Abstract
Coastal marine systems are active regions for the production and emission of nitrous oxide (N2O), a potent greenhouse gas. Due to the inherently high variability in different coastal biogeochemical cycles, the factors and mechanisms regulating coastal N2O cycling remain poorly understood. Hydroxylamine (NH2OH), a potential precursor of N2O, has received less attention than other compounds in the coastal areas. Here, we present the spatial distribution of N2O and the first reported NH2OH distribution in the South Yellow Sea (SYS) and the East China Sea (ECS) between March and April 2017. The surface N2O concentrations in the SYS and the ECS varied from 5.9 to 11.3 nmol L–1 (average of 8.4 ± 1.4 nmol L–1) and were characterized by offshore and north–south decreasing gradients. NH2OH showed patchy characteristics and was highly variable, fluctuating between undetectable to 16.4 nmol L–1. We found no apparent covariation between N2O and NH2OH, suggesting the NH2OH pathway, i.e., nitrification (ammonium oxidation), was not the only process affecting N2O production here. The high NH2OH values co-occurred with the greatest chlorophyll-a and oxygen levels in the nearshore region, along with the relationships between NO2–, NO3–, and NH2OH, indicating that a “fresh” nitrifying system, favoring the production and accumulation of NH2OH, was established during the phytoplankton bloom. The high N2O concentrations were not observed in the nearshore. Based on the correlations of the excess N2O (ΔN2O) and apparent oxygen utilization, as well as ΔN2O vs. NO3–, we concluded that the N2O on the continental shelf was mainly derived from nitrification and nitrifier denitrification. Sea-to-air fluxes of N2O varied from −12.4 to 6.6 μmol m–2 d–1 (−3.8 ± 3.7 μmol m–2 d–1) using the Nightingale et al. (2000) formula and −13.3 to 6.9 μmol m–2 d–1 (−3.9 ± 3.9 μmol m–2 d–1) using the Wanninkhof (2014) formula, which corresponds to 75–112% in saturation, suggesting that the SYS and the ECS acted overall as a sink of atmospheric N2O in early spring, with the strength weakening. Our results reveal the factors and potential mechanisms controlling the production and accumulation of NH2OH and N2O in the SYS and the ECS during early spring.
Highlights
Oceans represent the second important natural source of the climatically important trace gas nitrous oxide (N2O), a compound that plays a pivotal role in the global greenhouse effect and atmospheric chemistry generally (Stocker et al, 2013)
Since the first study of the N2O in the northeastern East China Sea (ECS) by Zhang et al (2008), researchers have carried out studies in the Yellow Sea (YS) (Yang et al, 2009), the East China Sea shelf (Wang et al, 2016), and the Yangtze River Estuary and its adjacent waters (Zhang et al, 2010)
The middle of transect B corresponded to the local maximum in both temperature and salinity, dominated by the Yellow Sea warm current (YSWC); the colder and less saline waters, carried by the coastal current, occupied both sides of the cross transect
Summary
Oceans represent the second important natural source of the climatically important trace gas nitrous oxide (N2O), a compound that plays a pivotal role in the global greenhouse effect and atmospheric chemistry generally (Stocker et al, 2013). The first record of the positive correlation of the concentrations of N2O and NH2OH was a recent study of the eastern tropical South Pacific and the equatorial Atlantic Ocean from Korth et al (2019) This revealed that NH2OH is a potential proxy for active nitrification and in situ N2O production in the open ocean. Since the first study of the N2O in the northeastern East China Sea (ECS) by Zhang et al (2008), researchers have carried out studies in the Yellow Sea (YS) (Yang et al, 2009), the East China Sea shelf (Wang et al, 2016), and the Yangtze River Estuary and its adjacent waters (Zhang et al, 2010) These studies found that disproportionately high N2O emissions occurred in the ECS and the YS, which are only a small proportion of world ocean areas. We integrated the analyses of N2O, NH2OH, and a variety of active biological elements (chlorophyll-a, O2, and nitrogenous nutrients) and discussed the potential in situ biological production processes of N2O
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