Nitrogen isotope constraint on the zonation of multiple transformations between dissolved and particulate organic nitrogen in the Changjiang plume

Xiuli Yan; Jin-Yu Terence Yang; Min Nina Xu; Hongjie Wang; Minhan Dai; Shuh-Ji Kao

doi:10.1016/j.scitotenv.2021.151678

Nitrogen isotope constraint on the zonation of multiple transformations between dissolved and particulate organic nitrogen in the Changjiang plume

Sci Total Environ. 2022 Apr 20:818:151678. doi: 10.1016/j.scitotenv.2021.151678. Epub 2021 Nov 15.

Authors

Xiuli Yan¹, Jin-Yu Terence Yang², Min Nina Xu³, Hongjie Wang⁴, Minhan Dai², Shuh-Ji Kao⁵

Affiliations

¹ Institute of Marine Science, Shantou University, Shantou 515063, China; State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China.
² State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China.
³ State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China; State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China.
⁴ State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China; University of Washington Cooperative Institute for Climate, Ocean, and Ecosystem Science, 7600 Sand Point Way NE, Seattle, WA 98115, United States of America.
⁵ State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China; State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China. Electronic address: sjkao@xmu.edu.cn.

PMID: 34793800
DOI: 10.1016/j.scitotenv.2021.151678

Abstract

Information on the sources and transformations of particulate organic N (PN) and dissolved organic N (DON) at the coastal interface remains insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. Here, we investigated the spatial distribution of concentrations and isotopic compositions of PN and DON in the Changjiang plume during the summer flood period. In average, DON and PN accounted for 25.6 ± 12.1% and 8.1 ± 9.1% (n = 55), respectively, of the total N pool, with the remaining N primarily in the form of nitrate (NO₃^-). Mean δ¹⁵N values were the lowest for DON (-0.1 ± 2.7‰, n = 58) and slightly higher for PN (2.0 ± 1.6‰, n = 101), and the highest for NO₃^- (6.5 ± 2.2‰, n = 67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among the three N pools. By applying a conservative mixing model, we found DON deficits (-3.5 ± 3.7 μmol L^-1, n = 43) and negative shift in δ¹⁵N_DON (-3.6 ± 2.2‰, n = 43) in the Changjiang plume, revealing nonconservative DON behaviors. In the offshore surface plume where Chlorophyll a was high, the most likely cause is the DON uptake by phytoplankton with a strong inverse isotope effect (around -40‰). This DON assimilation by phytoplankton contributed to ~16 ± 12% of the PN production, with the remaining supported by NO₃^- assimilation, producing an overall isotope effect of 4-9‰. However, in waters near the river mouth and at the bottom of the offshore plume where total suspended matter concentrations were high (>5 mg L^-1), the DON deficit was most likely induced by the selective adsorption of ¹⁵N enriched moieties of DON onto particulate surfaces (with an isotope effect of -20‰ to -5‰). Unlike dissolved organic carbon to behave conservatively in most estuaries, our results show that active transformations had occurred between the DON and PN pools in the Changjiang plume.

Keywords: Changjiang plume; Dissolved organic nitrogen; Isotope; Nitrogen dynamic; Particulate organic nitrogen.

MeSH terms

China
Chlorophyll A
Environmental Monitoring* / methods
Nitrogen Isotopes / analysis
Nitrogen* / analysis
Rivers

Substances

Nitrogen Isotopes
Nitrogen
Chlorophyll A