Guangdong Province Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang, China.
College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang, China.
PLoS One. 2019 Jan 2;14(1):e0209287. doi: 10.1371/journal.pone.0209287. eCollection 2019.
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.
采用夏季南海北部海域水柱样品，测定硝酸盐（NO3-）浓度及其双重同位素组成（δ15N-NO3-和δ18O-NO3-），以约束氮源及其循环过程。研究数据表明，珠江口附近沿海地区的上层水体中存在更高的硝酸盐浓度和δ15N-NO3-值。利用贝叶斯稳定同位素混合模型计算硝酸盐源的比例，结果表明，珠江口附近沿海地区上层水体中的硝酸盐主要受粪肥和污水（63%）、大气沉降（19%）、土壤有机氮（12%）和还原态氮肥（6%）的影响。对于外海的上层水体，低的 NO3-浓度和 δ15N-NO3-值，以及高的 δ18O-NO3-值，反映出 NO3-主要受黑潮水入侵（60%）、大气沉降（32%）和固氮/硝化（8%）的影响。底层水体中存在复杂的过程。硝化作用和浮游植物同化可能是硝酸盐浓度和 δ15N-NO3-值较高的原因。因此，本研究利用硝酸盐双重同位素来帮助说明南海北部海域硝酸盐源的空间变化和复杂的氮循环。
