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

Abstract

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 utilize 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 feedbacks. 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. A review fo current experimental evidence shows that although alternative N cycling routes may be important for plant nutrition, nutrient cycling, and ecosystem functioning, the many questions which are still open do not allow any firm conclusions to be drawn yet. Thus, one of the major challenges for mycorrhizal ecology in the coming years is to investigate this conceptual model and test its relevance under field conditions. This is a complicated task, because it requres detailed knowledge of the identity of various mycorrhizal taxa (especially the arbuscular mycorrhizal fungi), their specific function in plant nutrition, and knowledge of the dynamics of organic compouonds in the soil and their actual uptake by mycorrhizal plants. due ot recent advances in molecular ecology and in stable isotope applications for ecology, it should now be possible to unravel this mechanism at field scale under realistic conditions. Confirmation of our conceptual model would prove that mycorrhizal fungi play a curical role in the belowground controls on aboveground ecosystem functioning.