What Are Soils? -- Microbial Diversity in Soils -- Microorganisms and Soil Genesis -- Role of Microorganisms in Wear Down of Rocks and Minerals -- Humification and Mineralization in Soils -- Importance of Microorganisms for Soil Aggregation -- Microorganisms and Biogeochemical Processes in Soils -- Microbial Energetics in Soils -- Role of Microorganisms in Carbon Cycling in Soils -- Contribution of Bacteria to Initial Input and Cycling of Nitrogen in Soils -- Influence of Microorganisms on Phosphorus Bioavailability in Soils -- Biotic Interactions Involving Soil Microorganisms -- Interactions Between Mycorrhizal Fungi and Bacteria to Improve Plant Nutrient Cycling and Soil Structure -- Mycorrhizosphere: Strategies and Functions -- Interactions Between Microorganisms and Soil Micro- and Mesofauna -- Function of Microbes in Specific Soil Compartments -- Transgenic Rhizospheres of Crop Plants: Their Impact on Indigenous Soil Fungi -- Regulation of Microbial Activities in Functional Domains of Roots and Invertebrates -- Microorganisms of Biological Crusts on Soil Surfaces -- Microorganisms in Toxic Metal-Polluted Soils -- Techniques to Investigate Soil Microorganisms -- Marker Genes in Soil Microbiology -- Assessing Functions of Soil Microbes with Isotopic Measurements. Soils would not exist without the complex and heterogeneous activities of microorganisms. For the third volume of Soil Biology, an international board of renowned scientists shed light on the significant role of these organisms. The following key topics are covered: Microorganisms in bioerosion, humification, mineralization and soil aggregation; Microbial energetics and microbes in biogeochemical processes such as carbon and nitrogen cycles and phosphorus bio-availability; Interactions in the mycorrhizosphere, e.g. between mycorrhizal fungi and bacteria; Impact of microbes on plant nutrient cycling and the possible effects of transgenic rhizospheres on soil fungi; Functions of microbes in specific soil compartments such as soil surface or toxic metal polluted soils; Regulation of microbial activities in functional domains that are influenced by biotic or abiotic factors; Use of marker genes and isotopes as examples for modern techniques in soil microbiology.