Nitrogen Oxide Fluxes Research Articles

Winter and summer nitrous oxide and nitrogen oxides fluxes from a seasonally snow-covered subalpine meadow at Niwot Ridge, Colorado

The soil emission rates (fluxes) of nitrous oxide (N2O) and nitrogen oxides (NO + NO2 = NO x ) through a seasonal snowpack were determined by a flux gradient method from near-continuous 2-year measurements using an automated system for sampling interstitial air at various heights within the snowpack from a subalpine site at Niwot Ridge, Colorado. The winter seasonal-averaged N2O fluxes of 0.047–0.069 nmol m−2 s−1 were ~15 times higher than observed NO x fluxes of 0.0030–0.0067 nmol m−2 s−1. During spring N2O emissions first peaked and then dropped sharply as the soil water content increased from the release of snowpack meltwater, while other gases, including NO x and CO2 did not show this behavior. To compare and contrast the winter fluxes with snow-free conditions, N2O fluxes were also measured at the same site in the summers of 2006 and 2007 using a closed soil chamber method. Summer N2O fluxes followed a decreasing trend during the dry-out period after snowmelt, interrupted by higher values related to precipitation events. These peaks were up to 2–3 times higher than the background summer levels. The integrated N2O-N loss over the summer period was calculated to be 1.1–2.4 kg N ha−1, compared to ~0.24–0.34 kg N ha−1 for the winter season. These wintertime N2O fluxes from subniveal soil are generally higher than the few previously published data. These results are of the same order of magnitude as data from more productive ecosystems such as fertilized grasslands and high-N-cycling forests, most likely because of a combination of the relatively well-developed soils and the fact that subnivean biogeochemical processes are promoted by the deep, insulating snowpack. Hence, microbially mediated oxidized nitrogen emissions occurring during the winter can be a significant part of the N-cycle in seasonally snow-covered subalpine ecosystems.

Observations of HNO<sub>3</sub>, ΣAN, ΣPN and NO<sub>2</sub> fluxes: evidence for rapid HO<sub>x</sub> chemistry within a pine forest canopy

Abstract. Measurements of exchange of reactive nitrogen oxides between the atmosphere and a ponderosa pine forest in the Sierra Nevada Mountains are reported. During winter, we observe upward fluxes of NO2, and downward fluxes of total peroxy and peroxy acyl nitrates (ΣPNs), total gas and particle phase alkyl and multifunctional alkyl nitrates (ΣANs(g+p)), and the sum of gaseous HNO3 and semi-volatile NO3− particles (HNO3(g+p)). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday ΣNOyi fluxes in winter are –4.9 ppt m s−1. The signs and magnitudes of these wintertime individual and ΣNOyi fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of ΣANs(g+p), and upward fluxes of HNO3(g+p), ΣPNs and NO2 with signs and magnitudes that are unlike most, if not all, previous observations and analyses of fluxes of individual nitrogen oxides. The results imply that the mechanisms contributing to NOy fluxes, at least at this site, are much more complex than previously recognized. We show that the observations of upward fluxes of HNO3(g+p) and σPNs during summer are consistent with oxidation of NO2 and acetaldehyde by an OH x residence time of 1.1×1010 molec OH cm−3 s, corresponding to 3 to 16×107 molecules cm−3 OH within the forest canopy for a 420 to 70 s canopy residence time. We show that ΣAN(g+p) fluxes are consistent with this range in OH if the reaction of OH with ΣANs produces either HNO3 or NO2 with a 6–30% yield. Calculations of NO fluxes constrained by the NO2 observations and the inferred OH indicate that NOx fluxes are downward into the canopy because of the substantial conversion of NOx to HNO3 and σPNs in the canopy. Even so, we derive that NOx emission fluxes of ~15 ng(N) m−2 s−1 at midday during summer are required to balance the NOx and NOy flux budgets. These fluxes are partly explained by estimates of soil emissions (estimated to be between 3 and 6 ng(N) m−2 s-1). One possibility for the remainder of the NOx source is large HONO emissions. Alternatively, the 15 ng(N) m−2 s−1 emission estimate may be too large, and the budget balanced if the deposition of HNO3 and σPNs is slower than we estimate, if there are large errors in either our understanding of peroxy radical chemistry, or our assumptions that the budget is required to balance because the fluxes do not obey similarity theory.

