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FIGURE 3.16 Tissue phosphorus concentrations of blueball leaves (in March) and flowers and seeds at the end of the growing season for control plants (open bars) and plants treated with a soil application of benomyl (hatched bars) to reduce the arbusculr mycorrhizal colonization of the roots. Pairs of bars sharing the same letter are not significantly different from each other. Data from Merryweather and Fitter (1996).

Leaf Flower Seed

FIGURE 3.16 Tissue phosphorus concentrations of blueball leaves (in March) and flowers and seeds at the end of the growing season for control plants (open bars) and plants treated with a soil application of benomyl (hatched bars) to reduce the arbusculr mycorrhizal colonization of the roots. Pairs of bars sharing the same letter are not significantly different from each other. Data from Merryweather and Fitter (1996).

systems to optimize the nutrients available. Inorganic nutrients are frequently more available in arbuscular mycorrhizal-dominated environments. In this situation there does not seem to be the need for diversification of function, hence the low number of fungal taxa that have evolved the mycorrhizal habit. There may be more differences among fungal species within the arbuscular mycorrhizal community than initially appears evident, however. Dodd (1994) cites the work of Jakobsen et al. (1992a,b) that shows greater soil volume exploitation by the mycorrhizal fungus Acaulaospora laevis than Glomus sp. and thus the ability of the former species to obtain phosphorus from a greater distance from the root surface. The role of mycorrhizal diversity, particularly in ectomycorrhizae, will be discussed in the following section. It may be that in the arbuscular mycorrhizal symbiosis the host plant plays a more important role in determining the nature of the function of the mycorrhizal effect, where it may be growth, phosphorus content, or plant fitness, a subject that will be discussed in more depth in Chap. 5.

Faunal grazing on arbuscular mycorrhizal fungal extraradical hyphae has been shown to reduce the efficiency of the mycorrhizae in acquiring nutrients, particularly phosphorous, for the plant. Warnock et al. (1982) showed that there was a strong interaction between collembolan density and the growth of the host plant. The effect of this grazing is likely to be more important in agroecosystems, in which the diversity of soil fauna is reduced and high densities of collembola can occur in the absence of predators. In addition it has been shown that nematode feeding on mycorrhizal fungal hyphae can also reduce the effectiveness of the mycorrhizal association and has the effect of altering a plant's competitive fitness (Brussard et al., 2001).

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