## 2 Materials and Methods
### \({}^{15}\)N\({}_{2}\)O Sampling, Analysis, and Isotopocule Mapping Approach  
On 2, 5, 8, 15, 23, 39, 48, 51, and 54 days after onset of incubation (DAO), samples for analyses of N\({}_{2}\)O isotopomers were flushed into 100 mL crimp-top vials with butyl rubber septa. Samples were analyzed on a gas-isotope ratio mass spectrometer (Delta plus XP, Finnigan MAT, Bremen, Germany) coupled to a trace gas pre-concentration unit Precon (Thermo Electron Cooperation, Bremen, Germany) via a GC/GP Interface (Thermo Electron Cooperation, Bremen, Germany). In this setup, \(m/z\) 44, 45, and 46 of the intact N\({}_{2}\)O\({}^{+}\) molecular ions and \(m/z\) 30 and 31 of the NO\({}^{+}\) fragment ions are measured simultaneously [29], and \(\delta^{15}\)N\({}^{\text{bulk}}\)N\({}_{2}\)O, \(\delta^{15}\)N\({}^{\alpha}\)N\({}_{2}\)O, and \(\delta^{18}\)O\({}_{\text{N2O}}\) values were determined [30]. \(\delta^{15}\)N\({}^{\beta}\)N\({}_{2}\)O values were calculated based on the following Equation (4):  
\[\delta^{15}\text{N}^{\text{bulk}}\text{{}_{N2O}}=(\delta^{15}\text{N}^{\alpha} \text{{}_{N2O}}+\delta^{15}\text{N}^{\beta}\text{{}_{N2O}})/2 \tag{4}\]  
Site preference (\(\delta^{15}\)N\({}^{\text{SP}}\)N\({}_{2}\)O) was calculated as the difference between \(\delta^{15}\)N\({}^{\alpha}\)N\({}_{2}\)O and \(\delta^{15}\)N\({}^{\beta}\)N\({}_{2}\)O. We used the scrambling factor of 0.096 determined by [PERSON] et al. (2018) [31] to correct measured data [32]. \(\delta^{18}\)O of soil water was \(-\)6.7 \(\delta\)%.  
We applied the isotopocule mapping approach by [PERSON] et al. (2017) [22] to calculate the fraction of residual unreduced N\({}_{2}\)O (rN\({}_{2}\)O) and the N\({}_{2}\)O fraction from heterotrophic bacterial denitrification (f\({}_{\text{BD}}\)) based on the sample position in the \(\delta^{15}\)N\({}^{\text{SP}}\)/\(\delta^{18}\)O map using a mixing equation for the bacterial fraction (6) and the Rayleigh equation for N\({}_{2}\)O reduction (5):  
\[\text{r}_{\text{N2O}}=\text{e}^{((\delta\tau-\delta 0)/\eta\text{red})} \tag{5}\]  
\[\delta_{0,\text{sample}}=\text{g}_{\text{DD}}\ast\text{f}_{\text{BD}}+\delta_{ \text{f}D/\text{Ni}}\ast(1-\text{f}_{\text{BD}}) \tag{6}\]where \(\delta_{\rm r}\) is the isotopic signature of residual N2O after partial reduction, \(\delta_{0}\) is the isotopic signature of initial N2O before reduction, and \(\eta_{\rm red}\) is the isotopic fractionation factor associated with N2O reduction to N2. Two main scenarios were considered: (1) N2O emitted from bacterial denitrification is first reduced to N2 and residual N2O is then mixed with N2O originating from nitrification or fungal denitrification (Scenario 1, reduction-mixing). Alternatively, (2) N2O from bacterial denitrification and nitrification or fungal denitrification is first mixed and then partially reduced to N2 (Scenario 2, mixing-reduction). Recently, non-overlapping signatures for N2O produced by nitrification or fungal denitrification were proposed [23], and we calculated both scenarios for mixing of bacterial denitrification with either nitrification or fungal denitrification. A detailed description of the calculations can be found in [33]. We used the isotopic fractionation factors proposed by [23] (Supplementary Table S1), which were corrected for \(\xi^{18}\)O of soil water (\(-6.7\) \(\delta\)%) for mapping and calculations.  
In addition, calculations can be based on minimum or maximum end-member values, fractionation factors, and reduction factors, leading to a total of 14 different scenarios [31]. In our study, we used mean values for mixing, fractionation, and reduction whenever possible. However, as during anoxic incubation, samples were distributed outside the mean mixing-reduction area, we used minimum reduction values (mean mixing, mean fractionation) for 51 and 54 DAO. When calculations yielded values < 0 or > 1 for fbD or rN2O, these values were removed from the dataset before calculating means and plotting.