Evidence of parallel denitrification and nitrite oxidation in the ODZ of the Arabian Sea from paired stable isotopes of nitrate and nitrite

Abstract

[1] The Arabian Sea is a major oceanic nitrogen sink, and its oxygen-deficient zone (ODZ) extends from 150 m to 1200 m water depth. To identify the dominant transformation processes of reactive nitrogen and to quantify the amounts of nitrogen turned over in the different reactions of the nitrogen cycle, we use paired data on stable isotope ratios of nitrogen and oxygen in nitrate and nitrite measured at four near-coastal and five open ocean stations in the Arabian Sea. We find significant nitrate reduction and denitrification between 100 m and 400 m in the open Arabian Sea, which are most intense in the eastern and northern part of the basin, and estimate that about 50% of initial nitrate is being reduced either to dinitrogen gas (denitrification) or to nitrite (nitrate reduction) in the core zone of denitrification. Nitrite accumulates in concentrations above 4 μM in the water column of the eastern and northern Arabian Sea. Large differences in isotopic ratios of nitrate and nitrite and a decoupling of their stable nitrogen and oxygen isotopes can be explained by the reoxidation of nitrite. The observed decoupling of the paired isotopes may be due to the exchange of oxygen of nitrite with oxygen from ambient water. In agreement with model estimates from the literature, about 25% of the nitrate initially reduced to nitrite is returned to the nitrate pool by nitrification in the upper and lower denitrification layer while 40% is denitrified.Citation: Gaye, B., B. Nagel, K. Da¨hnke, T. Rixen, and K.-C. Emeis (2013), Evidence of parallel denitrification and nitrite oxidation in the ODZ of the Arabian Sea from paired stable isotopes of nitrate and nitrite, Global Biogeochem. Cycles, 27,

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

“…The resulting average NO − 3 loss of 15.4 ±4.3 Tg N yr −1 is within the medium range of previously published denitrification rates ranging from approximately 2 to 4 Tg N yr −1 (Deuser et al, 1978;Naqvi et al, 1982;Somasundar et al, 1990) to values of 10 and 30 Tg N yr −1 (e.g. Bange et al, 2000;Gaye et al, 2013;Howell et al, 1997) and even reaching values of 60 Tg N yr −1 (4.28 × 10 12 mol N yr −1 ). While the lower estimates were mainly derived from NO − 3 losses obtained and interpolated through different methods and nitrogen isotopic mass balance calculations, the high rate of 60 Tg N yr −1 was based on the excess N 2 concentrations derived from N 2 / Ar ratios (Codispoti, 2007;Codispoti et al, 2001b).…”

“…In order to avoid negative values, we multiplied N * by −1. Accordingly, increasing NO (Casciotti and McIlvin, 2007;Gaye et al, 2013). The standard deviation for IAEA-N3 was 0.3 ‰, which is in the same range for δ 15 N NO 3 for at least duplicate measurements of the samples.…”

“…The correlation between oxygen concentrations and N * (which was not found at the western stations) confirms that denitrification was significant below an oxygen concentration of approximately 1 µM. Since denitrification does not utilize oxygen, the decreasing oxygen concentrations associated with increasing N * indicates that denitrification was also coupled to processes using up oxygen such as nitrite oxidation and/or aerobic respiration (Gaye et al, 2013).…”

Enzyme‐level interconversion of nitrate and nitrite in the fall mixed layer of the Antarctic Ocean

“…In tracer studies, this has been shown through the addition of 15 N-labeled NO 2 À to seawater samples collected from suboxic zones and the subsequent detection of 15 N-labeled NO 3 À after incubation under anoxic conditions [Füssel et al, 2012;Beman et al, 2013;Peng et al, 2015]. In natural abundance studies, the same conclusion of NO 2 À oxidation is reached through the observation of a decoupling of NO N in suboxic waters; these observations have been attributed to forcings from the combined isotope effects of NO 3 À reduction, NO 2 À reduction and oxidation, and anammox Casciotti, 2009;Casciotti et al, 2013;Gaye et al, 2013;Buchwald et al, 2015;Martin and Casciotti, 2016]. Again, the perplexing aspect of interpretations suggesting NO 2 À oxidation in ODZ waters is the lack of appropriate oxidized species in the water column to serve as electron acceptors [Buchwald et al, 2015;Peng et al, 2015].…”

“…A counterintuitive finding in the ocean's subsurface oxygen-deficient zones (ODZs), both from 15 N tracer studies [Lipschultz et al, 1990;Füssel et al, 2012;Beman et al, 2013;Peng et al, 2015] and natural abundance isotope observations Casciotti et al, 2013;Gaye et al, 2013;Buchwald et al, 2015], is that NO 2 À appears to undergo oxidation to NO 3 À despite the lack of appropriate oxidants in the environment. In tracer studies, this has been shown through the addition of 15 N-labeled NO 2 À to seawater samples collected from suboxic zones and the subsequent detection of 15 N-labeled NO 3 À after incubation under anoxic conditions [Füssel et al, 2012;Beman et al, 2013;Peng et al, 2015].…”