Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification - a pure culture study

RATIONALE: The contribution of fungal denitrification to the emission of the greenhouse gas nitrous oxide (N2O) from soil has not yet been sufficiently investigated. The intramolecular 15Nsite preference (SP) ofN2Ocould provide a tool to distinguish between N2O produced by bacteria or fungi, since in previous studies fungi exhibited much higher SP values than bacteria. METHODS: To further constrain isotopic evidence of fungal denitrification, we incubated six soil fungal strains under denitrifying conditions, with either NO3 or NO2 as the electron acceptor, and measured the isotopic signature (d18O, d15Nbulk and SP values) of the N2O produced. The nitrogen isotopic fractionation was calculated and the oxygen isotope exchange associated with particular fungal enzymes was estimated. RESULTS: Five fungi of the order Hypocreales produced N2O with a SP of 35.1 ± 1.7‰after 7 days of anaerobic incubation independent of the electron acceptor, whereas one Sordariales species produced N2O from NO2 only, with a SP value of 21.9 ± 1.4‰. Smaller isotope effects of 15Nbulk were associated with larger N2O production. The d18Ovalues were influenced by oxygen exchange between water and denitrification intermediates, which occurred primarily at the nitrite reduction step. CONCLUSIONS: Our results confirm that SP of N2O is a promising tool to differentiate between fungal and bacterial N2O from denitrification. Modelling of oxygen isotope fractionation processes indicated that the contribution of the NO2 and NO reduction steps to the total oxygen exchange differed among the various fungal species studied. However, more information is needed about different biological orders of fungi as they may differ in denitrification enzymes and consequently in the SP and d18O values of the N2O produced.