There are also indirect estimates of the dominance of fungal denitrification in alkaline soils after the application of bacterial or fungal

inhibitors (Castaldi & Smith, 1998; Laughlin & Stevens, 2002; Crenshaw et al., 2008). Fungi are thought to contribute to N2O production through nitrite or nitrate reduction, as denitrification or codenitrification (Bollag & Tung, 1972; Shoun et al., 1992; Tanimoto et al., 1992), under low oxygen (O2) conditions, for example ‘initially Nutlin-3a cost aerobic’ culture vessels (Zhou et al., 2001). Fungal denitrification occurs in the fungal mitochondria (Kobayashi et al., 1996), whereas bacterial denitrification is restricted to the cell membrane. However, the universality of this trait within all fungal groups is unknown. The symbiotic mutualistic ectomycorrhizal fungi dominate the microbial biomass in acidic temperate and boreal PI3K phosphorylation forest soils (Smith & Read, 2008).

These fungi form symbiotic associations with tree roots (e.g. pine, birch, poplar): in return for carbon (C) derived from host-plant photosynthesis, the fungi forage and acquire nutrients for their host via the extensive fungal mycelial network. Although fungi, in general terms, were proposed as a source of N2O in acidic forest soils (Bleakley & Tiedje, 1982), the ability of the ectomycorrhizal fungal group to produce N2O or their contribution to soil N2O fluxes remains unknown. Ectomycorrhizal fungi can grow on certain nitrogen (N) sources, proteins, amino acids, ammonium and nitrate (Finlay et al., 1992); however, the N reduction pathway in ectomycorrhizal fungi is poorly understood compared with other fungal groups. For example, the presence of the nitrate reductase enzyme in 68 ectomycorrhizal fungal species Florfenicol was only recently confirmed (Nygren et al., 2008). Here, we provide the first evidence of the ability of two ectomycorrhizal fungi, Paxillus involutus (Batsch) Fr. and Tylospora fibrillosa (Burt.) Donk, which are highly competitive when inorganic N concentrations are high (Brandrud, 1995; Carfrae et al.,

2006), to produce N2O through nitrate reduction under low O2 conditions. N2O production by these fungi was compared with that of the known fungal denitrifier, F. lichenicola [CBS 483.96; Centraalbureau voor Schimmelcultures (CBS), the Netherlands] previously known as Cylindrocarpon tonkinense IFO 30561 (Shoun et al., 1992; Usuda et al., 1995; Watsuji et al., 2003). The production of N2O was examined in fungi P. involutus 8 (Batsch) Fr. (from Sheffield University), T. fibrillosa F23 3AT (Burt.) Donk (isolated from Sitka spruce root tips) and F. lichenicola CBS 483.96, under aseptic pure culture conditions. The growth medium was modified Melin–Norkrans liquid medium (Marx, 1969) with glucose, ammonium and malt extract omitted and the pH adjusted to 5.6.