Nitrogen Oxide Fluxes Research Articles

Driving force of tidal pulses on denitrifiers-dominated nitrogen oxide emissions from intertidal wetland sediments

Intertidal wetland sediments are an important source of atmospheric nitrogen oxides (NOx), but their contribution to the global NOx budget remains unclear. In this work, we conducted year-round and diurnal observations in the intertidal wetland of Jiaozhou Bay to explore their regional source-sink patterns and influence factors on NOx emissions (initially in the form of nitric oxide) and used a dynamic soil reactor to further extend the mechanisms underlying the tidal pulse of nitric oxide (NO) observed in our investigations. The annual fluxes of NOx in the vegetated wetland were significantly higher than those in the wetland without vegetation. Their annual variations could be attributed to changes in temperature and the amount of organic carbon in the sediment, which was derived from vegetated plants and promoted the carbon-nitrogen cycle. Anaerobic denitrifiers had advantages in the intertidal wetland sediment and accounted for the major NO production (63.8 %) but were still limited by nitrite and nitrate concentrations in the sediment. Moreover, the tidal pulse was likely a primary driver of NOx emissions from intertidal wetlands over short periods, which was not considered in previous investigations. The annual NO exchange flux considering the tide pulse contribution (8.93 ± 1.72 × 10–2 kg N ha–1 yr–1) was significantly higher than that of the non-pulse period (4.14 ± 1.13 × 10–2 kg N ha–1 yr–1) in our modeling result for the fluxes over the last decade. Therefore, the current measurement of NOx fluxes underestimated the actual gas emission without considering the tidal pulse.

Spatiotemporal Variations of Soil Reactive Nitrogen Oxide Fluxes across the Anthropogenic Landscape.

Volatile reactive nitrogen oxides (NOy) are significant atmospheric pollutants, including NOx (nitric oxide [NO] + nitrogen dioxide [NO2]) and NOz (nitrous acid [HONO] + nitric acid [HNO3] + nitrogen trioxide [NO3] + ...). NOy species are products of nitrogen (N) cycle processes, particularly nitrification and denitrification. Biogenic sources, including soil, account for over 50% of natural NOy emissions to the atmosphere, yet emissions from soils are generally not included in atmospheric models as a result of a lack of mechanistic data. This work is a unique investigation of NOy fluxes on a landscape scale, taking a comprehensive set of land-use types, human influence, and seasonality into account to determine large-scale heterogeneity to provide a basis for future modeling and hypothesis generation. By coupling 16S rRNA amplicon sequencing and quantitative polymerase chain reaction, we have linked significant differences in functional potential and activity of nitrifying and denitrifying soil microbes to NOy emissions from soils. Further, we have identified soils subject to increased N deposition that are less microbially active despite increased available N, potentially as a result of poor soil health from anthropogenic pollution. Structural equation modeling suggests human influence on soils to be a more significant effector of soil NOy emissions than land-use type.

Pandemic restrictions in 2020 highlight the significance of non-road NOx sources in central London

Abstract. Fluxes of nitrogen oxides (NOx=NO+NO2) and carbon dioxide (CO2) were measured using eddy covariance at the British Telecommunications (BT) Tower in central London during the coronavirus pandemic. Comparing fluxes to those measured in 2017 prior to the pandemic restrictions and the introduction of the Ultra-Low Emissions Zone (ULEZ) highlighted a 73 % reduction in NOx emissions between the two periods but only a 20 % reduction in CO2 emissions and a 32 % reduction in traffic load. Use of a footprint model and the London Atmospheric Emissions Inventory (LAEI) identified transport and heat and power generation to be the two dominant sources of NOx and CO2 but with significantly different relative contributions for each species. Application of external constraints on NOx and CO2 emissions allowed the reductions in the different sources to be untangled, identifying that transport NOx emissions had reduced by >73 % since 2017. This was attributed in part to the success of air quality policy in central London but crucially due to the substantial reduction in congestion that resulted from pandemic-reduced mobility. Spatial mapping of the fluxes suggests that central London was dominated by point source heat and power generation emissions during the period of reduced mobility. This will have important implications on future air quality policy for NO2 which, until now, has been primarily focused on the emissions from diesel exhausts.

