Isotopic composition of water-soluble nitrate in bulk atmospheric deposition at Dongsha Island: sources and implications of external N supply to the northern South China Sea

Abstract

Abstract. Increased reactive nitrogen (Nr, NO3− + NH4++ + dissolved organic nitrogen) emission from the Asian continent poses profound threats to ecosystem safety from terrestrial throughout the ocean proper. To quantify atmospheric Nr input, diagnose its sources, and evaluate influence on marine nitrogen cycle of the South China Sea (SCS), an oligotrophic marginal sea adjacent to the emission hot spot China, we conducted measurements of concentrations of nitrate and ammonium as well as other major ions and dual isotopes of nitrate (δ15NNO3 and δ18ONO3) in atmospheric deposition collected from Dongsha Island off southern China. The δ15NNO3 and δ18ONO3 for dry deposition averaged at −2.8‰ and +58.8‰, ranging from −7.5 to +3.7‰ and from ∼ +17 to +88‰, respectively. Wet deposition, although with limited samples, showed a similar flux-weighted mean in δ15NNO3 (−2.6‰) yet a significantly higher mean in δ18ONO3 (+78.8‰). The dual isotope ratios showed an anti-correlation and an inverse seasonality; the δ15NNO3 values were higher in summer compared to those in winter, while the δ18ONO3 values were higher in winter than those in summer. In winter, not only dual isotopic compositions of nitrate but also the ammonium and nitrate dry deposition fluxes were relatively uniform, demonstrating a persistent influence of fossil fuel combustion sourced from Asian continental outflows via the northeasterly monsoon winds. More variable isotopic values in summer likely suggest varying sources and dynamical formation processes of dry deposition nitrate. Biomass burning and lightning are suggested to be responsible for the observed higher δ15NNO3 values in summer. Atmospheric nitrate and ammonium deposition together was estimated to be ∼50 mmol N m−2 year−1, with the dominance of nitrate in dry deposition but ammonium slightly higher in wet deposition. If not including this additional fertilization of atmospheric inorganic nitrogen deposition to enhance the carbon sequestration, CO2 release out of the SCS would be double than that of the present amount, 460 ± 430 mmol C m−2 year−1. Our study demonstrates that atmospheric deposition may serve as an important external Nr supplier to the SCS; however it is difficult to separate the isotopic signal from N2 fixation (−2–0‰) due to their similarity in δ15NNO3. More studies related to isotopic composition of nitrogen speciation in atmospheric Nr deposition, their relative contributions and source identification, and the role of typhoons in the SCS are required.

Diapycnal Fluxes of Nutrients in an Oligotrophic Oceanic Regime: The South China Sea

“…According to previous studies, the summation of the N 2 fixation rate (0.055 ± 0.071 mmol m −2 d −1 ; Kao et al, ) and the atmospheric N deposition rate (0.048 ± 0.024 mmol m −2 d −1 ; Yang et al, ) is ~0.10 mmol m −2 d −1 in the summer season. Given the oligotrophic nature of the system under study, these externally supplied nutrients can only impact the very surface of the ocean, likely limited to the NDL (Bombar et al, ).…”

Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

“…The distribution of δ 15 N NO3 in the northern SCS presented minimum values of 4.8 ± 0.3‰ ( n = 14) in the subsurface (~100–125 m; Figure ). This is a common pattern observed in other tropical and subtropical oligotrophic oceans, primarily as a result of the accumulation of lighter N signals ( 14 N‐enriched) from the atmosphere (e.g., N 2 fixation and atmospheric N deposition) and preferential remineralization of low δ 15 N organic matter [e.g., Bourbonnais et al ., ; Casciotti et al ., ; Kao et al ., ; Liu et al ., ; Yang et al ., ]. In addition, it is in line with the only δ 15 N NO3 profile with a subsurface minimum of 4.6 ± 0.2‰ observed at the SEATS site during April 1997 [ Wong et al ., ].…”

Sources of reactive nitrogen in marine aerosol over the Northwest Pacific Ocean in spring

“…Averaged dry depositions of NH 4 + and NO 3 − in 2014 were 2 ~ 5 times higher than those of 2015 (Table 3) for all the aerosols. The dry deposition fluxes of NH 4 + and NO 3 − in dust aerosol were obviously higher than those in background aerosol in the NWPO (Table 3, (Yang et al, 2014). Here, we follow the previous approach in both Atlantic (Knapp et al, 2010) and South China Sea (Yang et al, 2014) to estimate the proportion(X) of lowing the thermocline δ 15 N-NO 3 − in the NWPO which may be caused by the atmospheric dry deposition of Nr: the NWPO may be more pronounced.…”

“…The detailed analytical steps of stable nitrogen isotope can be found in Archana et al (2016) and Yang et al (2014). Briefly, NO 3 − was reduced to N2O by denitrifying bacteria, Pseudomonas aureofaciens (ATCC 13985), then the stable nitrogen isotope of N2O was analyzed by using GasBench II connected to a continuous flow isotope ratio mass spectrometer (IRMS,…”