N deposition :: Mycorrhizal fungi

Impact Type: Deposition of pollutant

Key Concerns:

There is widespread evidence that nitrogen deposition reduces the vitality and species diversity of ectomycorrhizal fungi. However, direct effects of nitrogen are also likely due to competition with other soil flora and the reduced allocation of carbon from the host to the roots and mycorrhizal fungus (possibly reflecting the increased need to assimilate foliar absorbed N).

Changes in the ectomycorrhizal fungi of forests have been reported for the Netherlands (which is a region of high N deposition) (Arnolds 1991, van der Eerden et al. 1998), with a strong decline in the visible fruiting bodies and conversely an increase in the fruiting bodies of wood decaying (saprotrophic) and parasitic fungi. Several authors have reported that nitrogen fertilisation reduces the species diversity of mycorrhizal vegetation (Termorshuizen and Schaffers 1987, van der Eerden et al. 1998)(Wöllecke et al. 1999).

In experimental studies of forest soils subject to increased nitrogen deposition, several authors have reported a decease in colonisation by ectomycorrhizal fungi (e.g. Alexander and Fairley 1983, Taylor and Alexander 1989) although others such as Nilsen et al. (1998) observed no effect. There is a similar debate on the effects of nitrogen deposition on mycorrhizal fungi in heathland/moorland systems (Cairney and Meharg 1999). For example, Johansson (1992) and Caporn et al. (1995) found little effect on percentage colonisation, while Yesmin et al. (1996b) found a significant decrease in ectomycorrhizal colonisation. Yesmin et al. (1996b) also found a decrease of field mycorrhizal associations along a gradient of increasing nitrogen deposition in the Scotland. In a very comprehensive review, that cites many studies, Wallend & Kottke (1998) concluded the most prominent N effect was on the sporocarps i.e. production above-ground fruit bodies. Generalist species i.e. those forming a symbiosis with >1 tree species are less affected than specialist species, especially those in symbiosis with conifers. Loss of Cortinarius and Russulas has been reported. There are optimum soil water N concentrations for the formation of extra-radical mycelium.

Numbers of mycorrhizal tips seem to be less sensitive indicators although numbers of short roots typical of those colonised by fungi can be drastically reduced by N fertilisation. (Ahlstrom et al 1988). Assimilation of N by the fungal partner significantly increases its carbon demand. Ek (1997) reported 54-180% increases in Paxillus involutus on birch in response to inorganic N additions.

Additional Comments:

Mycorrhizal fungi are an important part of the soil system, forming associations with plant roots to improve nutrient uptake. There are many species of mycorrhizal fungi contributing to soil biodiversity. Although at present the functional importance of this biodiversity for ecosystem function is not transparent we do know that fungal populations change successionally as trees age and litter quality changes (Mason et al) (Cairney and Meharg, 1999). Many of the fruit bodies, sporocarps observable in the field are the reproductive structures of mycorrhizal fungi. Reduction in mycorrhizal vitality may therefore be most apparent in loss of above ground diversity of fungal fruiting bodies. Cairney and Meharg (1999) discuss the work of Heijne et al. (1994) who found that in the specific case of additional NH₃ deposition, arbuscular mycorrhizal associations increased, while ectomycorrhizal colonisation is decreased (Termorschuizen 1993).