An overview of snow photochemistry: evidence, mechanisms and impacts

Abstract. It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3–4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NOx flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

Emissão de óxidos de nitrogênio associada à aplicação de uréia sob plantio convencional e direto

O objetivo deste trabalho foi avaliar emissões de NO e N2O até cinco dias após a primeira fertilização de cobertura com uréia em milho, em Latossolo Vermelho argiloso distrófico, sob plantio convencional e direto. A adubação de cobertura foi de 60 kg ha-1 de N. O experimento foi conduzido na Embrapa Cerrados, Planaltina, DF, com delineamento de blocos ao acaso, com três repetições, sendo o terceiro cultivo de milho, em rotação com soja. Os fluxos de NO e N2O foram medidos em câmaras de PVC instaladas em cada parcela. Houve emissão alta de NO imediatamente após (5,4 ng N cm-2 h-1) e no terceiro dia (4,8 ng N cm-2 h-1) após aplicação de uréia e irrigação. Um dia após fertilização, a emissão de NO reduziu-se a 1,9 ng N cm-2 h-1, e cinco dias depois, alcançou 1,2 ng N cm-2 h-1. Os fluxos de N2O ficaram abaixo do limite de detecção de 0,6 ng N cm-2 h-1. Não houve diferença significativa entre os plantios convencional e direto quanto à emissão dos óxidos de nitrogênio.

NITRAPYRIN DELAYS DENITRIFICATION ON MANURED SOILS

Excessive application of manure may lead to NO3− leaching to groundwater and fluxes of nitrogen oxides to the atmosphere. Nitrification inhibitors such as nitrapyrin (N-serve; 2-chloro-6-(trichloromethyl)pyridine) may help to conserve manure N in the root zone by limiting NO3− supply to denitrifiers. The objective of this study was to test the effect of nitrapyrin on the timing and amounts of denitrification and N2O fluxes in manured soils under conditions favorable to denitrification. The study consisted of a laboratory incubation of soils under aerobic conditions. Three agricultural soils and a sand were included in the study, all with high moisture and initial NO3−-N content. Each soil received three treatments: 1) manure plus nitrapyrin (190 mg nitrapyrin kg−1 soil), 2) manure alone (0.15 mg manure N g−1), and 3) soil alone controls. Nitrapyrin was mixed with the manure before addition to soil. Destructive samplings were carried out weekly for 10 weeks. At each sampling, soil-extractable mineral N, microbial biomass N, denitrified N, and N2O fluxes were measured. Nitrapyrin was effective in reducing nitrification, thus enhancing soil NH4+-N accumulation and possibly reducing the potential for nitrate leaching. Although nitrapyrin was effective in reducing nitrification in manured soils, the effect on soil mineral N and potential N supply to plants varied across soils because of the interaction between nitrification, denitrification, and N immobilization. Neither manure nor nitrapyrin consistently affected net N mineralization in the five different soil types. Microbial N immobilization and/or denitrification were strong sinks of N that reduced net N mineralization. Nitrapyrin did not affect cumulative denitrification, but some soils had delayed denitrification when nitrapyrin was added. Manure had a strong effect on N2O fluxes and denitrified N in some soils, but the effects of nitrapyrin were inconsistent. Nitrapyrin significantly reduced microbial N immobilization in two agricultural soils. The observed reductions in microbial biomass may affect N availability beyond the time frame of the experiment because less N will be available for remineralization.

Fluxes of nitrogen oxides over a temperate deciduous forest

Eddy covariance flux measurements of NO, NO2, and O3 were obtained above the mixed deciduous canopy at Harvard Forest in central Massachusetts from April to November 2000. Net deposition of NOx was observed throughout the measurement period, with average velocity of ∼0.2 cm s−1. At night, NO2 is deposited at a rate that depends nonlinearly on NO2 concentration and cannot be explained by N2O5 hydrolysis, suggesting HONO formation by heterogeneous disproportionation of NO2. During the day, photochemically driven NOx fluxes conform to the predicted behavior based on gradients of light and eddy diffusivity through the canopy, with residual net flux attributable to both stomatal and nonstomatal processes. The results were consistent with a compensation point for NO2 near 1.5 nmol mol−1. These results were confirmed by independent evidence from NO, NO2, and O3 profiles acquired at the site over several years. If the rate of NOx deposition observed at this site in April through November continues during the winter, it would have a larger potential impact on tropospheric chemistry because mixing depths are shallow and chemical NOx oxidation is slow during winter; the impact at night is important for the same reasons. The results contradict widely used parameterizations of NO2 deposition that both overestimate stomatal uptake and do not allow for surface uptake when stomates are closed.

