Qixing Ji

Nitrous oxide production by nitrification and denitrification in the Eastern Tropical South Pacific oxygen minimum zone

The Eastern Tropical South Pacific oxygen minimum zone (ETSP-OMZ) is a site of intense nitrous oxide (N2O) flux to the atmosphere. This flux results from production of N2O by nitrification and denitrification, but the contribution of the two processes is unknown. The rates of these pathways and their distributions were measured directly using 15N tracers. The highest N2O production rates occurred at the depth of peak N2O concentrations at the oxic-anoxic interface above the oxygen deficient zone (ODZ) because slightly oxygenated waters allowed (1) N2O production from both nitrification and denitrification and (2) higher nitrous oxide production yields from nitrification. Within the ODZ proper (i.e., anoxia), the only source of N2O was denitrification (i.e., nitrite and nitrate reduction), the rates of which were reflected in the abundance of nirS genes (encoding nitrite reductase). Overall, denitrification was the dominant pathway contributing the N2O production in the ETSP-OMZ.

Nitrogen substrate–dependent nitrous oxide cycling in salt marsh sediments

Nitrous oxide (N2O) is important to Earth’s climate because it is a strong absorber of radiation and an important ozone depletion agent. Increasing anthropogenic nitrogen input into the marine environment, especially to coastal waters, has led to increasing N2O emissions. Identifying the nitrogen compounds that serve as substrates for N2O production in coastal waters reveals important pathways and helps us understand their control by environmental factors. In this study, sediments were collected from a long-term fertilization site in Great Sippewissett Marsh, Falmouth, Massachusetts. The 15N tracer incubation time course experiments were conducted and analyzed for potential N2O production and consumption rates. The two nitrogen substrates of N2O production, ammonium and nitrate, correspond to the two production pathways, nitrification and denitrification, respectively. When measurable nitrate was present, despite ambient high ammonium concentrations, denitrification was the major N2O production pathway. When nitrate was absent, ammonium became the dominant substrate for N2O production, via nitrification and coupled nitrification-denitrification. Net N2O consumption was enhanced under low oxygen and nitrate conditions. N2O production and consumption rates increased with increasing levels of nitrogen fertilization in long-term experimental plots. These results indicate that increasing anthropogenic nitrogen input to salt marshes can stimulate sedimentary N2O production via both nitrification and denitrification, whereas episodic oxygen depletion results in net N2O consumption.

Long‐term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments

Salt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient over-enrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15 N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen / 80% helium). Nitrification and denitrification rates were more than ten-fold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long-term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal. Long-term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments: NO 3 - reduction in salt marsh sediments (PDF Download Available). Available from: https://www.researchgate.net/publication/305480944_Long-term_fertilization_alters_the_relative_importance_of_nitrate_reduction_pathways_in_salt_marsh_sediments_NO_3_-_reduction_in_salt_marsh_sediments [accessed Jun 6, 2017].

Nitric Oxide Isotopic Analyzer Based on a Compact Dual-Modulation Faraday Rotation Spectrometer

We have developed a transportable spectroscopic nitrogen isotopic analyzer. The spectrometer is based on dual-modulation Faraday rotation spectroscopy of nitric oxide isotopologues with near shot-noise limited performance and baseline-free operation. Noise analysis indicates minor isotope (15NO) detection sensitivity of 0.36 ppbv·Hz−1/2, corresponding to noise-equivalent Faraday rotation angle (NEA) of 1.31 × 10−8 rad·Hz−1/2 and noise-equivalent absorbance (αL)min of 6.27 × 10−8 Hz−1/2. White-noise limited performance at 2.8× the shot-noise limit is observed up to ~1000 s, allowing reliable calibration and sample measurement within the drift-free interval of the spectrometer. Integration with wet-chemistry based on acidic vanadium(III) enables conversion of aqueous nitrate/nitrite samples to gaseous NO for total nitrogen isotope analysis. Isotopic ratiometry is accomplished via time-multiplexed measurements of two NO isotope transitions. For 5 μmol potassium nitrate samples, the instrument consistently yields ratiometric precision below 0.3‰, thus demonstrating potential as an in situ diagnostic tool for environmental nitrogen cycle studies.

