Atmospheric Nitrogen Sources Research Articles

The δ15N values of epilithic mosses indicating the changes of nitrogen sources in Guiyang (SW China) from 2006 to 2016–2017

In order to trace changes in atmospheric nitrogen sources in Guiyang (SW China) from 2006 to 2016–2017, tissue nitrogen content and bulk stable isotope (δ15N) values of epilithic moss (Haplocladium microphyllum), collected monthly between April 2016 and March 2017 along an urban-rural gradient (urban center, semi-urban, suburban, and rural) in Guiyang, Guizhou (SW China), were determined. These were then compared with previous surveys in 2006. The nitrogen concentrations (1.4 to 3.7%) and δ15N values (−8.3 to −2.7‰) of the mosses (sampled in 2016–2017) showed an obvious downward trend from the urban center to the rural area, suggesting that 15N enrichment in urban areas is associated with traffic-related NOxN. On the contrary, 15N-depleted mosses collected in the rural area were primarily affected by atmospheric NHy-N from agricultural activities. Our results showed an opposite trend for δ15N variation in 2016/2017 compared to that in 2006, this may reflect a historical change in atmospheric nitrogen sources. A Bayesian isotope mixing model was then applied to estimate the contributions of atmospheric nitrogen sources. The results indicated that the dominant atmospheric nitrogen compounds and sources in the urban area had switched from NHy-N (emissions from coal combustion and untreated waste and sewage) in 2006 to NOx-N (from traffic emissions) in 2016/2017. These findings are critical for assessing the air quality of cities and for controlling atmospheric nitrogen pollution.

Compound-specific \u03b415N composition of free amino acids in moss as indicators of atmospheric nitrogen sources

Haplocladium microphyllum moss samples were collected in Nanchang, China. Free amino acid (FAA) concentrations and N isotope compositions (δ15NFAA) in the samples were determined and compared with the bulk N concentrations and δ15Nbulk values. The aim was to determine whether δ15NFAA values in moss (which are very variable) indicate the sources of atmospheric N. The δ15NFAA values among individual FAA varied widely (from −19.3‰ to +16.1‰), possibly because of the different sources of N and isotope fractionation in amino acids metabolic pathways. Total 15N-enrichment for the individual FAAs was equal to total 15N-depletion relative to δ15Nbulk. The concentration-weighted mean δ15N value for total FAAs (TFAA) (δ15NTFAA) was −3.1‰ ± 3.2‰, which was similar to δ15Nbulk (−4.0‰ ± 2.9‰). We concluded that a N isotope balance occurred during amino acid metabolism and that little isotope disparity occurred between the concentration-weighted TFAA and bulk N. We concluded that δ15NTFAA ≈ δ15Nbulk ≈ δ15Nsource. The mean δ15Nalanine (−4.1‰), δ15Nglutamate (−4.2‰), and δ15Nlysine (−4.0‰) were similar to the mean δ15Nbulk, which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ15Nalanine, δ15Nglutamate, and δ15Nlysine in moss can be used to indicate the sources of atmospheric N deposition.

Using nitrogen concentration and isotopic composition in lichens to spatially assess the relative contribution of atmospheric nitrogen sources in complex landscapes

Reactive nitrogen (Nr) is an important driver of global change, causing alterations in ecosystem biodiversity and functionality. Environmental assessments require monitoring the emission and deposition of both the amount and types of Nr. This is especially important in heterogeneous landscapes, as different land-cover types emit particular forms of Nr to the atmosphere, which can impact ecosystems distinctively. Such assessments require high spatial resolution maps that also integrate temporal variations, and can only be feasibly achieved by using ecological indicators. Our aim was to rank land-cover types according to the amount and form of emitted atmospheric Nr in a complex landscape with multiple sources of N. To do so, we measured and mapped nitrogen concentration and isotopic composition in lichen thalli, which we then related to land-cover data. Results suggested that, at the landscape scale, intensive agriculture and urban areas were the most important sources of Nr to the atmosphere. Additionally, the ocean greatly influences Nr in land, by providing air with low Nr concentration and a unique isotopic composition. These results have important consequences for managing air pollution at the regional level, as they provide critical information for modeling Nr emission and deposition across regional as well as continental scales.

Modeling nitrogen-gas, -liquid, -solid chemistries at low temperatures (173–298 K) with applications to Titan

