f.x. meixner biogeochemistry department, max planck institute … · 2014-05-18 · the surface...

The surface exchange of nitric oxide

F.X. Meixner

Biogeochemistry Department, Max Planck Institute for Chemistry,

P.O.Box J060, D-JJOJOMo/Mz, Germany

EMail: [email protected]

Abstract

Among the different components of the surface exchange of nitric oxide anthropogenic aswell as biogenic emissions are overwhelming. While inventories for anthropogenic NO% sour-ces are presently among the most thorough compilations of pollutants' emissions, our know-ledge on biogenic NO emissions is comparatively poor. A brief overview is given about (a)the importance of NO% emissions for tropospheric chemistry, (b) the present estimate of glo-bal NO emissions, (c) techniques suitable for the mesurement of NO surface exchange, (d)microbial and abiotic processes regulating the soil-biogenic NO emission and its main influ-encing factors, and (e) the present state of modeling (global) soil-biogenic NO inventories.

1 Introduction

Among the presently best defined atmospheric effects of odd nitrogen tracegases (NO, NO:, HNOz, ROiNOz, NO]*, NzOs, HNOs, NOg) are (a) the roleof nitric oxide (NO) in tropospheric photo-chemistry ("triggering" the produc-tion/destruction of ozone; Crutzen\ Chameides et #/.*), and (b) the atmosphe-ric acidification by (photo-) chemical oxidation of NO and nitrogen dioxide(NOz) to gaseous and p articulate nitrate (HNOg, NOg) Effects of atmosphericodd nitrogen compounds on terrestrial biota are both of direct and indirect na-ture. Dry and wet N-deposition downwind of industrialized and agriculturalareas add very large amounts of nitrogen (10-40 kg ha^yr^ for Europe and eas-tern North America) to otherwise natural ecosystems and may dramaticallychange their (nitrogen-limited) status (e.g. Vitousek ). In those regions of theglobe, where the atmospheric NO concentration channels the oxidation of vola-tile hydrocarbons towards the production of ozone, one may expect substantialreductions in productivity and/or mortality in natural ecosystems and significantlosses in agriculture due to the adverse effects of ozone (Reich & Amundsen ).

Transactions on Ecology and the Environment vol 13, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

326 Measurements and Modelling in Environmental Pollution

2 Global emissions and troposheric chemistry of NO

Fossil-fuel combustion from energy production and automotive exhaust stillcontribute most to NO* emissions (NO% = NO + NO:) on a global scale. Thecorresponding global source strength has been recently estimated by Kasibhatlaet al™- to 21.3 x 10* kg-N yr*. Nitrogen oxides inventories for anthropogenicsources represent one of the most thorough compilations of emission informa-tion as compared to other atmospheric pollutants (Graedel et al. ). "Biomassburning" also contributes to global NO% emissions. Its source strength is pre-sently estimated to range between 8.5 - 12 x 10* kg N yr*, thus presenting ap-prox. 50% of current fossil-fuel combustion emissions (Levy et a/. , Andreaeet a/.*). However, future fire scenarios will be largely determined by the human-induced (e.g. population pressure) feed-back mechanisms between the bio-sphere and atmosphere (Goldammer ). There are also natural sources of NOx :lightning, microbial processes in soil, oxidation of atmospheric NH%, strato-

spheric injection, and photolytic processes in the oceans. While the NO% emissi-ons from biogenic NO production in soil are equal to and perhaps greater thanthose from lightning, they are certainly much larger than the remaining threementioned above (Williams et a/.**). However, the global soil NO source is cur-rently estimated to be around 20 x 10* kg-N yr* (Davidson*), respectively andis therefore comparable to the global NOx emissions from fossil-fuel combus-tion.

Nitric oxide, once emitted either by anthropogenic or biogenic processes,takes part in various homogeneous and heterogeneous chemical reactions in thetroposphere (see Warneck ). Different pathways - involving several odd nitro-gen compounds (NO:, HNOz, RX NOz, NOg*, N:Os) - lead to the principalend product of NO-oxidation, namely nitrate (gaseous HNOs, NOg bound toaerosol particles). Most of these compounds interact with terrestrial and marineecosystems through different surface exchange mechanisms, which comprisedry and wet deposition, as well as (biogenic) emission. Mainly due to their dif-ferent chemical reactivity/solubility, the significance of each surface exchangecomponent is also different for a particular odd nitrogen compound. Generally,wet deposition is most important for NOs", and dry deposition for NO](ROzNOz, NOs*), while the atmospheric loss of HNO^ and HNOj is likewise

wet and dry removal (Meixner ). However, for NO at ambient concentrations,emission from soils is observed more frequently than deposition.

