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 (N₂O) production and emissions. In order to understand changes in the magnitude of N₂O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N₂O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N₂O production to oxygen and organic matter was investigated using ¹⁵N 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 N₂O production with a mean of 8.7 nmol L^-1 d^-1 but up to 118±27.8 nmol L^-1 d^-1 just below the oxic-anoxic interface. The highest N₂O production from ammonium oxidation (AO) of 0.16±0.003 nmol L^-1 d^-1 occurred in the upper oxycline at O₂ concentrations of 10-30 μmol L^-1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N₂O formation (i.e., N₂O with one N atom from NH⁺₄ and the other from other substrates such as NO^-₂) was the dominant species, comprising 70 %-85% of total produced N₂O from NH⁺₄, regardless of the ammonium oxidation rate or O₂ concentrations. Oxygen responses of N₂O production varied with substrate, but production and yields were generally highest below 10 μmol L^-1 O₂. Particulate organic matter additions increased N₂O production by denitrification up to 5-fold, suggesting increased N₂O production during times of high particulate organic matter export. High N₂O yields of 2.1 % from AO were measured, but the overall contribution by AO to N₂O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N₂O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N₂O sources in response to increasing oceanic deoxygenation.