Abstract
Abstract. 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 NH4+ and the other from other substrates such as NO2-) was the dominant species, comprising 70 %–85 % of total produced N2O from NH4+, 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.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas (IPCC, 2013) and precursor for nitric oxide (NO) radicals, which can catalyze the destruction of ozone in the stratosphere (Crutzen, 1970; Johnston, 1971) and is the single most important ozone-depleting emission (Ravishankara et al, 2009)
Our goal was to determine the impact of O2 and particulate organic matter (POM) on N2O production rates using 15N tracer experiments in combination with quantitative PCR and functional gene microarray analysis of the marker genes, nirS for denitrification and amoA for ammonium oxidation (AO) by archaea, to assess how the abundance and structure of the community impacts N2O production rates from the different pathways. 15N-labeled NH+4 and NO−2 were used to trace the production of single-labeled (45N2O) and double-labeled (46N2O) N2O to investigate the importance of hybrid N2O production during AO along an O2 gradient
The highest N2O production rates from NO−2 and NO−3 were found at or below the oxic–anoxic interface, whereas the highest N2O production from AO was slightly shallower in the oxycline
Summary
Nitrous oxide (N2O) is a potent greenhouse gas (IPCC, 2013) and precursor for nitric oxide (NO) radicals, which can catalyze the destruction of ozone in the stratosphere (Crutzen, 1970; Johnston, 1971) and is the single most important ozone-depleting emission (Ravishankara et al, 2009). The expansion of ODZs is predicted in global change scenarios and has already been documented in recent decades (Stramma et al, 2008; Schmidtko et al, 2017). This might lead to further intensification of marine N2O emissions, which will constitute a positive feedback on global warming (Battaglia and Joos, 2018). Decreasing N2O emissions have been predicted based on reduced nitrification rates due to reduced primary and export production (Martinez-Rey et al, 2015; Landolfi et al, 2017) and ocean acidification (Beman et al, 2011; Breider et al, 2019). The parametrization of N2O production and consumption in global ocean models is crucial for realistic future predictions, and better understanding of their controlling mechanisms is needed
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