Progress in Quantifying Rates and Product Ratios of Microbial Denitrification Using Stable Isotope Approaches

Although it is known since long that microbial denitrification plays a central role in N cycling in soils due to loss of nutrient N, emissions of N₂O and lowering of N leaching, few data at the field scale are available due to the difficulty in measurement. In recent years, stable isotope signatures of N₂O such as d¹⁸O, average d¹⁵N (d¹⁵N^bulk) and ¹⁵N site preference (SP = difference in d¹⁵N between the central and peripheral N positions of the asymmetric N₂O molecule) have been used to constrain the atmospheric N₂O budget and to characterize N₂O turnover processes including N2O production and reduction by microbial denitrification. However, the use of this approach to study N₂O dynamics in soils requires knowledge of isotope fractionation factors for the various partial processes involved, e.g. N₂O production by nitrification or fungal/bacterial denitrification, and N₂O reduction by bacterial denitrification.

Here we present recent progress on the principles of isotope fractionation modeling to estimate N₂O reduction and on the role of microbial groups and their specific impact on isotope values. Moreover, we report and discuss approaches to determine isotope values of produced N₂O prior to its reduction as well as enrichment factors of N₂O reduction. Finally, a variety of results from lab and field studies will be shown were N₂O reduction estimates by isotope fractionation modeling are validated by independent measurements using ¹⁵N tracing or He/O₂ incubations. Methodical improvements to increase sensitivity of the ¹⁵N tracing approach will be briefly addressed.

We conclude that up to now SP of soil-emitted N₂O proved to be suitable to constrain the product ratio of denitrification if N₂O fluxes are dominated by bacterial denitrification. Although this approach is not yet precise enough for robust quantification of N₂ fluxes, improved precision can be obtained in future, if further progress in understanding the control of fractionation factors of production and reduction and identifying N₂O formation by processes other than bacterial denitrification is achieved.