Nature of the problem Reactive nitrogen (N • r) is of fundamental importance in biological and chemical processes in the atmosphere-biosphere system, altering the Earth's climate balance in many ways. Th ese include the direct and indirect emissions of nitrous oxide (N 2 O), atmospheric N r deposition and tropospheric ozone formation (O 3), both of which alter the biospheric CO 2 sink, N r supply eff ects on CH 4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NO x) and ammonia (NH 3). Human production and release of N • r into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an eff ect European N r emissions are having on net warming or cooling. Approaches Th is chapter summarizes current knowledge of the role of N • r for global warming. Particular attention is given to the consequences of atmospheric N r emissions. Th e chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric N r emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe. Th e use of N • r fertilizers has major additional eff ects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect eff ects are not assessed here. Key fi ndings/state of knowledge Due to its multiple, complex eff ects on biospheric and atmospheric processes, the importance of N • r for the European greenhouse gas balance has so far received insuffi cient attention. Th e main warming eff ects of European anthropogenic N • r emissions are estimated to be from N 2 O (17 (15-19) mW/m 2) and from the reduction in the biospheric CO 2 sink by tropospheric O 3 (4.4 (2.3-6.6) mW/m 2). Th e main cooling eff ects are estimated to be from increasing the biospheric CO 2 sink by atmospheric N r deposition at −19 (−30 to −8) mW/m 2 and by light scattering eff ects of N r containing aerosol (−16.5 (−27.5 to −5.5) mW/m 2), in both cases resulting from emissions of NO x and NH 3. Th e production of O • 3 from European emissions of NO x is estimated to have a modest warming eff ect (2.9 (0.3-5.5) mW/m 2), which is largely off set by the cooling eff ect of O 3 in reducing the atmospheric lifetime of CH 4 (−4.6 (−6.7 to −2.4) mW/m 2), giving an uncertain net warming of +1.7 (−6.4 to +3.1) mW/m 2). Overall, including all of these terms, European N • r emissions are estimated to have a net cooling eff ect, with the uncertainty bounds ranging from a substantial cooling eff ect to a small warming eff ect (−15.7 (−46.7 to +15.4) mW/m 2). Major uncertainties/challenges Th e largest uncertainties concern the aerosol and N • r fertilization eff ects, and the estimation of the European contributions within the global context. Published estimates suggest that the default N • 2 O emission factor of 1% used by IPCC for indirect emissions from soi...

Le processus de nitrification joue un rôle essentiel dans le cycle de l'azote dans les milieux aquatiques naturels. La mesure de l'activité nitrifiante est une étape obligée pour bien comprendre et quantifier les flux d'azote dans ces milieux. Ce travail présente une réévaluation de la méthode de mesure de l'activité nitrifiante autotrophe par la méthode d'incorporation de bicarbonate marqué au 14C et son application pour estimer des biomasses de bactéries nitrifiantes. La validité générale de la méthode a été démontrée par des tests menés sur des inhibiteurs de nitrification qui ont montré que l'utilisation combinée de N-serve (5 ppm) et de chlorate (10 mM) inhibait de manière complète et spécifique l'oxydation d'azote et l'incorporation de carbone des deux groupes de bactéries nitrifiantes. Un facteur de rendement (carbone incorporé par azote oxydé) de 0,1 mole C/mole N a également été déterminé sur des cultures pures de bactéries nitrosantes et nitratantes. Pour l'activité potentielle, en particulier, les conditions optimales pour la mesure d'activité nitrifiante ont également été établis: un pH entre 7 et 8, une température entre 20 et 30°C, une concentration en ammonium d'au moins 1 mmol/l et en oxygène d'au moins 6 mg/l. Une relation entre les mesures d'activité nitrifiante potentielle et la biomasse des bactéries nitrifiantes a été établie sur culture pure. Elle montre que dans les conditions de mesures de l'activité potentielle, 1 µg C de bactéries nitrifiantes oxyde 0,04 µmol N/hBy regenerating oxidised forms of nitrogen (nitrate), the nitrification process plays an important role in the nitrogen cycle of aquatic environments. The measurement of the activity and biomass of nitrifying bacteria is thus essential to understand and quantify the general nitrogen fluxes in those environments. Different methods of measuring the nitrifying activity exist. The first methods developed were based on the use of specific nitrification inhibitors: N-serve, allyl thio-urea, acetylene, methylfluoride and dimethyl ether, as most used. They consist in measuring differences of ammonium, nitrite and nitrate dynamics in an inhibited and control sample during time. These methods can be applied as long as the inhibitors are specific for nitrifying bacteria, and activities are high enough to allow the measurement of concentration variations during incubation times which are not too long. At the present time, the most used methods are dealing with isotopic tracers: 14C or 15N. 15N methods allow the direct measurement of the nitrifying activity, while 14C methods represent the measurement of a biomass production which can be converted into a substrate oxidation rate by the use of a yield factor. This factor is considered to be constant in the standard incubation conditions. The most frequently used enumeration methods of nitrifying bacteria are not very satisfactory. Classical culture techniques (most probable number) and immunofluorescence techniques are known to greatly underestimate the numbers of active organisms. Recently de...

No abstract

No abstract

Gross primary production, community respiration and reaeration coefficient were determined during an annual cycle on the Viroin River (South Belgium), based on the daily variations of dissolved oxygen concentration.Reaeration coefficient remains remarkably constant (0.26 h-') during the year in spite of discharge variations. The autotrophic community is dominated by 'Ranunculusfluitans'. Primary production parallels the variations of total solar radiations. It ranges from 0 in winter to 8 g 02 m 2 d-in summer. In spring and summer, respiration variations parallel those of primary production (average value: 10 g 02 m 2 d-1 ); in the dry autumn, decomposition of dying macrophytes considerably enhances the community respiration (15 g 02 m 2 d-l).A P/R diagram is used to characterize the trophic state of the Viroin.