3 Measurement techniques for NO surface exchange

Theories, techniques, and equipment originally developed for the study ofturbulent exchange of sensible heat, water vapour and momentum have beenextended to investigations of surface exchange of gases and particles and havebecome widely used during the last 15 years (Fowler & Duyzer ). Detailed as-sessments, including practical and theoretical limitations, of how to measuredry deposition or emission rates of pollutants by micrometeorological methods


Measurements and Modelling in Environmental Pollution 327

are given by Baldocchi et al.\ Fowler & Duyzer", Davidson & Wu*, and Len-schow for example. Applying the so-called gradient method, the turbulentvertical flux of a trace gas is obtained from measurements of vertical gradientsof concentration and of an "exchange coefficient", which is determined alsofrom vertical gradients of air temperature, humidity, and horizontal wind speed.The determination of vertical gradients is usually approached by measurementsat least at two levels over the surface. The magnitude of vertical concentrationgradients is generally very small. In the case of nitric oxide, the detection ofvertical fluxes requires a precision in measurement of concentration of 1 % (re-lative) or better. Applying the most direct micrometeorological method, the so-called eddy correlation method, the turbulent vertical flux of a substance is de-rived from measurements of fluctuations (in the range of 0.001-20 Hz) in itsconcentration and also of the instantaneous vertical wind velocity. While themeasurement at one level over the surface is basically sufficient, sensitive andfast concentration analyzers are required, i.e. response and lag times must beless than 1 second. Micrometeorological measurement of vertical fluxes usuallyassumes constancy of flux with height, i.e. the flux measured at a certain heightabove the surface is equal to the actual surface flux. This assumption isexpected to hold under specific ("good fetch") conditions (Businger"). Aproperly measured flux at the chosen sampling point above the surface providesthe average vertical flux over the "fetch" distance/area. The advantage of mi-crometeorological methods is, that whole ecosystems can be studied on scalesof several hectares without interference to the actual exchange process (Fowler& Duyzer ). Eddy correlation as well as gradient methods have been used tostudy vertical fluxes of NO, NO: and HNOg (e.g. Meixner ).

Using enclosure (cuvette-, chamber-, box-) methods, a transparent oropaque box enclosure is placed over bare soil and soil covered by low vegeta-tion, respectively (see Livingston & Hutchinson ). The air volume inside theenclosures/chambers may or may not be mechanically stirred. So-called closed(or static) chambers cover the plant/soil intermittently during measurementswith enough air flow-through for the analyzers; the exchange rate is determinedfrom the temporal change of concentration inside the chamber (e.g. Davidson eta/.™). Using so-called dynamic chambers, the flow rate of air through thechamber and the difference between inlet and outlet concentration are measuredin order to calculate the flux towards or away from the enclosed surface (e.g.Meixner et al™). The relative advantages of static versus dynamic chambershave been recently discussed by Hutchinson & Livingston**. In most applicati-ons, the dynamic chamber is flushed with ambient air (e.g. Remde et a/. ,Meixner et al™\ while Williams et al™ used "zero-air" (NO free air). How-ever, the net surface exchange of NO is dependent on ambient NO concentra-tion: above a certain NO concentration level ("compensation point" level, seeConrad*) there is uptake of NO, whereas below that concentration level emissi-on will occur. It is therefore essential that the NO concentration inside achamber is as close as possible to the ambient concentration (see Johansson**);using "zero-air" chambers, any surface uptake of NO can never be detected.Enclosure methods have been questioned: (a) the enclosure itself may affect



soil and plants in it, and (b) the point measurement by an enclosure is notnecessarily representative of whole ecosystems. Folorunso & Rolston^ as wellas Williams & Davidson^ have shown that flux spatial heterogeneity may bethe overwhelming problem for enclosure methods.