Methane and Nitrogen Oxide Fluxes in Tropical Agricultural Soils: Sources, Sinks and Mechanisms

Tropical soils are important sources and sinks of atmospheric methane (CH4) and major sources of oxides of nitrogen gases, nitrous oxide (N2O) and NOx (NO+NO2). These gases are present in the atmosphere in trace amounts and are important to atmospheric chemistry and earth’s radiative balance. Although nitric oxide (NO) does not directly contribute to the greenhouse effect by absorbing infrared radiation, it contributes to climate forcing through its role in photochemistry of hydroxyl radicals and ozone (O3) and plays a key role in air quality issues. Agricultural soils are a primary source of anthropogenic trace gas emissions, and the tropics and subtropics contribute greatly, particularly since 51% of world soils are in these climate zones.

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N2O) and nitric oxide (NO) are not well understood. We examined emissions of N2O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N2O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N2O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha−1· y−1). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha−1· y−1), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha−1· y−1). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified.

Nitrogen oxide fluxes between corn (Zea mays L.) leaves and the atmosphere

In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NOx emissions. Leaf chamber measurements were conducted of NO and NO2 fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NOx emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO2 flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3ppbv, average fluxes were zero. At ambient NO concentrations above 1ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0pmolm−2s−1) as to be insignificant in the NOx inventory for this site. In field-grown plants, NO2 was emitted to the atmosphere at ambient NO2 concentrations below 0.9ppbv (the NO2 compensation point), with the highest rate of emission being 50pmolm−2s−1 at 0.2ppbv. NO2 was assimilated by corn leaves at ambient NO2 concentrations above 0.9ppbv, with the maximum observed uptake rate being 643pmolm−2 s−1 at 6ppbv. When fluxes above 0.9ppbv are standardized for ambient NO2 concentration, the resultant deposition velocity was 1.2±0.1mms−1. When scaled to the entire corn canopy, NO2 uptake rates can be estimated to be as much as 27% of the soil-emitted NOx. In greenhouse-grown and field-grown leaves, NO2 deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700nm), whether measured above or below the NO2 compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO2 deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO2 uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO2 uptake concern both stomatal and mesophyll components.

Soil‐atmosphere nitrogen oxide fluxes: Effects of root disturbance

Chambers are the most common method used to sample soil‐atmosphere fluxes of trace gases. Working in tree plantations in Costa Rica and in subtropical forest in Puerto Rico, we performed controlled experiments in order to evaluate whether installation of the chamber bases into the soil affects the soil‐atmosphere flux of nitrogen oxides. Installation of chambers severed roots. We found a short‐term disturbance effect related to chamber installation. Fluxes of nitrogen oxides increased by as much as a factor of 4 during a period of about 1 month following chamber installation in the soil. Within 6 weeks of chamber base installation, fluxes fell to control levels. Given the timescale of disturbance and recovery, root mortality and decomposition is the most likely cause of the observed effect.

Spatial and temporal variability of nitrogen oxide and methane fluxes from a fertilized tree plantation in Costa Rica

Nitric oxide (NO), nitrous oxide (N2O), and methane (CH4) are naturally produced and consumed by soil biogeochemical processes. Naturally high variation between trace gas fluxes may temporarily increase due to agricultural management. We studied spatial and temporal variability of fluxes in the context of a 3‐year field experiment established to identify and quantify N2O fluxes and controlling factors using automated field measurements. We measured trace gas fluxes, soil temperature, and moisture from fertilized and unfertilized balsa (Ochroma lagopus) plantations. Combining spatial and temporal sampling we evaluate if automatically measured time series of N2O emissions are representative of overall mean fluxes from fertilized loam under balsa. Soil trace gas fluxes were measured manually at 36 randomly distributed sampling locations per plot. Mean plot emissions were evaluated against fluxes measured by seven chambers commonly used for routine bimonthly manual measurements and against N2O emissions measured by two automated chambers at 4.6‐hour sampling intervals. Trace gas fluxes were highly variable over 40 × 40 m plots. Nitrogen oxide fluxes were mainly spatially independent. Fertilization increased nitrogen oxide emissions but did not introduce spatial dependency of flux data. Within about 6 weeks fluxes approached pre‐fertilization level again. Given high spatial variation of nitrogen oxide fluxes we find that automatically measured N2O fluxes represent the nature of the flux response well and are in the range of fluxes indicated by spatial sampling. When soils were relatively dry fertilization inhibited CH4 uptake.