Spatially and temporally resolved measurements of NO<sub><i>x</i></sub> fluxes by airborne eddy covariance over Greater London

Abstract. Flux measurements of nitrogen oxides (NOx) were made over London using airborne eddy covariance from a low-flying aircraft. Seven low-altitude flights were conducted over Greater London, performing multiple overpasses across the city during eight days in July 2014. NOx fluxes across the Greater London region (GLR) exhibited high heterogeneity and strong diurnal variability, with central areas responsible for the highest emission rates (20–30 mg m−2 h−1). Other high-emission areas included the M25 orbital motorway. The complexity of London's emission characteristics makes it challenging to pinpoint single emissions sources definitively using airborne measurements. Multiple sources, including road transport and residential, commercial and industrial combustion sources, are all likely to contribute to measured fluxes. Measured flux estimates were compared to scaled National Atmospheric Emissions Inventory (NAEI) estimates, accounting for monthly, daily and hourly variability. Significant differences were found between the flux-driven emissions and the NAEI estimates across Greater London, with measured values up to 2 times higher in Central London than those predicted by the inventory. To overcome the limitations of using the national inventory to contextualise measured fluxes, we used physics-guided flux data fusion to train environmental response functions (ERFs) between measured flux and environmental drivers (meteorological and surface). The aim was to generate time-of-day emission surfaces using calculated ERF relationships for the entire GLR; 98 % spatial coverage was achieved across the GLR at 400 m2 spatial resolution. All flight leg projections showed substantial heterogeneity across the domain, with high emissions emanating from Central London and major road infrastructure. The diurnal emission structure of the GLR was also investigated, through ERF, with the morning rush hour distinguished from lower emissions during the early afternoon. Overall, the integration of airborne fluxes with an ERF-driven strategy enabled the first independent generation of surface NOx emissions, at high resolution using an eddy-covariance approach, for an entire city region.

Nitrogen cycling microbiomes are structured by plant mycorrhizal associations with consequences for nitrogen oxide fluxes in forests.

Volatile nitrogen oxides (N2O, NO, NO2, HONO, …) can negatively impact climate, air quality, and human health. Using soils collected from temperate forests across the eastern United States, we show microbial communities involved in nitrogen (N) cycling are structured, in large part, by the composition of overstory trees, leading to predictable N‐cycling syndromes, with consequences for emissions of volatile nitrogen oxides to air. Trees associating with arbuscular mycorrhizal (AM) fungi promote soil microbial communities with higher N‐cycle potential and activity, relative to microbial communities in soils dominated by trees associating with ectomycorrhizal (ECM) fungi. Metagenomic analysis and gene expression studies reveal a 5 and 3.5 times greater estimated N‐cycle gene and transcript copy numbers, respectively, in AM relative to ECM soil. Furthermore, we observe a 60% linear decrease in volatile reactive nitrogen gas flux (NOy ≡ NO, NO2, HONO) as ECM tree abundance increases. Compared to oxic conditions, gas flux potential of N2O and NO increase significantly under anoxic conditions for AM soil (30‐ and 120‐fold increase), but not ECM soil—likely owing to small concentrations of available substrate (NO3‐) in ECM soil. Linear mixed effects modeling shows that ECM tree abundance, microbial process rates, and geographic location are primarily responsible for variation in peak potential NOy flux. Given that nearly all tree species associate with either AM or ECM fungi, our results indicate that the consequences of tree species shifts associated with global change may have predictable consequences for soil N cycling.

Fluxes of nitrogen oxides above a subtropical forest canopy in China.

