N2O isotope fractionation method to estimate N2 fluxes from soils – methodical development and validation
Denitrification often causes high losses of reactive nitrogen from agricultural soils, but is hardly predictable and up to now difficult to measure in the field. Is it possible to use stable isotopic signatures of N2O from soil to estimate N2 fluxes from denitrification? We tackle this question using controlled process studies in the laboratory and in the field
Denitrification reduces the plant-available nitrogen in soils and it is an essential process for the emission of the greenhouse gas N2O. Denitrification represents thus a key process in the N balance, especially in fertilized agricultural ecosystems. Due to methodical limitations in the measurement of N2, denitrification is very difficult to determine at the field scale. The production and reduction of N2O by denitrification and associated isotope effects result in a specific isotopic signature of the N2O molecule, comprising of the relative enrichment or depletion in 15N and in heavy O isotopes (17O, 18O) and of the site specific distribution of 15N within the linear N2O molecule. The isotopic siganture of N2O is controlled by the following variables: the 15N and 17O/18O signatures of precursor compounds (nitrate, soil water), the O-exchange with the soil water during N2O production, the rates of N2O production and N2O reduction to N2 as well as the isotope effects (fractionation) of the various sub-processes. Theoretically, the reduction of N2O to N2 - and consequently the N2 emission - can be derived from the isotopic signature of the emitted N2O, if the other parameters, affecting this signature, can be determined reliably enough. It is our aim to evaluate this methodological approach, i.e. the N2O isotope fractionation method for the determination of N2 emissions, in laboratory experiments with soils.
The N2O isotope fractionation method for the determination of N2 emissions is tested by laboratory soil incubations. Initially, incubation experiments are carried out to determine one or more control parameters directly. Some of these parameters are determined for the first time (isotope effect of the O-exchange with H2O) or for the first time with a new method (O exchange with H2O based on the 17O anomaly in the produced N2O). An existing isotope fractionation model is expanded in order to simulate and validate the isotopic signature of the emitted N2O based on the parameters determined before. Finally, soil incubation experiments with different soils and varied process conditions are performed with approaches that allow the simultaneous determination of N2O isotopic signatures and the magnitude of the N2 emission. By comparing the measured N2 emissions with the respective values calculated with the isotope fractionation model, it is examined to which extent the N2O fractionation method is suitable for the determination of N2 emissions and provides a new option for the detection of N2 emissions at the field scale.
10.2011 - 3.2015
Project funding number: DFG We 1904/4-1
Project status: finished