Nitrous oxide, nitric oxide, and methane fluxes from soils following clearing and burning of tropical secondary forest

Conversion of humid tropical forest to agriculture significantly alters trace gas emissions from soils. We report nitrous oxide (N2O), nitric oxide (NO), and methane (CH4) fluxes from secondary forest soils prior to and during deforestation, and throughout the first agricultural cropping. Annual average nitrogen oxide emissions from forest soils were 1.5 ng N cm−2 h−1 for N2O and 0.9 ng N cm−2 h−1 for NO. Forest clearing increased the level of extractable nitrate in soils and average nitrogen oxides fluxes (2.7 ng N cm−2 h−1 for N2O, and 8.1 ng N cm−2 h−1 for NO). Immediately after biomass burning, short‐term peaks of N2O and NO (123 ng N cm−2 h−1 for N2O, and 41 ng N cm−2 h−1 for NO) were superimposed on generally increased fluxes. Peak emissions declined within 3 days after burning. Postburn fluxes stayed higher than measured on adjacent forest sites for 3–4 months (averages for postburn fluxes were 17.5 ng N cm−2 h−1 for N2O, and 19.2 ng N cm−2 h−1 for NO). Increased N2O and NO emissions after clearing and until cropping were probably due to a combination of increased rates of nitrogen cycling and higher gaseous diffusion in drying soils. Compared to emissions from young pastures in the region, fluxes of nitrogen oxides from unfertilized agricultural areas were low (3.9 ng N cm−2 h−1 for N2O and 3.4 ng N cm−2 h−1 for NO), probably due to nitrogen uptake by fast growing corn plants and losses by leaching with draining soil water in the wet season. Variation in CH4 fluxes was high for all land use periods. Forest soils consumed an average of 1.0 mg CH4 m−2 d−1, which slightly increased in drier soils after clearing (1.2 mg CH4 m−2 d−1). Postburn CH4 consumption by soils was slightly reduced (0.8 mg CH4 m−2 d−1) compared to forest soils. Unfertilized agricultural soils consumed less CH4 than forest soils.

Effects of pasture management on N2O and NO emissions from soils in the humid tropics of Costa Rica

Emissions of nitrous oxide (N2O) and nitric oxide (NO) from agricultural soils in the tropics are important in the global budgets of these trace gases. We made monthly measurements of N2O and NO emissions from pastures with three different management systems on volcanic soils in northwestern Costa Rica: traditional (no N input from fertilizer or legumes), pastures with a grass‐legume combination, and pastures fertilized with 300 kg N ha−11 yr−1. Average annual N2O emissions were 2.7 ng N cm−2 h−1 from the traditional pastures, 4.9 ng N cm−2 h−1 from the grass‐legume pastures, and 25.8 ng N cm−2 h−1 from the fertilized pastures. Average annual NO emissions were 0.9, 1.3, and 5.3 ng N cm−2 h−1 from traditional, grass‐legume and fertilized pastures, respectively. In a separate experiment the effects of ammonium, nitrate, and urea‐based fertilizer mixtures on nitrogen oxide fluxes were compared. We measured nitrogen oxide fluxes following four different fertilization events. Nitrogen oxide fluxes were among the highest ever measured. The difference in soil water content between the fertilization events had a far greater effect on N2O and NO emissions than the effect of fertilizer composition. We conclude that the concept of “emission factors” for calculating N2O and NO emissions from different types of N fertilizer is flawed because environmental factors are more important than the type of N fertilizer. To estimate fertilizer‐induced N2O emission in tropical agriculture, stratification according to soil moisture regime is more useful than stratification according to fertilizer composition.

