Abstract
N2O is a powerful greenhouse gas contributing both to global warming and ozone depletion. While fungi have been identified as a putative source of N2O, little is known about their production of this greenhouse gas. Here we investigated the N2O-producing ability of a collection of 207 fungal isolates. Seventy strains producing N2O in pure culture were identified. They were mostly species from the order Hypocreales order—particularly Fusarium oxysporum and Trichoderma spp.—and to a lesser extent species from the orders Eurotiales, Sordariales, and Chaetosphaeriales. The N2O 15N site preference (SP) values of the fungal strains ranged from 15.8‰ to 36.7‰, and we observed a significant taxa effect, with Penicillium strains displaying lower SP values than the other fungal genera. Inoculation of 15 N2O-producing strains into pre-sterilized arable, forest and grassland soils confirmed the ability of the strains to produce N2O in soil with a significant strain-by-soil effect. The copper-containing nitrite reductase gene (nirK) was amplified from 45 N2O-producing strains, and its genetic variability showed a strong congruence with the ITS phylogeny, indicating vertical inheritance of this trait. Taken together, this comprehensive set of findings should enhance our knowledge of fungi as a source of N2O in the environment.
Highlights
N2O is a powerful greenhouse gas contributing both to global warming and ozone depletion
To assess how the N2O producing ability is distributed within fungi, 207 fungal strains comprising 23 orders and 54 genera were screened by incubating the strains in liquid culture under conditions that were previously reported to favour fungal denitrification[21]
The N2O-producing activities were much lower when nitrate was used as an electron acceptor (,0.1 mg N2ON g21 fungal biomass; F 5 108.55, P, 0.0001), supporting previous studies showing that nitrite rather than nitrate is preferable for fungal denitrification[23]
Summary
To assess how the N2O producing ability is distributed within fungi, 207 fungal strains comprising 23 orders and 54 genera were screened by incubating the strains in liquid culture under conditions that were previously reported to favour fungal denitrification[21]. Seven strains belonging to the Trichoderma, Fusarium, Penicillium and Phialocephala genera produced N2O in at least one of the soils, up to a maximum amount of 82.1 ng N2O-N g21 h21 (Fig. 2), which was significantly higher (F 5 15590, P , 0.0001) than the amount produced by chemical denitrification in the non-inoculated sterile soils This is comparable to previous studies focusing on fungal contribution[15,16,17,42] or net N2O production from soil[43,44,45]. The fungal N2O-producing capacity observed in our study could be influenced by biotic interactions occurring in the natural ecosystems between fungi or with bacteria and other organisms Both copper-containing nitrite reductase (encoded by the nirK gene) and P450nor (nitric oxide reductase) are key enzymes involved in fungal denitrification. Further studies are clearly warranted to elucidate the significance of denitrification in fungi and its consequences for N2O emissions
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