The increase in anthropogenic nitrogen (N) emissions to the environment has led to serious eutrophication in marine ecosystems with increasing oxygen deficient zones (Hypoxia) and more harmful algae blooms. The Baltic Sea, one of the world's largest brackish water bodies, suffers from eutrophication by large N loads from rivers and oxygen deficiency in the central, deep basins. Therefore studies on the identification of major nitrate sources to the Baltic Sea and on the nitrogen loss and turnover in the hypoxic waters were performed.
Data availability. The microarray data presented here are available via GEO (Gene Expression Omnibus; http://www.ncbi.nlm.nih.gov/ geo/, NCBI, 2020) at NCBI (National Center for Biotechnology Information) under GEO accession no GSE142806. The N2O production rates were archived in the PANGAEA data archive (https: //doi.pangaea.de/10.1594/PANGAEA.914948, Frey et al., 2020). The N2O data are available from the Marine Methane and Nitrous Oxide (MEMENTO) database (https://memento.geomar.de/de/n2o, last access: 16 April 2020; Kock and Bange, 2015). ; Supplement. The supplement related to this article is available online at: https://doi.org/10.5194/bg-17-2263-2020-supplement. ; 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. ; German Research Foundation (DFG) SFB754 ; Deutscher Akademischer Austausch Dienst (DAAD) 57350888 ; China Scholarship Council 201406330054 ; Universidad de Granada ; European Union (EU) 704272 ; Villum Foundation 16518 ; National Science Foundation (NSF) OCE-1657663 ; Spanish Government 014/02917
