Redwood Reader

A good basic understanding of the soil and its inhabitants is available at BLM's Soil Biological Communities. This is the page about mycorrhiza, links there take you to bacteria, nematodes and so forth. Most plants are VA mycorhizzal, most forest trees ectomycorhizzal, also producers of soil glue. They are the majority of the commercial mushroom industry, and of course, responding to elevated CO2.

Mycorrhizal Fungi
What They Are and A Few Interesting Facts
http://www.blm.gov/nstc/soil/index.html
Mycorrhizal fungi colonize the roots of many plants. Mycorrhizal fungi don’t harm the plant; on the contrary, they develop a "symbiotic" relationship that helps the plant be more efficient at obtaining nutrients and water. In return, the plant provides energy to the fungus in the form of sugars.
Here’s how that symbiotic relationship works. The fungus is actually a network of filaments that grow in and around the plant root cells, forming a mass that extends considerably beyond the plant’s root system. This essentially extends the plant’s reach to water and nutrients, allowing it to utilize more of the soil’s resources.
There are two main categories of mycorrhizae common to western rangelands in the United States.
Vesicular-arbuscular mycorrhizae or VAM. VAM is a type of endomycorrhizae (endo = inside), and is the most widespread of the mycorrhizae. These fungi actually reside inside the cells of the plant root. They’re typically found associated with most grasses, forbs, shrubs, and a few trees such as juniper. They are generalists, have only a few species, and are slow to disperse.
Ectomycorrhizae (ecto = outside) grow around the root and between the root cells, but unlike VAM, the fungus doesn’t actually penetrate the root cells. The fungus also forms a considerable mass in the soil surrounding the plant roots. The fruiting, or reproductive bodies, of these fungi are sometimes visible as something we all recognize— mushrooms! Ectomycorrhizae are commonly associated with forest trees of temperate regions. On rangelands, they are found in riparian areas (the places next to water), open woodlands, and shrub oak communities. They are host-specific, have many species, and can disperse far and quickly.
Not all fungi are mycorrhizal. There are also fungi that help decompose the organic matter in litter and soil. However, they play a lesser role than bacteria in this important process in semi-arid and arid rangeland soils.
Why They Are Important
Some plants are "mycorrhizal-obligate," meaning that they can’t survive to maturity without their fungal associate. Important mycorrhizal-obligate plants in western North America are sagebrush, bitterbrush, and some native bunchgrasses.
Mycorrhizae are particularly important in assisting the host plant with the uptake of phosphorus and nitrogen, two nutrients vital to plant growth.
Mycorrhizae actually increase the surface area associated with the plant root, which allows the plant to reach nutrients and water that might not be available otherwise. Put simply, mycorrhizae extends the plant’s reach, allowing it to get to more of what it needs to survive. That makes the plant stronger, especially during drought periods. Stronger individuals means that the community is more resilient to disturbance. Some mycorrhizae may even protect their host plant against unwanted pathogens.
Learn More!

Allen, M.F. 1991. The ecology of mycorrhizae. Cambridge Univ. Press. New York. 184 pp.

Ingham, Elaine R. 1998. The soil biology primer, soil fungi. USDA, Natural Resources Conservation Service, Soil Quality Institute.

Read, D.J. 1991. Mycorrhiza in ecosystems. Experientia 47: 376-391.

Wicklow-Howard, M. 1994. Mycorrhizal ecology of shrub-steppe habitat in Proceedings-Ecology and Management of Annual Rangelands. USDA, Forest Service. Inter-mountain Research Station. General Technical Report INT-GTR-313. pp. 207-210.

Wicklow-Howard, M. 1998. The role of mycorrhizal fungi in rangelands. pp. 23-25 in Rosentreter, R. and A. DeBolt, editors. The Ellen Trueblood Symposium. Technical Bulletin No. 98-1, Bureau of Land Management, Boise, Idaho.