Molecular diatomic nitrogen (henceforth, “nitrogen”) is a major gas on Venus, Earth, Mars, Titan, Triton, and Pluto; a major condensed liquid component on Titan; and major condensed ices on Triton and Pluto. Nitrogen also occurs as a component of air gas hydrates in Earth’s polar ice sheets. The Solar System’s nitrogen originally might have been produced by condensation of nitrogen ice in the outer Solar Nebula disk, or it might have first condensed as and then decomposed from nitrogen gas hydrate or ammonia. The specific objectives of this study were to add nitrogen into the FREZCHEM model (which already included ammonia) and explore some roles of nitrogen on planetary satellites, especially Titan, where nitrogen is a major atmospheric gas and a component of Titan’s hydrocarbon-rich lakes and rain. Nitrogen gases, aqueous phases, and gas hydrates were added to FREZCHEM. Nitrogen, methane, and carbon dioxide gas hydrates were parallel with respect to temperature, which allowed extensions to 173K, including for mixed N2–CH4 and N2–CO2 gas hydrates. Simulations from 273 to 173K used a surface Titan pressure of 1.467bars with a major mixing gas of nitrogen at 1.394bars (95%) and methane at 0.073bars (5%). These gas concentrations eventually led to formation of N2–CH4 gas hydrate at 178K, which suggests that these gas hydrates could be forming on the surface of Titan today (at 94K) and in sub-surfaces with higher temperatures. On Earth, air gas hydrates are common deep within polar ice sheets and are dominated by atmospheric N2/O2 gas compositions; and as a consequence, gas hydrates do not control, but are controlled by, atmospheric N2/O2 gas compositions. Given that Earth and Titan have similar atmospheric nitrogen (0.78 vs. 1.39bars) and icy environments, on neither body are gas hydrates likely the main source of atmospheric nitrogen; but rather, atmospheric gases are likely the source of minor amounts of nitrogen in gas hydrates on Earth and Titan.

Nitrogen isotope variations in camphor (Cinnamomum Camphora) leaves of different ages in upper and lower canopies as an indicator of atmospheric nitrogen sources

Nitrogen isotopic composition of new, middle-aged and old camphor leaves in upper and lower canopies has been determined in a living area, near a motorway and near an industrial area (Jiangan Chemical Fertilizer Plant). We found that at sites near roads, more positive δ15N values were observed in the camphor leaves, especially in old leaves of upper canopies, and ∆δ15N=δ15Nupper−δ15Nlower>0, while those near the industrial area had more negative δ15N values and ∆δ15N<0. These could be explained by two isotopically different atmospheric N sources: greater uptake from isotopically heavy pools of atmospheric NOx by old leaves in upper canopies at sites adjacent to roads, and greater uptake of 15N-depleted NHy in atmospheric deposition by leaves at sites near the industrial area. This study presents novel evidence that 15N natural abundance of camphor leaves can be used as a robust indicator of atmospheric N sources.

Nitrogen Fixation in Surface Soils and Vegetation in an Arctic Tundra Watershed: A Key Source of Atmospheric Nitrogen

Atmospheric nitrogen (N) fixation is a key N input to arctic ecosystems, but relatively few estimates of annual N-fixation rates are available. We measured N-fixation of plant-soil cores by the acetylene reduction technique at different topographic positions in an upland tundra watershed, Imnavait Creek, through two growing seasons in order to evaluate spatial and temporal variation in N-fixation. We also examined the effects of light and temperature on N-fixation to estimate annual N-fixation rates of surface soil in this watershed using field meteorological data. Surface soil at Imnavait Creek had significant acetylene reduction potential throughout the watershed (generally 6 to 10 μmol C2H4 m−2 h−1), indicating that N-fixing organisms were present everywhere. Although acetylene reduction potential was roughly constant through the growing season, moisture, temperature and light intensity strongly affected the measured acetylene reduction rates in laboratory incubations. In addition, the relatively few samples that included the lichen, Peltigera apthosa, had significantly greater acetylene reduction potential, although the overall influence of Peltigera on N-fixation in this watershed seems to be small. The N input via N-fixation at Imnavait Creek was estimated at 80 to 131 mg N m−2 yr−1, indicating that N-fixation contributed 85 to 90% of total watershed N inputs.

Deposition of atmospheric ammonia to moorlands

Micrometeorological methods were applied to measure fluxes of atmospheric ammonia (NH 3) to moorlands. Measurements were made in a wide variety of surface conditions and included both Calluna vulgaris (L.) Hull and Eriophorum vaginatum L. dominated sites. NH 3 was found to deposit rapidly to all the sites investigated, providing large deposition velocities ( V d, typically 10–40 mm s −1) and usually minimal surface resistances ( r c). A small number of measurements were made in frozen conditions and suggest a possible exception to this pattern with mean r c of 50–200 s m −1. The effect of vegetation drying was also investigated and a possible increase in r c observed, though this was small (< 10 s m −1). The results are interpreted in terms of the processes controlling exchange; it is shown that NH 3 deposition is predominantly to the leaf surfaces and that the net NH 3 compensation point approaches zero. Annual estimates show that dry deposition of NH 3 is a major source of atmospheric nitrogen to moorland ecosystems. For two typical UK sites subject to background air concentrations, NH 3 dry deposition is of similar magnitude to equivalent NH 4 + inputs in wet deposition. In the vicinity of emission sources, NH 3 dry deposition is expected to dominate inputs of atmospheric nitrogen.

Dissolved gas evidence for denitrification in the Lincolnshire Limestone groundwaters, eastern England

High concentrations of dissolved nitrogen gas are present in some groundwaters within the confined zone of the Lincolnshire Limestone aquifer. Atmospheric nitrogen sources are shown to be from air equilibration at groundwater recharge and air entrainment during recharge. Nitrogen arising from these two processes can be identified and quantified by the pattern of noble gas dissolution in the groundwaters. Nitrogen which is in excess of the amounts due to these two processes is attributed to denitrification. The isotopic composition of this nitrogen gas derived from denitrification has variably light δ 15N values.