers
Manure
Nitrates
Nitrogen Isotopes
Nitrogen-15
Oxygen Isotopes
Oxygen-18
Sewage
Soil
Water Pollutants, Chemical

Grants and funding

This work was supported by the National Natural Science Foundation of China (41476066, 41466010, 41676008), Guangdong Natural Science Foundation of China (2016A030312004), International Science and Technology Cooperation Project (GASI-IPOVAI-04), National Key Research and Development Plan (2016YFC1401403), the Project of Enhancing School with Innovation of Guangdong Ocean University (GDOU2014050201, GDOU2013050215, GDOU2013050315, GDOU2013050201), and the Fund of Key Laboratory of Global Change and Marine-Atmospheric Chemistry, SOA, (GCMAC1503) to FC. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.