The role of ectomycorrhizal fungi in denitrification

Abstract

The contribution of EcM fungi to forest denitrification has been over-looked, despite the effects EcM fungi have on soil nitrate and C availability, two important factors controlling N2O emissions.  Although fungal denitrification has been proposed as a significant source of N2O in forest soils, the ability for EcM fungi to denitrify is unknown.  Here, I test the hypotheses that EcM fungi regulate forest N2O production both indirectly by supplying free-living microbes with C through exudation and mycelial turnover, and directly by denitrifying themselves. Soil incubations demonstrated the importance of the quality of C sources released by EcM mycelium in driving denitrification.  Comparing N2O production from EcM and non-mycorrhizal seedlings showed the potential for the presence of EcM mycelia to increase reduction of 15N-nitrate to 15N-N2O and 15N-N2.  The link to EcM C was confirmed in bacterial culture: denitrification by Paracoccus denitrificans 1222 was greatest when using C from extracts of dead mycelium of Paxillus involutus and extrudates produced when the fungus was in symbiosis with a host plant.  Therefore, EcM fungi indirectly increase denitrification by providing high quality C.  The potential for a direct contribution by EcM fungi to N2O production under oxygen-limited conditions was demonstrated in pure cultures of Tylospora fibrillosa and Paxillus involutus.  These findings enabled me to develop a schematic model describing the ecological significance of the role of EcM fungi in denitrification in relation to inorganic N availability.  Overall, my work provides the first evidence that EcM fungi have the potential to play a key role in N2O emissions.EThOS - Electronic Theses Online ServiceGBUnited Kingdo