Acid deposition :: Mycorrhizal fungi

Impact Type: Deposition of pollutant

Key Concerns:

Acidification appears to exert different effects dependoing on the fungal taxa and the site (Cainey & Meharg 1999). Mycorrhizal fungi can protect their hosts from heavy metal toxicity i.e. mycorrhizal infection mitigates some of the toxic effects of acid deposition allowing their hosts to be more tolerant of acid conditions. The negative action of acid deposition on mycorrhizal is not fully understood.

Acid deposition has often been reported to have negative effects on the diversity of mycorrhizal fungi. Vosatka and Dodd (1998) found effects of simulated acid rain at all stages of development in three arbuscular mycorrhizal fungi species, including spore germination, colonisation of host roots, growth of the mycelium and below ground competition with plant roots. They found varying sensitivity of the different species, with the most robust species appearing to survive in acid damaged soils, by maintaining association with Deschampsia flexuosa (wavy hair grass), the cover of which increased because of the thinner tree canopy.

There is strong circumstantial evidence for a depressive effect of acid and/or ammonium deposition upon the extent of ericoid mycorrhizal infection of Calluna roots (Yesmin et al. 1996a, b). However, it is currently not clear to what extent this is actually damaging to mycorrhizal fungi in the field.

The relative contribution of different effects of acid deposition leading to reductions in mycorrhizal fungi is likely to vary with situation. Vosatka et al. (1999) found that simulated acid rain, combined with increased Aluminium concentrations (as often occur in acidified soils), negatively influenced the physiology of three arbuscular mycorrhizal fungi species. Such effects are not just important for the mycorrhizas, since other studies have shown that mycorrhizas tend to protect plant roots from high concentrations of toxic metals in soils (Dueck et al. 1986, Galli et al. 1994). Brunner (2001) reports that the fungal hyphae contain vacular polyphosphates that bind Al (and other heavy metals) protecting roots from the toxic effects of Al3+.

Effects on the fruiting bodies of ectomycorrhizal fungi in association with Spruce have also been observed in response to acid mist (Deans et al. 1994). The exposure was designed to simulate the impacts of cloud deposition which occurs frequently on high altitude sites in Scotland. In a field simulation study fruit bodies of corrinarius species (Ingleby pers comm) were fewer in the acid treated plots (NH₄NO₃ + H₂SO₄pH2.5).

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 on the roles of mycorrhizal fungi is not fully clear (Cairney and Meharg 1999), many of the fruiting bodies observed 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.

Shafer et al. (1996) found major interactions between simulated acid rain, arbuscular mycorrhizal fungi and soil rhizobium fungi (associated with clover), which led them to conclude that it is difficult to provide quantitative conclusions of acid rain effects in model systems, because most studies have not considered such interactions.

Van Breemen et al. (2000) report on the potential for mychorrizal weathering of minerals, and note that this may be a mechanism whereby mycorrhizae tend to protect the roots of trees from the effects of acid deposition.