Dry deposition of Nitrogen (N) in forests is commonly estimated from inferential method and/or throughfall measurements, with inevitable uncertainty. In this study, we applied an aerodynamic gradient method to directly measure the nitrogen oxides (NOx) flux above the canopy of a subtropical forest in southeastern China for two consecutive years. The flux and transfer velocity generally reached the maximum absolute values in the midday, with the largest diurnal maximum of absolute flux values observed in the winter of 2015 and that of transfer velocity in the autumn of 2015. The annual average transfer velocity was -0.79 and -0.38cms-1 in 2015 and 2016, respectively. Although the net downward NOx fluxes predominated for both years, upward flux (net emission) of NOx was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NOx concentration seemed to be the most important factor controlling the NOx exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NOx flux and concertation was explored, and it was observed that the deposition of NOx was offset by possible underlayer emission of NOx when the ambient NOx concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NOx for the two years was 6.28±0.06kgNha-1 a-1, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NOx flux. This indicated the importance of stomatal uptake of NOx in nitrogen budget in subtropical forests.

Biochar can increase nitrogen use efficiency of Malus hupehensis by modulating nitrate reduction of soil and root

A surplus of nitrogen (N) fertilizer can reduce plant nitrogen use efficiency (NUE), however, biochar can improve nutrient uptake by altering soil properties and root growth. Therefore, the objective was to determine how biochar alter soil nitrate (NO3−-N) availability and root development, and then to affect the plant NUE. The biochar was prepared by heating the apple branches at 700 °C and low oxygen. The effects of biochar on NO3−-N leaching and nitrogen oxides fluxes in orchard soil treated with five biochar rates (0, 0.5%, 1%, 2% and 4%) were investigated, the nitrate reduction in the soil and roots, and the NUE of Hupeh Crabapple (Malus hupehensis Rehd.) were measured in two soils amended (or not) with biochar, at six N fertilizer rates, using the solution culture, amino benzene sulfonate and subtraction method, respectively. The results showed that 0.5–4% (w/w) biochar decreased the leaching ratio of NO3−-N (9.9–68.7%) and nitrogen oxides flux (6.3–19.2%) in the soil. Soil denitrification intensity were most significantly decreased in the presence of 1% biochar when N fertilizer was applied at 0–450 mg NO3−-N kg−1; the soil nitrate reductase (NR) activity decreased at the application of 0–50 mg NO3−-N kg−1 in the presence of biochar, The root NR activity and root activity of M. hupehensis were significantly increased in the presence of 1% biochar at the application of 0–300 mg NO3−-N kg−1. In the biochar-amended soil, when the highest root activity and NR activity were obtained, N fertilizer application rate were reduced by 33.3% and 66.7%, compared to the non-amended soil, respectively. The NUE of M. hupehensis was increased by biochar-amended from 6.77% to 261.53%, comparied with the non-amended soils. These results indicated that biochar amendment could decrease nitrate leaching and nitrate reduction in soil, and promote nitrate absorption and reduction by root, thereby increasing the biomass and NUE of M. hupehensis when the input of N fertilizer was reduced from 450 to 300 mg NO3−-N kg−1.

Seasonal and Spatial Variation of Nitrogen Oxide Fluxes from Human-Disturbance Coastal Wetland in the Yellow River Estuary

Anthropogenic activities strongly affect greenhouse gases emissions in coastal wetland. In situ field study was conducted to explore the effects of anthropogenic activities on the N2O emissions, used static, manual stainless steel chambers in four seriously disturbed coastal wetlands (west side of the seawall, WSS; oil field existed, OF; Spartina alterniflora coastal marsh, SCM; and aquaculture pond, ACP) in the Yellow River estuary. Results showed that N2O emissions showed significant seasonal variation (p < 0.05). The maximum values were found in August at WSS and ACP, in December and April at SCM and OF, while minimal values were observed in April, February, October, and August, respectively. The annual average N2O fluxes from WSS, OF, SCM, and ACP were 13.24, 9.83, 8.11 and 2.70 μg N2O m−2 h−1, and the variance analysis results showed that the difference between WSS and ACP reached significant level (p < 0.05). For each month, significant differences were observed between WSS and ACP during June, December and February, WSS and SCM during October, and SCM and ACP during December (p < 0.05). Basing on previous natural wetland measurements, we found that seawall construction and S. alterniflora invasion significantly affected the N2O fluxes in the Yellow River estuary, especially seawall construction which accelerated N2O emissions due to its block of seawater migration, resulting in changes of soil physicochemical properties. Pearson correction analysis showed that significant correlations were observed between N2O emissions and electrical conductivity at WSS and OF, while other environmental factors (temperature, water content, total carbon, total nitrogen, C/N ratio) had no significant effect, which indicated that electrical conductivity was the most important factor that affected N2O emissions in these sampling sites. Our results suggested that there could be considerable changes of N2O emissions as a response of artificial disturbance, particularly seawall construction.

