# Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru

Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (inline-formulaN2O) production and emissions. In order to understand changes in the magnitude of inline-formulaN2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to inline-formulaN2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of inline-formulaN2O production to oxygen and organic matter was investigated using inline-formula15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest inline-formulaN2O production with a mean of 8.7 nmol Linline-formula−1 dinline-formula−1 but up to inline-formula118±27.8 nmol Linline-formula−1 dinline-formula−1 just below the oxic–anoxic interface. The highest inline-formulaN2O production from ammonium oxidation (AO) of inline-formula0.16±0.003 nmol Linline-formula−1 dinline-formula−1 occurred in the upper oxycline at inline-formulaO2 concentrations of 10–30 inline-formulaµmol Linline-formula−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid inline-formulaN2O formation (i.e., inline-formulaN2O with one N atom from inline-formula $M21inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimgfc7f721ac75f3b099ec2891c91941f4b bg-17-2263-2020-ie00001.svg24pt15ptbg-17-2263-2020-ie00001.png and the other from other substrates such as inline-formula $M22inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 2}^{-}$ 25pt16ptsvg-formulamathimg57d92df6ed78c83b450a25437527dccc bg-17-2263-2020-ie00002.svg25pt16ptbg-17-2263-2020-ie00002.png ) was the dominant species, comprising 70 %–85 % of total produced inline-formulaN2O from inline-formula $M24inlinescrollmathmlchem{\mathrm{normal NH}}_{normal 4}^{+}$ 24pt15ptsvg-formulamathimgae42caf6f031f0c6641dd1d09f6cb28c bg-17-2263-2020-ie00003.svg24pt15ptbg-17-2263-2020-ie00003.png , regardless of the ammonium oxidation rate or inline-formulaO2 concentrations. Oxygen responses of inline-formulaN2O production varied with substrate, but production and yields were generally highest below 10 inline-formulaµmol Linline-formula−1inline-formulaO2. Particulate organic matter additions increased inline-formulaN2O production by denitrification up to 5-fold, suggesting increased inline-formulaN2O production during times of high particulate organic matter export. High inline-formulaN2O yields of 2.1 % from AO were measured, but the overall contribution by AO to inline-formulaN2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important inline-formulaN2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic inline-formulaN2O sources in response to increasing oceanic deoxygenation.

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Frey, Claudia / Bange, Hermann W. / Achterberg, Eric P. / et al: Regulation of nitrous oxide production in low-oxygen waters off the coast of Peru. 2020. Copernicus Publications.

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