Acetylene and oxygen as inhibitors of nitrous oxide production in Nitrosomonas europaea and Nitrosospira briensis: a cautionary tale

Abstract

Autotrophic ammonia-oxidizing bacteria produce nitrous oxide (N2O) as a by-product of nitrification or as an intermediate of nitrifier denitrification. In soil incubations, acetylene (C2H2) and large partial pressures of oxygen (O2) are used to distinguish between these sources. C2H2 inhibits ammonia oxidation and should therefore inhibit N2O production by both nitrification and nitrifier denitrification. O2 suppresses the reduction pathway of nitrifier denitrification. However, doubts concerning the reliability of C2H2 and O2 as inhibitors have arisen recently. Therefore, in this study we tested the influence of C2H2 and large partial pressures of O2 alone and in combination on N2O production in pure cultures of the ammonia oxidizers Nitrosomonas europaea and Nitrosospira briensis. C2H2 largely inhibited nitrite production in both ammonia oxidizers and N2O production by N. europaea. Surprisingly, it did not affect the N2O production in N. briensis. The variable response of ammonia oxidizers to C2H2 might have consequences for the use of C2H2 as an inhibitor of nitrification in soils. Different partial pressures of O2 ranging from less than 10 kPa O2 to 100 kPa O2 were tested for their effectiveness in inhibiting N2O production via nitrifier denitrification. The partial pressure of 100 kPa O2 yielded minimal N2O production by both ammonia-oxidizing species and seemed to inhibit N2O emission from nitrifier denitrification to a large extent. However, a negative effect of 100 kPa O2 on ammonia oxidation itself could not be excluded. The applicability of both inhibitors in determining N2O production pathways in soils is discussed.