Genome-Scale, Constraint-Based Modeling of Nitrogen Oxide Fluxes during Coculture of Nitrosomonas europaea and Nitrobacter winogradskyi.

Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite, emits nitrogen (N) oxide gases (NO, NO2, and N2O), which are potentially hazardous compounds that contribute to global warming. To better understand the dynamics of nitrification-derived N oxide production, we conducted culturing experiments and used an integrative genome-scale, constraint-based approach to modelN oxide gas sources and sinks during complete nitrification in an aerobic coculture of two model nitrifying bacteria, the ammonia-oxidizing bacterium Nitrosomonas europaea and the nitrite-oxidizing bacterium Nitrobacter winogradskyi. The model includes biotic genome-scale metabolic models (iFC578 and iFC579) for each nitrifier and abiotic N oxide reactions. Modeling suggested both biotic and abiotic reactions are important sources and sinks of N oxides, particularly under microaerobic conditions predicted to occur in coculture. In particular, integrative modeling suggested that previous models might have underestimated gross NO production during nitrification due to not taking into account its rapid oxidation in both aqueous and gas phases. The integrative model may be found at https://github.com/chaplenf/microBiome-v2.1. IMPORTANCE Modern agriculture is sustained by application of inorganic nitrogen (N) fertilizer in the form of ammonium (NH4+). Up to 60% of NH4+-based fertilizer can be lost through leaching of nitrifier-derived nitrate (NO3-), and through the emission of N oxide gases (i.e., nitric oxide [NO], N dioxide [NO2], and nitrous oxide [N2O] gases), the latter being a potent greenhouse gas. Our approach to modeling of nitrification suggests that both biotic and abiotic mechanisms function as important sources and sinks of N oxides during microaerobic conditions and that previous models might have underestimated gross NO production during nitrification.

Acyl-Homoserine Lactone Production in Nitrifying Bacteria of the Genera Nitrosospira, Nitrobacter, and Nitrospira Identified via a Survey of Putative Quorum-Sensing Genes.

The genomes of many bacteria that participate in nitrogen cycling through the process of nitrification contain putative genes associated with acyl-homoserine lactone (AHL) quorum sensing (QS). AHL QS or bacterial cell-cell signaling is a method of bacterial communication and gene regulation and may be involved in nitrogen oxide fluxes or other important phenotypes in nitrifying bacteria. Here, we carried out a broad survey of AHL production in nitrifying bacteria in three steps. First, we analyzed the evolutionary history of AHL synthase and AHL receptor homologs in sequenced genomes and metagenomes of nitrifying bacteria to identify AHL synthase homologs in ammonia-oxidizing bacteria (AOB) of the genus Nitrosospira and nitrite-oxidizing bacteria (NOB) of the genera Nitrococcus, Nitrobacter, and Nitrospira Next, we screened cultures of both AOB and NOB with uncharacterized AHL synthase genes and AHL synthase-negative nitrifiers by a bioassay. Our results suggest that an AHL synthase gene is required for, but does not guarantee, cell density-dependent AHL production under the conditions tested. Finally, we utilized mass spectrometry to identify the AHLs produced by the AOB Nitrosospira multiformis and Nitrosospira briensis and the NOB Nitrobacter vulgaris and Nitrospira moscoviensis as N-decanoyl-l-homoserine lactone (C10-HSL), N-3-hydroxy-tetradecanoyl-l-homoserine lactone (3-OH-C14-HSL), a monounsaturated AHL (C10:1-HSL), and N-octanoyl-l-homoserine lactone (C8-HSL), respectively. Our survey expands the list of AHL-producing nitrifiers to include a representative of Nitrospira lineage II and suggests that AHL production is widespread in nitrifying bacteria.IMPORTANCE Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite by nitrifying microorganisms, plays an important role in environmental nitrogen cycling from agricultural fertilization to wastewater treatment. The genomes of many nitrifying bacteria contain genes associated with bacterial cell-cell signaling or quorum sensing (QS). QS is a method of bacterial communication and gene regulation that is well studied in bacterial pathogens, but less is known about QS in environmental systems. Our previous work suggested that QS might be involved in the regulation of nitrogen oxide gas production during nitrite metabolism. This study characterized putative QS signals produced by different genera and species of nitrifiers. Our work lays the foundation for future experiments investigating communication between nitrifying bacteria, the purpose of QS in these microorganisms, and the manipulation of QS during nitrification.

