The Role of Various Types of Mycorrhizal Fungi in Nutrient Cycling and Plant Competition

More than 90% of terrestrial plant species associate with mycorrhizal fungi. These fungi play an important role in the mineral nutrition of plants. This chapter focuses on the ability of various mycorrhiza-types: arbuscular mycorrhizal fungi (AMF), ecto-mycorrhizal fungi (EMF) and ericoid mycorrhizal fungi to utilise different soil nutrient sources (EMF and ericoid mycorrhizal fungi: organic N; AMF: inorganic P) and the consequences of this ability for plant competition and plant-soil feed-backs. A conceptual model is presented which shows that this differentiation in the use of various inorganic and organic nutrient sources may create positive feed-backs between plant species dominance, litter chemistry, litter decomposition, and the dominant mycorrhiza type. However, the mycorrhizal impact, especially of ericoid mycorrhizal fungi and ecto-mycorrhizal fungi, on this triad of plant nutrition, plant competition, and ecosystem functioning can be strongly reduced under high levels of atmospheric nitrogen input. As increased N input leads to a relative P shortage in ecosystems, it can be expected that arbuscular mycorrhizal fungal species become progressively more dominant. However, the effects of increased N supply on AMF colonisation are very variable. Studies show that positive, negative or no effects are found in almost equal proportions.