Mutualisms are interspecific relationships beneficial to both organisms involved. While these relationships have been described as mathematically unstable, a diverse array of cross-kingdom partnerships has existed throughout evolutionary history. Soil mutualists have great impact on above- and belowground community dynamics across a wide range of ecosystems. Organisms in soil collaborate with a wide variety of plants to perform nutrient acquisition services in exchange for plant-derived carbohydrates. While the relationships were originally perceived as bacteria in symbiotic relationships for N acquisition and fungi involved with P acquisition, more recent studies have indicated that fungi are actually involved in the acquisition of almost any limiting nutrient in soil, depending on partnering species (Allen, 1991; Smith and Read, 1997).

Mycorrhiza, the relationship between a plant root and fungus, is one of the most important soil mutualisms. It may be one of the oldest relationships participated in by plants (Brundrett, 2002; Stubblefield and Taylor, 1988). There is evidence that this relationship evolved and was lost multiple times in different divisions in the Kingdom Fungi and in different groups of plants. Mycorrhizal fungi have the ability to acquire nutrients directly from decomposing litter (Leake and Read, 1997) and from live animals such as springtails (Klironomos and Hart, 2001). They can also influence plant-water relations (Allen, 1991) and reduce attack on roots by pathogenic fungi (Gange et al., 1994; Azcon-Aguilar and Barea, 1992). Each of these relationships alters the aboveground community directly by changing rates of reproduction and death of participant species and indirectly by altering competition among plant species.

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