Combined application of the natural abundance stable isotope and 15N tracing methods to quantify denitrification in agricultural soils in field studies

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Quantifying denitrification in arable soils is crucial in predicting the microbial consumption of nitrogen fertilizers and the associated nitrous oxide (N2O) emissions, but still challenging due to difficulties in direct measurements of dinitrogen (N2) emissions and due to possible co-existence of numerous microbial N-transformations. Isotopomer analyses of emitted N2O (d15N, d18O and SP=Site Preference of 15N within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N2. However, such interpretations are often ambiguous because: (i) the majority of basic knowledge about isotopic fractionation factors originate from laboratory experiments and (ii) systematic studies on method transfer into field conditions are still lacking. This proposal aims at combining laboratory and field studies to obtain information on the possible transferability of the methods developed under laboratory conditions to the field scale. The general aim is to develop a method for in situ quantification of the entire denitrification process in agricultural soils. The finally developed method will be solely based on the natural abundance stable isotope approach, i.e., combined isotopomer analyses of N2O and isotopic analyses of soil N-compounds. To precisely determine the isotopic fractionation factors associated with denitrification and possible accompanying processes valid for field conditions, detailed microcosm experiments under controlled conditions and 15N-tracing as a reference method are proposed. 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 isotopomer 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. Additionally, 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 a method 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 estimation of the N2O reduction contribution.