Air‐snow exchange of HNO3 and NOy at Summit, Greenland

Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol‐associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3− in snow may be reemitted to the atmosphere after deposition. Here we report on extensive measurements of HNO3, including vertical gradients between 1.5 and 7 m above the snow, made during the summers of 1994 and 1995 at Summit. These HNO3 data are compared with NO3− concentrations in surface snow and the first measurements of the concentrations and fluxes of total reactive nitrogen oxides (Ny) on a polar glacier. Our results confirm that HNO3 concentrations are quite low (mean 0.5 nmol m−3) during the summer, while NO3− is the dominant ion in snow. Daytime peaks in HNO3− appear to be due at least partly to emissions from the snow, an assertion supported by gradients indicating a surface source for HNO3− on many days. Observed short‐term increases in NO3− inventory in the snow can be too large to be readily attributed to deposition of HNO3− suggesting that deposition of one or more other N oxides must be considered. We found that the apparent fluxes of HNO3 and NOy were in opposite directions during about half the intervals when both were measured, with more cases of HNO3 leaving the snow, against an NOy flux into the snow, than the reverse. The concentrations of NOy are generally about 2 orders of magnitude greater than those of HNO3; hence deposition of only a small, non‐HNO3, fraction of this pool could dominate NO3− in snow, if the depositing species converted to NO3−, either in the snowpack or upon melting for analysis.

Scaling the turbulent transport of chemical compounds in the surface layer under neutral and stratified conditions

AbstractA model of the vertical transport of chemically reactive species is presented which is applicable to all possible stability conditions in the atmospheric surface‐layer. the non‐dimensional formulation adopted allows any chemical scheme to be included. an equation hierarchy is presented for a fully second‐order closure description of the process. the model describes comprehensively the turbulence/chemistry process by calculating the mean concentrations, the turbulent fluxes and concentration covariances of the chemical species. It explicitly takes into account the chemical transformation in first‐ and second‐order variable equations. the model is applied to the NO‐NO2‐O3 cycle and to the so‐called ‘night‐time’ chemical scheme of nitrogen oxides. an analysis of the flux budget indicates that the relative importance of the chemistry term in the flux equation varies with distance from the surface. For atmospheric neutral conditions, it is suggested that the chemistry term in the flux equation can be used to make an a priori evaluation of the possible effect of chemistry upon the flux of a chemical species. the model also shows that under stable conditions, despite the greatly reduced activity of turbulence, differences can occur between the turbulent transport of an inert tracer and that of a chemically reactive species. Under any stratification and for typical surface‐layer fluxes of nitrogen oxides, the intensity of segregation is very low and can in general be neglected in model calculations.

Sulfur and reactive nitrogen oxide fluxes in the North Atlantic atmosphere

This paper compiles atmospheric budgets for the large‐scale cycling of sulfur and reactive nitrogen oxides in the atmosphere of the North Atlantic Ocean. Using model results and measurements reported in the literature, estimates are made of the following large‐scale fluxes: eastward from North America into the North Atlantic atmosphere, northward into the Arctic, eastward into and westward out of Europe, westward from Africa, and westward at the Caribbean. Marine emissions into the North Atlantic atmosphere and deposition to the ocean are estimated. Atmospheric transport from continental sources contributes 4.4–7.5 Tg S/yr and 1.7–3.4 Tg N/yr to the North Atlantic Ocean atmosphere, while 0.6–3.6 Tg S/yr comes from marine emissions. Atmospheric transport removes 1.4–1.9 Tg S/yr and 0.5–0.8 Tg N/yr, and deposition to the ocean accounts for 5.2–15.7 Tg S/yr and 1.6–5.7 Tg N/yr. The main input terms for sulfur are flow from North America, followed by marine dimethylsulfide emissions and flow from Africa; for nitrogen, North American and African inputs are comparable. Although outputs exceed inputs for both sulfur and nitrogen, the ranges of estimates overlap by approximately 75%. The spatial distribution of total sulfur deposition flux is generally consistent with known sources and flow regimes except for high values which occur near the European west coast and off the east coast of the Americas south of 30°N. The spatial distribution of total nitrogen deposition flux is more consistent with current knowledge, with the exception of high values across the 0–10° latitude band. The molar sulfur to nitrogen ratio of total deposition exceeds 2 north of 30°N but is close to or below 1 to the south. Both sulfur and nitrogen outputs exceed inputs in the northeastern and southwestern parts of the region, indicating that either inputs from adjacent continents are underestimated or that the deposition is overestimated.