Nitrogen Oxides in Early Earth's Atmosphere as Electron Acceptors for Life's Emergence

We quantify the amount of nitrogen oxides (NOx) produced through lightning and photochemical processes in the Hadean atmosphere to be available in the Hadean ocean for the emergence of life. Atmospherically generated nitrate (NO3-) and nitrite (NO2-) are the most attractive high-potential electron acceptors for pulling and enabling crucial redox reactions of autotrophic metabolic pathways at submarine alkaline hydrothermal vents. The Hadean atmosphere, dominated by CO2 and N2, will produce nitric oxide (NO) when shocked by lightning. Photochemical reactions involving NO and H2O vapor will then produce acids such as HNO, HNO2, HNO3, and HO2NO2 that rain into the ocean. There, they dissociate into or react to form nitrate and nitrite. We present new calculations based on a novel combination of early-Earth global climate model and photochemical modeling, and we predict the flux of NOx to the Hadean ocean. In our 0.1-, 1-, and 10-bar pCO2 models, we calculate the NOx delivery to be 2.4 × 105, 6.5 × 108, and 1.9 × 108 molecules cm-2 s-1. After only tens of thousands to tens of millions of years, these NOx fluxes are expected to produce sufficient (micromolar) ocean concentrations of high-potential electron acceptors for the emergence of life. Key Words: Nitrogen oxides-Nitrate-Nitrite-Photochemistry-Lightning-Emergence of life. Astrobiology 17, 975-983.

Soil nitrogen oxide fluxes from lowland forests converted to smallholder rubber and oil palm plantations in Sumatra, Indonesia

Abstract. Oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) plantations cover large areas of former rainforest in Sumatra, Indonesia, supplying the global demand for these crops. Although forest conversion is known to influence soil nitrous oxide (N2O) and nitric oxide (NO) fluxes, measurements from oil palm and rubber plantations are scarce (for N2O) or nonexistent (for NO). Our study aimed to (1) quantify changes in soil–atmosphere fluxes of N oxides with forest conversion to rubber and oil palm plantations and (2) determine their controlling factors. In Jambi, Sumatra, we selected two landscapes that mainly differed in texture but were both on heavily weathered soils: loam and clay Acrisol soils. Within each landscape, we investigated lowland forests, rubber trees interspersed in secondary forest (termed as jungle rubber), both as reference land uses and smallholder rubber and oil palm plantations as converted land uses. In the loam Acrisol landscape, we conducted a follow-on study in a large-scale oil palm plantation (called PTPN VI) for comparison of soil N2O fluxes with smallholder oil palm plantations. Land-use conversion to smallholder plantations had no effect on soil N-oxide fluxes (P = 0. 58 to 0.76) due to the generally low soil N availability in the reference land uses that further decreased with land-use conversion. Soil N2O fluxes from the large-scale oil palm plantation did not differ with those from smallholder plantations (P = 0. 15). Over 1-year measurements, the temporal patterns of soil N-oxide fluxes were influenced by soil mineral N and water contents. Across landscapes, annual soil N2O emissions were controlled by gross nitrification and sand content, which also suggest the influence of soil N and water availability. Soil N2O fluxes (µg N m−2 h−1) were 7 ± 2 to 14 ± 7 (reference land uses), 6 ± 3 to 9 ± 2 (rubber), 12 ± 3 to 12 ± 6 (smallholder oil palm) and 42 ± 24 (large-scale oil palm). Soil NO fluxes (µg N m−2 h−1) were −0.6 ± 0.7 to 5.7 ± 5.8 (reference land uses), −1.2 ± 0.5 to −1.0 ± 0.2 (rubber) and −0.2 ± 1.2 to 0.7 ± 0.7 (smallholder oil palm). To improve the estimate of soil N-oxide fluxes from oil palm plantations in this region, studies should focus on large-scale plantations (which usually have 2 to 4 times higher N fertilization rates than smallholders) with frequent measurements following fertilizer application.

