Abstract
Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to stratospheric ozone depletion. Besides microbial denitrification, abiotic nitrite reduction by Fe(II) (chemodenitrification) has the potential to be an important source of N2O. Here, using microcosms, we quantified N2O formation in coastal marine sediments under typical summer temperatures. Comparison between gamma-radiated and microbially-active microcosm experiments revealed that at least 15–25% of total N2O formation was caused by chemodenitrification, whereas 75–85% of total N2O was potentially produced by microbial N-transformation processes. An increase in (chemo)denitrification-based N2O formation and associated Fe(II) oxidation caused an upregulation of N2O reductase (typical nosZ) genes and a distinct community shift to potential Fe(III)-reducers (Arcobacter), Fe(II)-oxidizers (Sulfurimonas), and nitrate/nitrite-reducing microorganisms (Marinobacter). Our study suggests that chemodenitrification contributes substantially to N2O formation from marine sediments and significantly influences the N- and Fe-cycling microbial community.
Highlights
Nitrous oxide (N2O) is one of the most important long-lived greenhouse gases with an atmospheric lifetime of 131 ± 10 years[1]
Processes forming N2O include: (1) nitrification[7], (2) denitrification[8] by fungi, archaea, and bacteria, (3) dissimilatory nitrate reduction to ammonium (DNRA)[9], (4) nitrifier-denitrification[10,11] and (5) nitrite-induceddenitrification, e.g. by ferrous iron (Fe(II))[12,13]
We found that the addition of Fe(II), nitrate, and nitrite to the marine sediment and the resulting N2O formation caused a general microbial community shift based on DNA and RNA analysis (Fig. S2)
Summary
Nitrous oxide (N2O) is one of the most important long-lived greenhouse gases with an atmospheric lifetime of 131 ± 10 years[1]. Elevated levels of N2O have been observed in numerous studies examining iron- and nitrate-/nitrite-rich environments, e.g. soils[26], hypersaline ponds and brines in Antarctica[27], and marine coastal sediments[28,29,30] These high levels of N2O have been solely attributed to microbial denitrification, potentially overlooking the important contribution of chemodenitrification to the overall N2O formation. Based on microsensor measurements Wankel et al.[30] could show N2O formation at the interface of the nitrate and Fe(III) reduction zone within marine sediments These authors hypothesized that chemodenitrification could play a major role in N2O production and hinted towards connections between the biogeochemical Fe and N cycle. The focus of our study was to quantify the chemodenitrification-based N2O formation potential in natural marine sediments and to investigate the potential impact on the microbial community
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