ABSTRACT
In warmer, moist environments, in which nutrient cycling occurs at a more rapid pace, the major forms of nutrients in soil are in the inorganic phase in soil water. Arbuscular mycorrhizae dominate under these conditions in temperate grasslands and in tropical forests and grasslands. In these ecosystems of herbaceous-dominated plant communities, decomposition is rapid and organic matter rapidly becomes incorporated into the soil mineral matrix. The nutrient supply for plants is mainly through inorganic nutrients, mineralized by saprotrophic activity on the high resource quality plant residues. Phosphorus tends to be a limiting nutrient in these systems (Read, 1991b), and the arbuscular mycorrhizal associations of these plants appears to confer a greater efficiency in effecting plant acquisition of mineral nutrients (Hetrick, 1989). Jeffries and Barea (1994) reviewed the role of arbuscular mycorrhizal fungi in biogeochemical cycling and the maintenance of sustainable plant-soil interactions. Arbuscular mycorrhizae are of particular importance in agriculture (Gianinazzi and Schüepp, 1994), but discussion of these ecosystems is out of the scope of this book, except when specific principles relative to natural ecosystems are discussed. Jeffries and Barea (1994) discuss the influence of arbuscular mycorrhizae on biogeochemical cycling and sustainability by improving plant nutrition, preventing root pathogens, and improving soil structure by binding soil particles together with mycelia. As a consequence of the relatively high availability of inorganic to organic sources of nutrient in these soils, these mycorrhizal types have only
limited enzyme expression. It has, however, been shown that they are capable of producing phosphatase enzymes to solubilize poorly available phosphates in soil (Azcón et al., 1976; Singh and Kapoor, 1998). Indeed, Jayachandran et al. (1992) recorded the ability of nonmycorrhizal big bluestem grass (Andropogon gerardii) to access phosphorus from glycerophosphate and adenosine monophosphate, but not from phytic acid, RNA, ATP, or CMP (cytidine 2′-and 3′ monophosphate). In the presence of the arbuscular mycorrhizal fungus Glomus etunicatum plants were able at access all forms of organic phosphorus, and uptake into the plant was 500-to 600-fold higher in the mycorrhizal plants than in the nonmycorrhizal plants (Fig. 3.12). Bolan (1991) suggests that the arbuscular
mycorrhizal benefit for phosphate uptake into plants is due to three factors: (1) exploitation of a larger soil volume, (2) faster movement of phosphate into the root via fungal hyphae, and (3) the ability to solubilize complex inorganic forms of phosphate. He suggests that mycorrhizal fungi may help to overcome the three rate-limiting steps of phosphate uptake by increasing the rate of diffusion into plant roots, the phosphate
concentration at the root surface, and the rate of phosphate dissociation from the surface of soil particles (Fig. 3.13). The ability of arbuscular mycorrhizae to solubilize phosphate may be an important factor in permitting plants to grow in calcareous soils in which phosphate is limited because of complexing with heavy metal ions. Tyler (1994) shows that the inability of the calcifuge plant species Carex pilulifera, Deschampsia flexuosa, Holcus mollis, Luzula pilosa, Nardus stricta, and Veronica officinalis to grow on limestone is because of their inability to decouple the iron-phosphate complexes to derive both elements essential to their growth. Calcicole species, however, appear to have developed mechanisms of acquiring both P and Fe from these soils by the production of organic acids in the rhizosphere (Ström, 1997; Lee, 1999) (Table 3.14). Part of this ability may be linked to the arbuscular mycorrhizal