In the case of N0% surface exchange, both the micrometeorological as wellas the (dynamic) enclosure method face the problem of fast (photo-) chemicalinterconversions of the NO-NCVOs triad, which occur on time scales similar tothose of (a) the turbulent transport within the atmospheric boundary layer, or(b) the mean residence time of air in an enclosure (typically some 10 to 100seconds). However, measured fluxes can be corrected for these reactions (e.g.Kramm et a/. , Meixner et a/.).

4 Biogenic emission of nitric oxide

NO reveals a very low solubility in aqueous solutions. Therefore, any NOuptake by plants is expected to be very low. Reported deposition velocities forNO are generally less than 10^ m s^ (e.g. Neubert et al?*). However, soils areobserved to act as both sources and sinks of nitric oxide (Conrad*). On a globalscale, NO emission from natural and cultivated soil is dominating (Williams etal. ). Both biotic and abiotic processes are involved in the production and con-sumption of NO in soils. Numerous groups of soil microorganisms contributeto the production and consumption of NO through a variety of biochemicalprocesses. Besides bacterial denitrification and nitrification (which are most im-portant), also other microbial, hence also non-nitrifying and non-denitrifyingprocesses are reported to consume and to yield at least trace amounts of NO(see reviews of Conrad*, Williams et al*\ Davidson & Schimel ).

4.1 Denitrification

Denitrification is an anaerobic process. In the absence of oxygen bacteria(generally found in all soil and freshwater environments) utilize nitrate and ni-trite for their growth and reduce them (via NO) to N:O and molecular nitrogen.The absence of oxygen requires either high soil water contents, or large re-spiration and oxygen consumption rates in the soil. Readily oxidizable organiccarbon is a requirement for most denitrifying bacteria (heterotrophs). Denitrifi-cation occurs over a wide temperature range (maximum rate at -65-70 °C),roughly doubling for every 10°C rise, whereas the maximum of NO productionwas observed around 40-50°C. However, the ability of specific denitrifyingbacteria to produce and consume NO has been shown (Remde & Conrad ).

4.2 Nitrification

Nitrification involves the biological oxidation of nitrogen. A common formof this involves the oxidation ofNH/ to NO* (with NO: as intermediate), butthere are also bacteria which oxidize NtV to NO: and NO: to NOs Generally



chemoautotrophic, these bacteria require only CO], H:O, O] and either NH/ orNO: for growth (Galbally"). IfNH," or urea ((NH2):CO) is present, nitrifica-tion will occur in well aerated soils. Remde & Conrad^ have shown that NOproduction may be dominated by nitrification in a particular soil and by denitri-fication in another one. Hence, it is a priori not clear which process may beresponsible for NO metabolism in a particular soil.

4.3 Abiotic processes

Vaporization of HNOz from soil aqueous solution, chemical decompositionof HNO2 to NO and NO:, and photolysis in aqueous solution of MV (yieldingNO) are biotic processes which can emit NO (and/or NO:) from soils and sur-face waters. These processes and their (potential) importance for NO% emissionare reviewed in detail by Galbally^. However, the contribution of HNO% de-composition to NO% soil emission seems to be at least an order of magnitudesmaller that of the biological processes of nitrification and denitrification.

4.4 Bi-directional flux of nitric oxide

The bi-directional NO flux between soils and atmosphere observed in labo-ratory and field is due to the simultaneous action of NO producing and NOconsuming processes mentioned above (Conrad*). The relative rates of theseopposing processes determine the NO concentration within soil air (the "com-pensation point" concentration), and hence affect both the direction and themagnitude of the net NO flux to or from the atmosphere. According to Wil-liams et al*\ NO compensation points may range between 0-75 ppbv (fieldstudies) and 50-600 ppbv (laboratory studies) for aerobic soils, while under an-aerobic conditions enhanced levels were observed (1600-2200 ppbv, e.g. Rem-de et al ). Most recent results on the dependence of the NO compensationpoint on different controlling factors (such as soil temperature, soil nutrientcontent) is given by Ludwig^.

4.5 Factors influencing the biogenic NO emission

Soil temperature. NO emission from agricultural land generally show arather strong dependence on soil temperature (see Williams et al^ and refe-rences therein). In accordance with most microbial processes, including nitrifi-cation and denitrification, an approximate doubling of the NO emission rate foreach 10°C rise in soil temperature was found. However, Ludwig^ and Meixneret al™ found that the diel behaviour of NO emission closely followed the dailyvariation of soil temperature in the uppermost soil level only if the soil moisturewas not limiting (see below).

