Fungi and bacteria are capable of both nitrification and denitrification. In this study, we quantified the importance of bacteria and fungi as sources of nitrous oxide (N₂O) in soils. We have previously determined that in pure microbial culture experiments that Site Preference (SP) (difference in d¹⁵N between the central and outer atom in N₂O) is a conservative tracer of the microbial origins of N₂O. A SP value of 0 ‰ and 33 ‰ has been shown to be indicative of N₂O derived from denitrification and nitrification, respectively. Soil samples were collected from a semiarid grassland soil from central New Mexico. Soils were incubated in triplicates according to the following treatments: nitrogen fertilized or unfertilized, 35 or 75% water holding capacity (WHC), and exposed to one of three antibiotic treatments (control, cycloheximide (fungicide), and streptomycin (bactericide)).

Results/Conclusions

After 46 to 72 h, N₂O production was greatest in fertilized soils compared to unfertilized soils at 75% WHC, averaging 50.2 and 3.8 pmol h^-1 g^-1 in the control, 20.9 and 2.0 pmol h^-1 g^-1 with cycloheximide added, and 21.9 and 1.5 pmol h^-1 g^-1 with streptomycin added, respectively. The N₂O production at 35% WHC was only detectable from fertilized control and streptomycin treatments (2.5 vs. 2.7 pmol h^-1 g^-1). The flux data, therefore, suggest that both fungi and bacteria are important contributors to N₂O production from soils at 75% WHC and that fungal production predominates at 35% WHC. Site preference values at 75% WHC ranged between 3.8 and 6.3 ‰ for all treatments, with the exception of a value of 17.4 ‰ within the fertilized mesocosm with cyclohexamide added. The low SP values at 75% WHC, excluding the fertilized cycloheximide treatment, suggest that denitrification was the dominant pathway for N₂O production. The SP values for the fertilized control and streptomycin treatments at 35% WHC averaged 10.7 and 20.1 ‰, respectively, and suggest production from both nitrification and denitrification. Production from nitrification in the presence of streptomycin may be due to archaeal production as this compound may not inhibit archaea. Our data confirms the importance of fungi and bacteria, and potentially archaea, as N₂O producers in semiarid grassland soils, and the dominant pathway may largely be dependent on soil water content.