Urban eddy covariance measurements reveal significant missing NOx emissions in Central Europe

Nitrogen oxide (NOx) pollution is emerging as a primary environmental concern across Europe. While some large European metropolitan areas are already in breach of EU safety limits for NO2, this phenomenon does not seem to be only restricted to large industrialized areas anymore. Many smaller scale populated agglomerations including their surrounding rural areas are seeing frequent NO2 concentration violations. The question of a quantitative understanding of different NOx emission sources is therefore of immanent relevance for climate and air chemistry models as well as air pollution management and health. Here we report simultaneous eddy covariance flux measurements of NOx, CO2, CO and non methane volatile organic compound tracers in a city that might be considered representative for Central Europe and the greater Alpine region. Our data show that NOx fluxes are largely at variance with modelled emission projections, suggesting an appreciable underestimation of the traffic related atmospheric NOx input in Europe, comparable to the weekend-weekday effect, which locally changes ozone production rates by 40%.

Long-term measurements of NOx and O3 soil fluxes in a temperate deciduous forest

A long-term campaign devoted to the measurement of fluxes of nitrogen oxides and ozone at the surface of a temperate deciduous forest was carried out from 1st September 2012 to 31st December 2013, using automated dynamic chambers.Soil temperature and water content profiles were recorded near the chambers in order to clarify the short-term response of the fluxes to these two variables.Bulk soil resistances against NO, NO2 and O3 emission or deposition were also estimated in order to understand their daily and seasonal variations.Nitric oxide was almost exclusively emitted and the annual average soil emission of 27.4μgNm−2h−1 was higher than those for other deciduous European forests. The maximum daily peaks of NO emission exceeded 100μgNm−2h−1 during summertime and were close to zero in wintertime.On annual basis the N balance of the ecosystem as NOx showed a net emission towards the atmosphere of 1.1kgNha−1yr−1. This result can be explained by the strong N input through wet deposition observed at this site during the last decades. NO emission fluxes depended exponentially on soil temperature and decreased at high levels of soil water content during summertime, suggesting that the main contribution was due to aerobic nitrification processes.Soil deposition fluxes of NO2 and O3 were mainly governed by atmospheric processes related to the atmosphere/soil concentration differences.Soil resistances against NO2 and O3 deposition showed both diurnal and seasonal variations which disagree with the constant values often used in model studies.

Evaluation of N environmental risks on Andosols from an intensive dairy farming watershed using DNDC

Manure nitrogen (N) in the livestock sector has become a key driver of environmental change. The denitrification–decomposition (DNDC) model was used to evaluate N pollution strengths on Andosols with intensive dairy manure application in Upper Naka River Watershed, Japan. The calibrated model was capable of predicting Andosol N flows because the simulated soil mineral N content, soil nitrogen oxide (N2O) fluxes, denitrification rate, and crop N uptake matched the patterns and magnitudes of the field observations from a wide range of soil textures, as well as manure management and cropping systems. The simulations showed that current intensive manure application systems caused low crop N use efficiency and a large amount of NO3−-N leaching and N2O emission. The crop N use efficiency was 27%–42% and 37%–55% of input N for uplands and paddy rice, respectively. The uplands showed much more serious N environmental pollution risks with N leaching 123–362kgNha−1yr−1 and N2O emissions 6.53–11.8kgNha−1yr−1 than that in the lowland paddy rice with N leaching 17.4–103kgNha−1yr−1 and N2O emissions 0.59–2.77kgNha−1yr−1. Forage rice/barley crop systems have high N cleaning capability due to the greater crop N uptake which reached to 304kgNha−1yr−1. High precipitation stimulated more NO3−-N leaching. Sandy soil also showed higher N leaching and was unsuitable for paddy rice. Slurry application stimulated more N2O emission than compost manure. To mitigate the current high N pollution, the critical N application rate was recommended to be approximately 380, 470, 640, and 390kgNha−1yr−1 for loam sand planted with maize/grass, loam soil with maize/grass, forage rice/barley, and rice/fallow with winter manure application, respectively.

