Background and Objective

Denitrification is an anaerobic microbial process of successive reduction of nitrate (NO₃^-) and nitrite (NO₂^-) to N₂ through the following reaction steps: NO₃^- - NO₂^- - NO - N₂O - N₂. The intermediate product, nitrous oxide (N₂O), is partially not further reduced to N₂. The resulting emission to the atmosphere constitutes the main emission source of this greenhouse gas from agricultural soils.

Quantification of soil denitrification is crucial for mitigating nitrogen fertiliser loss as well as for reducing N₂O emission. Commonly applied analytical techniques in field studies enable us to analyse only N₂O. However, the measured N₂O-fluxes alone do not provide information on the entire N loss by soil denitrification because (i) the emissions due to other microbial processes (nitrification, co-denitrification) are indistinguishable, and (ii) the contribution of N₂O reduction to N₂ is unknown, since it is difficult to directly measure N₂ emissions due to the high atmospheric background.

To overcome this problem, the ¹⁵N gas flux method can be applied, which includes stable isotope analyses of gas fluxes after the addition of ¹⁵N-labelled NO₃^-. This method has been applied in numerous laboratory but few field studies, showing that the fraction of N₂O reduction to N₂ (i.e. the ‘product ratio’: N₂O/(N₂O+N₂)) during denitrification ranging from 0 to 1 is extremely variable. So far, due to technical limitations, no comprehensive data sets from field-based measurements of soil N₂ emissions are available. This reinforces the necessity of more in situ studies.

Isotopic analyses of N₂O is altered during the partial N₂O reduction to N₂. The magnitude of the observed change depends largely on the product ratio. Hence, the measurement of isotopic signatures of the emitted N₂O hasa great potential to determine the product ratio under field conditions. However, to enable this determination, the characteristic isotopic signatures for produced N₂O and the characteristic fractionation factors associated with N₂O reduction must be known. The reported ranges of apparent isotopic fractionation factors are very wide and an explanation of these large variations is still ambiguous. Hence, in this project N₂O isotopic measurements are coupled with ¹⁵N tracing as a reference method. The aim is to ultimately evaluate the N₂O isotopic method as a robust quantification method for field studies.

The advantage of the N₂O isotopic methods over the ¹⁵N tracing method lies in its much lower costs, easier application and avoidance of various artefacts associated with the ¹⁵N tracing method. The N₂O isotopic method has thus the potential for more widespread use.

Approach

To precisely determine the isotopic fractionation factors associated with denitrification and possible accompanying processes valid for field conditions, detailed microcosm experiments under controlled conditions with ¹⁵N-tracing as a reference method will be conducted. The fractionation factors associated with N₂O reduction will be validated for field conditions through direct determination of the product ratio (N₂O/(N₂+N₂O)) of denitrification from ¹⁵N tracing results obtained parallel to N₂O isotopic analyses. Moreover, a ¹⁵N-tracer model will be used to estimate the extent of the possibly co-occurring N-transformation processes (nitrification, ammonification, immobilisation and ammonia oxidation) and their impact on isotopic values of emitted N₂O. Natural abundance studies of soil N compounds carried out in parallel will aim to determine the isotopic fractionation characteristic for these processes. Moreover, the analysis of natural abundance isotopic signatures of soil nitrite will be conducted for the first time in order to better assess the process dynamics of this crucial compound in N-cycling.

As an ultimate result of this project, a complex isotopic model will be developed, which should provide an approach to quantify the whole denitrification process in agricultural soils under field conditions. In perspective, this model can be combined with the process-based modelling of N₂O emissions to help in better prediction of N₂O reduction in soils.

Our Research Questions

Is the isotopic fractionation during denitrification stable and predictable? Which factors control the magnitude of the respective isotope fractionation factors?

Do other processes beside denitrification significantly influence the isotopic signatures of soil NO₃^- and of the emitted N₂O? Is this a limitation for the field application of the N₂O isotopic method?

Can we quantify N₂O reduction to N₂ based on N₂O isotopic signatures? Which requirements have to be taken into account and how exact can this quantification be?

Results