Characterization of nitrogen oxide fluxes from soil of a fallow field in the central piedmont of North Carolina

Abstract NO x emissions from soils may contribute to the formation of 0 3 in rural areas, especially when there is substantial emissions of natural hydrocarbons from surrounding vegetation, such as in the southeastern United States. Soil NO, flux measurements were made from 6 June 1992 to 7 July 1992 in the central Piedmont region of North Carolina (Southeast Oxidants and Nitrogen Intensive Analysis site, SONIA) in an effort to determine the role of natural emissions of NO, on rural atmospheric photochemistry. The overall average NO and N0 2 emission rates, using a dynamic chamber technique, were found to be 1.79 ± 1.37 ng-N m −2 s −1 (range: 0.13 to 6.67 ng-N m − s −1 ') and -1.07 ± 0.87 ng-N m −2 s −1 (range: -6.71 to 3.16ng-Nm −2 s −1 respectively. Over 85% of the N0 2 flux measurements were negative indicating net deposition to the soil surface. No negative NO flux rates were observed. NO flux was correlated with soil temperature. There was a positive correlation between NO concentration near the soil surface (∼50 cm) and NO flux ( r =0.35). The NO compensation point (1.12 ppbv) was estimated from the relationship between NO emission rate and ambient NO concentrations measured at 10 m. Both positive and negative vertical gradients of NO concentration between 10 m and soil surface were detected. The positive vertical gradients are indicative of NO transport to the site from polluted air masses. A significant negative correlation between NO flux and ambient 03 concentration ( r =0.66), however, supports the hypothesis that soil emissions of NO contribute to local production of 03 in rural areas.

Effect of pasture age on soil trace-gas emissions from a deforested area of Costa Rica

MEASUREMENTS of the flux of nitrous oxide―an important green-house gas―from recently formed pasture in the Amazon basin have shown a threefold increase relative to the flux from the original forest soil1. Based on these measurements, Luizao et a1.1 esti-mated that present rates of conversion from forest to pasture supply up to 1 Tg of N2O—N to the atmosphere each year, corresponding to ≤25 % of the current imbalance between sources and sinks of this gas. But this estimate assumes that such conversion produces elevated fluxes that remain constant in time. To assess the validity of this assumption, we present measurements of tracegas fluxes from Costa Rican pastures of varying ages. Nitrogen oxide fluxes peak during the first ten years after conversion, but decline thereafter to values that are even lower than the original forest fluxes. We conclude that previous studies have overestimated the contribution of pastures to the global budget of nitrous oxide, and that accurate predictions of soil-atmosphere tracegas fluxes will require a detailed knowledge not only of current land use, but also of landuse history.

Fluxes of nitrogen oxides and ozone measured by eddy correlation over a shortgrass prairie

The fluxes of NO2, NOy and ozone were measured by eddy correlation at a grassland field site from March 5 to August 2, 1988. The nitrogen oxide fluxes were generally small and upward during the day. Overnight, the fluxes were zero. The average daytime NO2 flux increased from 2.2 ng (N) m−2 s−1 during March and April (Julian days 65 to 121) to 5.7 ng (N) m−2 s−1 in June and July (Julian days 153 to 193). The average NOy flux for the period June 21 to August 2, 1988, was 7.6 ng (N) m−2 s−1 (Julian days 173 to 215). The largest fluxes were observed after rainstorms from drying surfaces, when the flux increased exponentially with soil temperature. Fluxes from dry soils were small and were not a strong function of soil temperature. The deposition of ozone, measured as oxidant to correct for homogeneous gas phase chemistry, showed a strong diurnal variation. The average daytime deposition velocity was 0.36 cm s−1 and 0.22 cm s−1 overnight. Surface resistance calculations show that under conditions of adequate soil moisture, approximately half of the ozone was deposited on the inner surfaces of the foliage, the remainder being deposited on the soil, outer leaf walls, and to the dead plant material. When soil moisture is low, less ozone is deposited on the foliage presumably because of water‐stress‐induced stomatal closure.

Measurement of nitrogen oxide fluxes from soils: Intercomparison of enclosure and gradient measurement techniques

The present estimates of emissions of NOx from soils to the atmosphere are based on field studies that used enclosure techniques. While this technique is direct, convenient and precise, the technique may perturb the biogenic soil processes under study. In addition, the limited sample size may produce nonrepresentative results. In order to address these concerns, NO flux measurements made using an enclosure technique are intercompared with NO flux determinations made using a gradient technique. The measurements were made during the nighttime hours from August to October, 1985, at an unfertilized, uncultivated grassland site in Colorado, United States. The NO flux measured during this period ranged between 0.08 ng N m−2 s−1 and 21 ng N m−2 s−1. The flux measurements made by the two techniques, which are based on very different approaches as described herein, agree very well.