Snowpack-atmosphere gas exchanges of carbon dioxide, ozone, and nitrogen oxides at a hardwood forest site in northern Michigan

Abstract Snowpack-atmosphere gas exchanges of CO2, O3, and NOx (NO + NO2) were investigated at the University of Michigan Biological Station (UMBS), a mid-latitude, low elevation hardwood forest site, during the 2007–2008 winter season. An automated trace gas sampling system was used to determine trace gas concentrations in the snowpack at multiple depths continuously throughout the snow-covered period from two adjacent plots. One natural plot and one with the soil covered by a Tedlar sheet were setup for investigating whether the primary source of measured trace gases was biogenic (i.e., from the soil) or non-biogenic (i.e., from the snowpack). The results were compared with the “White on Green” study conducted at the Niwot Ridge (NWT) Long Term Ecological Research site in Colorado. The average winter CO2 flux ± s.e. from the soil at UMBS was 0.54 ± 0.037 µmol m-2 s-1 using the gradient diffusion method and 0.71 ± 0.012 µmol m-2 s-1 using the eddy covariance method, and in a similar range as found for NWT. Observed snowpack-O3 exchange was also similar to NWT. However, nitrogen oxides (NOx) fluxes from snow at UMBS were 10 times smaller than those at NWT, and fluxes were bi-directional with the direction of the flux dependent on NOx concentrations in ambient air. The compensation point for the change in the direction of NOx flux was estimated to be 0.92 nmol mol-1. NOx in snow also showed diurnal dependency on incident radiation. These NOx dynamics in the snow at UMBS were notably different compared to NWT, and primarily determined by snow-atmosphere interactions rather than by soil NOx emissions.

Observations of reactive nitrogen oxide fluxes by eddy covariance above two midlatitude North American mixed hardwood forests

Abstract. Significant knowledge gaps persist in the understanding of forest–atmosphere exchange of reactive nitrogen oxides, partly due to a lack of direct observations. Chemical transport models require representations of dry deposition over a variety of land surface types, and the role of canopy exchange of NOx (= NO + NO2) is highly uncertain. Biosphere–atmosphere exchange of NOx and NOy (= NOx + HNO3 + PANs + RONO2 + pNO3− + ...) was measured by eddy covariance above a mixed hardwood forest in central Ontario (Haliburton Forest and Wildlife Reserve, or HFWR), and a mixed hardwood forest in northern lower Michigan (Program for Research on Oxidants: Photochemistry, Emissions and Transport, or PROPHET) during the summers of 2011 and 2012 respectively. NOx and NOy mixing ratios were measured by a custom-built two-channel analyser based on chemiluminescence, with selective NO2 conversion via LED photolysis and NOy conversion via a hot molybdenum converter. Consideration of interferences from water vapour and O3, and random uncertainty of the calculated fluxes are discussed. NOy flux observations were predominantly of deposition at both locations. In general, the magnitude of deposition scaled with NOy mixing ratios. Average midday (12:00–16:00) deposition velocities at HFWR and PROPHET were 0.20 ± 0.25 and 0.67 ± 1.24 cm s−1 respectively. Average nighttime (00:00–04:00) deposition velocities were 0.09 ± 0.25 cm s−1 and 0.08 ± 0.16 cm s−1 respectively. At HFWR, a period of highly polluted conditions (NOy concentrations up to 18 ppb) showed distinctly different flux characteristics than the rest of the campaign. Integrated daily average NOy flux was −0.14 mg (N) m−2 day−1 and −0.34 mg (N) m−2 day−1 (net deposition) at HFWR and PROPHET respectively. Concurrent wet deposition measurements were used to estimate the contributions of dry deposition to total reactive nitrogen oxide inputs, found to be 22 and 40% at HFWR and PROPHET respectively.

