The majority of plants have mycorrhizal fungi associated with them. Mycorrhizal fungi are ecologically significant because they form relationships in and on the roots of a host plant in a symbiotic association. The host plant provides the fungus with soluble carbon sources, and the fungus provides the host plant with an increased capacity to absorb water and nutrients from the soil. Adverse conditions are a pervasive feature in both natural and agronomic soils. The soil environment is constantly changing with regard to moisture, temperature and nutrient availability. In addition, soil properties are often manipulated to improve crop yields. In many cases, soils may be contaminated through disposal of chemicals that are toxic to plants and microorganisms. The formation and function of mycorrhizal relationships are affected by edaphic conditions such as soil composition, moisture, temperature, pH, cation exchange capacity, and also by anthropogenic stressors including soil compaction, metals and pesticides. Arbuscular mycorrhizal fungi are of interest for their reported roles in alleviation of diverse soil-associated plant stressors, including those induced by metals and polychlorinated aliphatic and phenolic pollutants. Much mycorrhizal research has investigated the impact of extremes in water, temperature, pH and inorganic nutrient availability on mycorrhizal formation and nutrient acquisition. Evaluation of the efficacy of plant–mycorrhizal associations to remediate soils contaminated with toxic materials deserves increased attention. Before the full potential benefits of arbuscular mycorrhizal fungi to reclaim contaminated soils can be realized, research advances are needed to improve our understanding of the physiology of mycorrhizae subjected to adverse physical and chemical conditions. This paper will review literature and discuss the implications of soil contamination on formation and function of arbuscular mycorrhizal associations.

Current agricultural practices are responsible for the deposition of unwanted quantities of pesticides into streams throughout the United States. Previous studies in riparian areas have shown that microbial communities are capable of degrading herbicides faster in forest soils than in pasture soils. This study tested the influence of age of riparian forests on herbicide degradation in the soil. Active and total fungal and bacterial biomasses and mineralization of atrazine (2 chloro-4 [ethylamino] -6 [isopropylamino]-S-triazine) and 2,4-D (2,4-dichlorophenoxy-acetic acid) in the litter and top 10 cm of mineral soil were measured in forests 20-40, 60-90, and 120-300 years old on three riparian sites in autumn, winter, spring, and summer. Active and total fungal and active and total bacterial biomasses did not differ in the top 10 cm of mineral soil regardless of forest age. In litter, total bacterial biomass also did not differ with forest age throughout the year, but in spring and autumn, active bacterial biomass was greater in old-growth than in second- or young-growth litter samples, as were both active and total fungal biomasses. Atrazine mineralization in litter did not differ with forest age or season, but in mineral soil it was greater in old-growth than in second-or young-growth forest samples in all seasons. Mineralization of 2,4-D in litter was greater in old- and second-growth forest samples than in young-growth samples, except in winter; and in mineral-soil, it was greater in old-growth samples than in those from both younger forest types. Results indicate that microbial communities in old-growth riparian areas have a greater capacity to degrade herbicides than do such communities in second-or young-growth forests. Management of riparian forest to long rotations may tend to increase herbicide degradation and protection of lakes and streams.

Compounds produced by photosynthetic plants were shown to support the growth of PCB-degrading bacteria, and the organisms retained their ability to metabolize PCBs. These results indicate that the rhizosphere zone surrounding the roots of some plant species may selectively foster the growth of PCB-degrading microbes. Thus, introduction of a carefully selected plant species at PCB-contaminated sites has promise as a new means of enhancing and maintaining microbial degradation of PCBs.

The concentration of lignin in plant tissue is a major factor controlling organic matter degradation rates in forest ecosystems. Microbial biomass and lignin and cellulose decomposition were measured for six weeks in forest soil microcosms in order to determine the influence of pH, moisture, and temperature on organic matter decomposition. Microbial biomass was determined by chloroform fumigation; lignin and cellulose decomposition were measured radiometrically. The experiment was designed as a Latin square with soils of pH of 4.5, 5.5, and 6.5 adjusted to 20, 40, or 60% moisture content, and incubated at temperatures of 4, 12, or 24°C. Microbial biomass and lignin and cellulose decomposition were not significantly affected by soil acidity. Microbial biomass was greater at higher soil moisture contents. Lignin and cellulose decomposition significantly increased at higher soil temperatures and moisture contents. Soil moisture was more important in affecting microbial biomass than either soil temperature or soil pH.