Nitrogen deposition in precipitation to a monsoon-affected eutrophic embayment: Fluxes, sources, and processes
Introduction
Emission of reactive nitrogen, including oxidized and reduced inorganic nitrogen and organic nitrogen, to the atmosphere have increased dramatically because of food production and energy consumption (Duce et al., 2008; Galloway et al., 2003, 2008). Globally, nitrogen deposition increased slowly from the 1860s to the 1960s, then rapidly in the past half-century (Galloway et al., 2008). It has been predicted that atmospheric nitrogen deposition to the ocean will reach 77 Tg N·yr−1 by 2030, of which more than 45% derives from anthropogenic sources (Altieri et al., 2014; Duce et al., 2008). Such fluxes of anthropogenic nitrogen deposited to the ocean can impact coastal ecosystem functions, especially embayments downwind of densely populated areas (Galloway et al., 2008; Kim et al., 2014). Inputs of biologically available nitrogen from the atmosphere to coastal waters are comparable to that from riverine input (Duce et al., 1991; Spokes and Jickells, 2005). It is estimated that 10% to more than 40% of nitrogen entering estuarine and coastal waters derives from atmospheric sources and has been considered as a major driver of primary production (Owens et al., 1992; Paerl et al., 2002; Valigura et al., 2001). Nitrogen-containing compounds in precipitation provide phytoplankton with nutrients and potentially contribute to eutrophication in estuaries and coastal waters (Galloway et al., 2003).
Identifying the sources of nitrogen in precipitation and their biogeochemical fate is the key managing nitrogen inputs from the atmosphere. Dual isotopes, δ15N and δ18O, of nitrate have been used to identify the sources and chemical processes of nitrate in the atmosphere (Fang et al., 2011; Hoering, 1957; Michalski et al., 2015). δ15N−NO3 is viewed as a signal for nitrate sources as the N atoms are conservative during the formation of NO3− in the atmosphere (Morin et al., 2009; Widory, 2007). δ15N−NO3 abundance can be used to determine N provenance, for example: +6 to +25.6‰ for coal combustion (Felix et al., 2012; Heaton, 1990), −19.1 to +9.8‰ for vehicle (Walters et al., 2015), −7 to +12‰ for biomass burning (Fibiger and Hastings, 2016), −0.5 to +1.4‰ for lightning (Hoering, 1957), and −50 to −20‰ for biogenic emission (Li and Wang, 2008). δ18O-NO3- values implicate the oxidation pathway involved with oxidants as O3, free radical like hydroxy (OH), peroxy and halogen. There are two main pathways, (1) OH radical pathway (+55‰), 2/3 O3 and 1/3 OH radical; (2) N2O5 pathway (+102‰), 5/6 O3 and 1/6 H2O, for the formation of oxygen atoms in nitrate (Fang et al., 2011; Hastings et al., 2003). 76% and 18% of annual inorganic NO3− are formed via the OH pathway and the N2O5 pathway, respectively. Moreover, dimethylsulfide (DMS) and hydrocarbons (HC) (DMS/HC) pathway is another important pathway in forming oxygen atoms of nitrate in recent years, which accounts for 4% of the annual inorganic NO3− (Alexander et al., 2009; Fang et al., 2011). NO3− induced via the DMS/HC pathway have higher δ18O values than those via the OH pathway and the N2O5 pathway due to the involvement of O3 (Hastings et al., 2003).
China's coastal areas are hotspots of nitrogen emissions and deposition (Liu et al., 2011). A recent study reported that nitrogen deposition averaged 282 mmol∙(m2∙yr)−1 within China's coast during 2010–2012, of which wet deposition account for 54.6% (Luo et al., 2014). Daya Bay, located on the north coast of the South China Sea (SCS), is surrounded by densely populated land with rapid industrial growth along the Pearl River Delta (PRD), an economic hub in south China. Industrial development has increased considerably during the past 30 years along Daya Bay's coast, including the petrochemical industry, nuclear and coal-fired power plants, and marine aquaculture. The drastically increasing input of nutrients has induced eutrophication in Daya Bay (Yu et al., 2007). To our knowledge, the deposition flux of nitrogen in precipitation in rainy seasons (March to August in 2012) (Chen et al., 2014) and incomplete annual data (a field observation from August 2008 to July 2009, but no data from November 2008 to February 2009) (Zou et al., 2011) were reported. Daya Bay is a semi-enclosed bay with relative lower water exchange ability. And the nitrogen emission continues to increase in this region. On the basis of understanding the dry deposition flux (Wu et al., 2018) and riverine input (Ren et al., 2013) of nutrients in Daya Bay, therefore, it is urgent to provide annual data of wet deposition fluxes of nitrogenous species for systematic calculation the budget of nitrogen deposition and quantifying its ecological effects on Daya Bay. Moreover, Daya Bay is under the influence of the East Asian monsoon system, whereby strong northeast winds prevail in winter and southwest winds in spring. However, the sources of nitrogen deposited from the atmosphere to the Daya Bay under the co-effect of air pollution and monsoon climate are unknown. This research, which was conducted over one-year of field sampling, aims to provide a preliminary understanding of the input flux and sources of nitrogen in precipitation in Daya Bay under the combined effects of monsoon and anthropogenic pollution.
In this study, concentrations and fluxes of nitrogenous species, ion composition and dual isotopes (δ15N-NO3−, and δ18O-NO3−) of nitrate in precipitation in Daya Bay were measured to identify nitrogen inputs, estimate the sources of the main nitrogen inputs and discuss the depositional processes among different seasons.
Section snippets
Sampling description
Daya Bay is a semi-enclosed subtropical embayment located in 22.45–22.83°N, 114.50–114.89°E on the northern coast of SCS (Fig. 1). It is one of the largest bays of south China, with an area of ∼600 km2. The average annual temperature (22 °C) and precipitation (1948 mm) are controlled by subtropical and monsoonal climate of East Asia. 14 typhoons traverse the SCS in summer and autumn annually (Huang and Guan, 2012), half of which invade or influence Daya Bay on average. March to August is
Concentration of nitrogen in precipitation
The time series data of the nitrogen concentration and atmospheric nitrogen deposition flux in precipitation and rainfall for each month are illustrated in Fig. 2. The concentration of TDN varied from 29.3 to 556.5 μmol L-1, with an annual VWM concentration of 54.3 μmol L-1. TDN concentration present contrary variation trend and negatively correlated with rainfall (r = 0.58, p < 0.05). This is partially due to the strong dilution effects of the higher rainfall in rainy seasons (Laouali et al.,
Conclusions
The atmospheric deposition fluxes of TDN in precipitation was 132.5 mmol m−2∙yr−1 from October 2015 to September 2016, which accounted for 37.8% of the total input of nitrogen into Daya Bay. DIN accounted for 62.6% of the TDN, and NO3− accounted for 77.8% of the DIN. DON acted as a significant contributor to nitrogen in precipitation. Further study on the compositions and specific sources of DON should be clarified in the future due to the important role of DON. The TDN flux was higher in
Acknowledgements
This work was supported by the National Program on Key Basic Research Project of China (No.2015CB452901, No.2015CB452905), and National Natural Science Foundation of China (NSFC, No.41406128 and No. 41730529). Peter Macreadie's involvement was supported by an Australian Research Council Linkage Project (LP160100242). The MBRS was thanked for sampling supporting. We also gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model
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