Comparison of NOx Fluxes Measured by Eddy Covariance to Emission Inventories and Land Use

Uncertainty in emission inventories remains a critical limitation of air quality modeling and management. Using eddy covariance, we measured surface-atmosphere exchange fluxes of nitrogen oxides (NO(x)) at the neighborhood scale at 13 sites in the Norfolk, Virginia area to estimate emissions, to evaluate official inventories, and to quantify relationships between emissions and land use. Average daytime fluxes ranged from 0.4 μg m(-2) s(-1) at a site near open water to 9.5 μg m(-2) s(-1) at a site dominated by vehicle traffic. NO(x) fluxes were correlated with both road density and medium- plus high-intensity development, confirming that both motor vehicles and sources associated with development are responsible for NO(x) emissions in urban areas. Spatially averaged NO(x) fluxes measured by eddy covariance agreed to within 3% with the National Emission Inventory (NEI) but were 2.8 times higher than those in the corresponding grid cell of an emission inventory developed for air quality modeling. These average fluxes were 4.6, 4.5, and 1.7 μg m(-2) s(-1), respectively. Uncertainty in the inventories appears to be dominated by the nonroad mobile source category. It is especially important to know NO(x) emissions accurately because in certain photochemical regimes, reducing NO(x) emissions can exacerbate secondary pollutant formation.

Emissions of nitrous oxide, nitrogen oxides and ammonia from a maize field in the North China Plain

The exchange fluxes of nitrous oxide (N2O), nitrogen oxides (NOx) and ammonia (NH3) from a maize field with three different treatments were simultaneously measured using static and dynamic chambers in the North China Plain (NCP) from June 28 to October 11, 2009. The three treatments included control plot (CK, without crop, fertilization and irrigation), fertilizer N plot (NP) and wheat straw returning plus fertilizer N plot (SN). N-fertilizer application greatly stimulated the emissions of N2O, NOx and NH3, with durations of about 10 days for N2O and NO, and about 7 days for NH3. Fertilizer loss rates were 1.08% (NP plot) and 1.20% (SN plot) as N2O–N, were 1.93% (NP plot) and 0.76% (SN plot) as NO–N, and were 5.24% (NP plot) and 3.03% (SN plot) as NH3–N. In comparison with the NP plot, the significant low fertilizer loss rates as NO–N and NH3–N from the SN plot indicated that the wheat straw returning to the field could reduce NOx and NH3 emissions. The molar ratio of NO/N2O was greater than unity for most data during the pulse emission periods induced by fertilization, and thus, nitrification was the dominant process for N2O and NO emissions during these periods. Considering the significant amount (>80%) of N2O and NOx emissions occurred during the pulse emission periods, the emissions of NOx and N2O from the investigated field were mainly ascribed to nitrification process.

Ecosystem-scale biosphere–atmosphere interactions of a hemiboreal mixed forest stand at Järvselja, Estonia

During two measurement campaigns, from August to September 2008 and 2009, we quantified the major ecosystem fluxes in a hemiboreal forest ecosystem in Järvselja, Estonia. The main aim of this study was to separate the ecosystem flux components and gain insight into the performance of a multi-species multi-layered tree stand. Carbon dioxide and water vapor fluxes were measured using the eddy covariance method above and below the canopy in conjunction with the microclimate. Leaf and soil contributions were quantified separately by cuvette and chamber measurements, including fluxes of carbon dioxide, water vapor, nitrogen oxides, nitrous oxide, methane, ozone, sulfur dioxide, and biogenic volatile organic compounds (isoprene and monoterpenes). The latter have been as well characterized for monoterpenes in detail. Based on measured atmospheric trace gas concentrations, the flux tower site can be characterized as remote and rural with low anthropogenic disturbances. Our results presented here encourage future experimental efforts to be directed towards year round integrated biosphere–atmosphere measurements and development of process-oriented models of forest–atmosphere exchange taking the special case of a multi-layered and multi-species tree stand into account. As climate change likely leads to spatial extension of hemiboreal forest ecosystems a deep understanding of the processes and interactions therein is needed to foster management and mitigation strategies.