Evaluation of a Closed Tunnel for Field-Scale Measurements of Nitrous Oxide Fluxes from an Unfertilized Grassland Soil

Emissions of the major greenhouse gas NO from soils are characterized by huge spatial variability. An upscaling based on conventional small-scale chamber measurements is thus questionable and may involve a considerable amount of uncertainty. In this feasibility study, we evaluated the applicability of a large, closed tunnel for field-scale measurements of NO fluxes from an unfertilized grassland soil. The tunnel, coupled to an open-path Fourier transform infrared spectrometer, covered 500 m. During a 2-yr campaign, concurrent closed-chamber measurements (area of 0.045 m) were performed at the tunnel plot. The tunnel system enabled high-density and precise NO concentration measurements under dry, stable, nocturnal atmospheric conditions, but higher wind speeds and rain limited its application. To calculate an unbiased, predeployment NO flux from the increase of NO concentrations during tunnel deployment, we propose a novel approach based on inverse modeling (IMQ0). We show that IMQ0 is appropriate for the specific non-steady state tunnel setup. Compared with conventional models, which were developed for gas flux calculation from concentration gradients measured in vented closed chambers, IMQ0 is most accurate. Whereas NO fluxes obtained from the tunnel measurements were generally small and at a typical background level, the chamber measurements revealed high spatial and temporal variability of NO emissions, including slight NO uptake and precipitation-triggered emission peaks. The cumulative NO fluxes of both methods differed by one order of magnitude and were smaller for the tunnel measurements. We argue that the chambers were occasionally susceptible to detection of hotspots and hot moments of NO emission. However, these emissions were evidently not representative for the field scale. Compared with available greenhouse gas measurement techniques, we conclude that the tunnel may serve as a gap-filling method between small-scale chamber and ecosystem-level micrometeorological techniques, particularly during stable nocturnal conditions

Auswirkung saurer atmosphaerischer Depositionen bei Nadelwald auf Stoffanlieferung an das Grundwasser und Stoffumsetzungen in einem Aquifer aus basenarmen Sanden (Fallstudie Modellgebiet Fuhrberger Feld)

Im Modellgebiet Fuhrberger Feld wurde die Gefahr der Grundwasserversauerung durch Aziditaetseintrag aus sauren Sandboeden unter Kiefernwald in einen Lockergesteins-Aquifer systemorientiert untersucht. Die gemessene Stoffdeposition in dem Kieferforst ist relativ gering; z.B. liegt die Gesamtdeposition von Sulfat-S bei 13.5 kg/(ha*a). Die Sandboeden (meist Podsol-Gleye) sind stark versauert. Laborexperimente an Bodenproben und ungestoerten Bodensaeulen zeigen, dass diese Boeden Aziditaet und Sulfat in Form von Al-Hydroxo-Sulfat-Verbindungen speichern koennen. Vorratsabschaetzungen dieser Verbindungen weisen darauf hin, dass daraus noch fuer eine Reihe von Jahren Aziditaet und Sulfat an das Grundwasser angeliefert werden koennen. Dadurch wird der derzeitige Aziditaetseintrag in den Aquifer noch fuer eine entsprechende Zeit auf gleichem Niveau bleiben. Im Aquifer ist die Versauerungsfront unter Nadelwald bis in 5-6 m Tiefe vorgedrungen. Die Saeuren werden aber durch Pufferprozesse stetig neutralisiert. Zur Pufferung im Aquifer tragen Sulfatreduktion, Silikatverwitterung und Kationenaustausch bei. Die Rate der Sulfatreduktion wurde durch Isotopenmessungen ermittelt. Zur Quantifizierung der Silikatverwitterungsrate wurden Berechnungen mit dem Modell PROFILE durchgefuehrt und anhand von Stoffkonzentrationsmessungen im Grundwasser ueberprueft. Anhand der Daten fuer Aziditaetseintrag und Pufferraten im Aquifer konnte mit einem einfachen Modell das Vordringen der Versauerungsfront im Aquifer abgeschaetzt werden. Nach diesen Rechnungen wird eingetragene Saeure im Aquifer innerhalb von 10-30 Jahren neutralisiert. (orig.)In the model area Fuhrberger Feld the risk of groundwater acidification by input of acidity from sandy soils under pine forest into the sandy aquifer was investigated. Measured atmospheric solute deposition into the pine forest is relatively low; e.g. total deposition of sulfate-S is 13,5 kg/(ha*a). The sandy soils are strongly acidified. Laboratory experiments with soil samples and undisturbed soil columns showed that acidity and sulfate can be stored in this soils as Al-hydroxo-sulfate compounds. Estimation of the stored amount of these compounds makes evidence that acidity and sulfate originating from dissolution of these compounds will be leached into groundwater for some further years. Therefore, the input of acidity into the aquifer will be maintained at the actual level for the respective time span. Below pine forest acidity has penetrated the aquifer to a depth of 5 to 6 m. In the aquifer the acids are continuously buffered by sulfate reduction, silicate weathering, and cation exchange. The sulfate reduction rate was evaluated by isotope measurement. To quantify the rate of silicate weathering the PROFILE model was used. Based on data for input of acidity into the aquifer buffering rates migration of acidity in the aquifer was estimated. Preliminary results indicate that acidity is buffered in the aquifer within 10 to 30 years. (orig.)SIGLEAvailable from TIB Hannover: RN 8908(98-056) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekUmweltbundesamt, Berlin (Germany); Bundesministerium fuer Umwelt, Naturschutz und Reaktorsicherheit, Bonn (Germany)DEGerman