Dependence of nitrite oxidation on nitrite and oxygen in low‐oxygen seawater

Nitrite oxidation is an essential step in transformations of fixed nitrogen. The physiology of nitrite oxidizing bacteria (NOB) implies that the rates of nitrite oxidation should be controlled by concentration of their substrate, nitrite, and the terminal electron acceptor, oxygen. The sensitivities of nitrite oxidation to oxygen and nitrite concentrations were investigated using 15N tracer incubations in the Eastern Tropical North Pacific. Nitrite stimulated nitrite oxidation under low in-situ nitrite conditions, following Michaelis-Menten kinetics, indicating that nitrite was the limiting substrate. The nitrite half-saturation constant (Ks=0.254±0.161 μM) was 1-3 orders of magnitude lower than in cultivated NOB, indicating higher affinity of marine NOB for nitrite. The highest rates of nitrite oxidation were measured in the oxygen depleted zone (ODZ), and were partially inhibited by additions of oxygen. This oxygen sensitivity suggests that ODZ-specialist NOB, adapted to low oxygen conditions, are responsible for apparently anaerobic nitrite oxidation.

Nitrous oxide production in surface waters of the mid‐latitude North Atlantic Ocean

The ocean is a major source of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone-depleting agent. The oceanic flux of N2O varies regionally, and in the midlatitude North Atlantic, the production of N2O is poorly constrained. Incubation experiments with 15N-ammonium and 15N-nitrite revealed active N2O production from ammonium oxidation and nitrite reduction in the surface ocean, suggesting the midlatitude North Atlantic could be a net source for N2O, with a flux density of 0.06 µmol-N2O m−2 d−1 in the top 120 m. The peak of N2O production was detected in the upper 100 m, shallower than the depth at which highest rates of ammonium oxidation to nitrite occurred. Oxygen was not depleted in the water column, but its concentration minimum corresponded to highest N2O oversaturation and low in situ N2O production. The apparent N2O yield, i.e., the molar ratio of N2O-N production over nitrite production was 1.7% at the peak N2O production depths in the surface layer and decreased to less than 0.1% at peak ammonium oxidation depths. The majority of N2O production was apparently through “hybrid formation,” in which ammonium and nitrite each contribute one nitrogen atom to N2O formation, a process that is proposed to be mediated by ammonia oxidizing archaea.

Community Composition of Nitrous Oxide-Related Genes in Salt Marsh Sediments Exposed to Nitrogen Enrichment

Salt marshes provide many key ecosystem services that have tremendous ecological and economic value. One critical service is the removal of fixed nitrogen from coastal waters, which limits the negative effects of eutrophication resulting from increased nutrient supply. Nutrient enrichment of salt marsh sediments results in higher rates of nitrogen cycling and, commonly, a concurrent increase in the flux of nitrous oxide, an important greenhouse gas. Little is known, however, regarding controls on the microbial communities that contribute to nitrous oxide fluxes in marsh sediments. To address this disconnect, we generated profiles of microbial communities and communities of micro-organisms containing specific nitrogen cycling genes that encode several enzymes (amoA, norB, nosZ) related to nitrous oxide flux from salt marsh sediments. We hypothesized that communities of microbes responsible for nitrogen transformations will be structured by nitrogen availability. Taxa that respond positively to high nitrogen inputs may be responsible for the elevated rates of nitrogen cycling processes measured in fertilized sediments. Our data show that, with the exception of ammonia-oxidizing archaea, the community composition of organisms involved in the production and consumption of nitrous oxide was altered under nutrient enrichment. These results suggest that previously measured rates of nitrous oxide production and consumption are likely the result of changes in community structure, not simply changes in microbial activity.

Nitrogen and oxygen availabilities control water column nitrous oxide production during seasonal anoxia in the Chesapeake Bay

Nitrous oxide (N2O) is a greenhouse gas and an ozone depletion agent. Estuaries that are subject to seasonal anoxia are generally regarded as N2O sources. However, insufficient understanding of the environmental controls on N2O production results in large uncertainty about the estuarine contribution to the global N2O budget. Incubation experiments with nitrogen stable isotope tracer were used to investigate the geochemical factors controlling N2O production from denitrification in the Chesapeake Bay, the largest estuary in North America. The highest potential rates of water column N2O production via denitrification (7.5± 1.2 nmolN L−1 h−1) were detected during summer anoxia, during which oxidized nitrogen species (nitrate and nitrite) were absent from the water column. At the top of the anoxic layer, N2O production from denitrification was stimulated by addition of nitrate and nitrite. The relative contribution of nitrate and nitrite to N2O production was positively correlated with the ratio of nitrate to nitrite concentrations. Increased oxygen availability, up to 7 μmol L−1 oxygen, inhibited both N2O production and the reduction of nitrate to nitrite. In spring, high oxygen and low abundance of denitrifying microbes resulted in undetectable N2O production from denitrification. Thus, decreasing the nitrogen input into the Chesapeake Bay has two potential impacts on the N2O production: a lower availability of nitrogen substrates may mitigate short-term N2O emissions during summer anoxia; and, in the long-run (timescale of years), eutrophication will be alleviated and subsequent reoxygenation of the bay will further inhibit N2O production.