Oxygen isotopic signature of N2O for distinguishing between bacterial and fungal denitrification

Abstract:

The isotopic composition of the greenhouse gas N₂O (δ¹⁵N^bulk, δ¹⁸O and ¹⁵N site preference (SP) of N₂O) can be used to distinguish N₂O production pathways. So far, controls of δ¹⁸O values are not sufficiently explored due to complex fractionation processes and varying extent of O-exchange with soil water. However, it can potentially serve as another isotopic parameter, beside SP values, enabling to differentiate between bacterial and fungal N₂O production.

In the study presented here, natural isotopic signature of N₂O and O-exchange between denitrification intermediates and water for the first time was analyzed simultaneously from three bacterial and three fungal pure cultures. Anaerobic incubations with nitrite for fungi and nitrate for bacteria as electron acceptors were conducted. Treatments with three waters differing in ¹⁸O signature were used to determine O-exchange. ¹⁵N labeled electron acceptors served to determine the ongoing production pathway. After an incubation time of five, ten and 14 days gas samples were taken and analyzed with GC-IRMS.

Aside from one fungus all others produced N₂O by denitrification only. As expected, SP values of N₂O produced by fungi were much higher compared to bacterial N₂O. During fungal denitrification O-exchange was high (78 to 93%) and O isotope effects were stable over time and species and depended on O signature of water (42 to 48‰). In contrast, bacteria showed a much larger range of O-exchange (15 to 86%) with varying O isotope effects (14 to 39‰). Modelling O fractionation during denitrification revealed that O-exchange occurring by different enzymatic steps (nitrite reductase or nitric oxide reductase) could be responsible for the observed inconsistent O fractionation effects of bacteria compared to fungi. Thus, O fractionation of bacteria seems to be very complex and needs further investigation. Generally, fungal denitrification seems to be characterized by higher O fractionation effect than bacterial denitrification.