Soil nutrient content. The availability of organic and inorganic nitrogen insoils has a strong impact on NO emission rates. However, whether NOemission is more related to soil NKLf or NOg content, is still an open question(e.g. Ludwig ). Generally, both underlying processes for NO production, ni-



trification and denazification are often substrate limited. Therefore, it may beassumed that soil NH/ and/or NOs" pool sizes could serve as indicators of theN transformation rate, and thus as predictors for NO emission rates. The inputof animal waste and/or mineral fertilizer resulting in both enhanced nitrificationand denitrification activity, are responsible for enhanced N0% emission rates ofrecently fertilized fields and for grazed pastures (when compared to ungrazed)(Williams et al. ). Generally, NO fertilizer loss (due to volatilization) occurswithin the first few days after application and amounts to some tenths to a fewpercent of the applied nitrogen (e.g. Ludwig^, Meixner et al.).

Soil moisture. NO production, NO consumption and (diffusional) transportof NO in soils are affected by the soil water content. According to Davidson**,soil water content is one of the most important, but least well-defined, environ-mental quantities which control soil NO emissions; the soil water content (a)mainly controls the oxygen supply (which governs the relative importance of ni-trification versus denitrification), and (b) affects the diffusive transport of ga-seous and dissolved reactants and products (hence influencing rates and pro-duct ratios of microbial activities). Abrupt changes in soil moisture (e.g. afterheavy rainfalls) cause large bursts ("pulsing") of NO emission, which may be 1-3 orders of magnitude higher than those observed before. As already indicatedby Johansson & Sanhueza^* and Cardenas et a/.*, NO emissions from low-lati-tude ecosystems (tropical savannas) are dominated by seasonal variability inrainfall (see Meixner et al ).

Plant cover. It has been shown that NO emissions from plant covered soilsare reduced when compared to those from bare soils (see references in Meix-ner ). A 75% "canopy reduction" of soil NO emission was estimated for tropi-cal rainforests (Jacob & Wofsy ), where during the night a significant portionof NO is converted (by reaction with Og) within the canopy into NO2 (whichmay be deposited on the plants' cuticles).

5 Modeling of soil-biogenic NO emissions

Based on their field studies over different American ecosystems Williams etal^ developed an algorithm for the estimation of NO emission for a variety ofsoil categories. The NO emission flux is described by F(NO)=A exp(B*Tsoii),where T^n is the soil temperature (°C), Bis a dependency coefficient which isassumed to be constant across biomes, and A is a biome fitting parameter(ngN m'V ) which is deduced/interpolated from observed relationships of NOemission versus nitrate content of soils. A similar algorithm, extended for thethe effects of "pulsing" and "canopy reduction", was used by Yienger & Levy**to construct global patterns of soil-biogenic NOx emissions. In contrast, Potteret al^ reported on an ecosystem modeling approach that integrated global sa-tellite, climate, vegetation, and soil data sets to examine conceptual controls onnitrogen trace gas (NO, N:O, %) emissions from soils.

Using "A"-parameters of Williams et al. and mean soil temperatures ob-served during their field experiments (grazed pasture, wheatfield, heather =



forest soil), Meixner et al^ calculated corresponding NO emission fluxes.Whereas for the pasture and the heathland sites the NO emission is always un-derestimated (at least by a factor of 4), the calculated wheatfield emissionsoverestimate the observed NO emissions up to a factor of 15. The obviousdiscrepancy (a) is certainly due to "A"-parameters which are constant withtime, and (b) may also be due to the statistically weak deduction of ' "-para-meters (see Williams et al^ and references therein). It is suggested that a simp-le "biome-fitting" factor as used by Williams et a/., is by far not sufficient to de-scribe temporal and geographical variation of the behaviour of individualbiomes as long as this parameterization is not based on specific (microbial)processes which produce and consume NO in soil (see Potter et a/. , Yang &Meixner ).

Acknowledgements

The presented work was financed by Max Planck Society and partially was supported byBundesminister fur Forschung und Technologic (contract 07EU713-7) and the Commissionof the European Communities (contracts EV4V-0167-C, EV4V-00883-C).

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