Intramolecular nitrogen isotope distribution in N2O from soil air along depth profiles in a Spruce forest (Fichtelgebirge, Germany)
P 3.3 in Ökosysteme: Funktion und Leistungen
The atmospheric trace gas N2O contributes to the global warming and the depletion of the ozone layer. The major source are soils. Its concentration and isotope signature (δ15NbulkN2O, δ18ON2O) in soil air changes in a characteristic manner along depth profiles (Goldberg et al. 2008). This information can be used to localize sites of N2O production and consumption within the soil (Goldberg & Gebauer 2009, Goldberg et al. 2010). Unfortunately very little information about the distribution of the heavy N isotope in the asymmetric linear structure of N2O (N-N-O) like the δ15Nα- and δ15Nβ values and consequential the 15NN2O site preference is currently available for soil air samples from depth profiles (Park et al. 2011). During production or consumption of N2O by microorganisms different kinetic fractionations are possible (Toyoda & Yoshida 1999). It is expected that the site preference increases with ongoing reduction of N2O to N2 (Park et al. 2011). Thus, the intramolecular distribution of nitrogen isotopes is a tool to further improve our current understanding about the fate of N2O along soil profiles. The aim of our study was to fill this information gap by analyzing the isotopomeric signature of N2O in soil profiles (Haplic Podsol) of a Norway spruce forest in the Fichtelgebirge (Germany).
The soil air was collected with evacuated glass vessels (100mL) using soil gas samplers installed in seven different depths (-10 cm to -100 cm) in four replicates. The samples were analyzed via PreCon-GC-IRMS coupling concerning the δ15NbulkN2O, δ18ON2O, δ 15NαN2O and δ 15NβN2O values and the N2O concentration.
Highest N2O concentrations were found in the subsoil at -60 cm. These high N2O concentrations were accompanied by the most 15Nbulk-depleted N2O isotope signatures and by least pronounced 15NN2O site preferences. N2O concentrations, 15Nbulk depletion of N2O decreased and 15NN2O site preference increased in air collected from the top soil. Thus, all three measured parameters indicate preferential N2O production in the subsoil and preferential N2O consumption in the top soil.
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