Soil comprises mineral particles of different sizes, shapes and chemical characteristics, together with the soil BIOTA and organic compounds in various stages of decomposition. The formation of clay–organic
matter complexes and the stabilization of clay, sand and silt particles through the formation of aggregates are the dominant structural characteristics of the soil matrix. Soil-matrix-component aggregates range from approximately 2 mm or more (macroa ggregates) to fractions of a micrometer for bacteria and colloidal particles (for models see REF. 2). Prokaryotes are the most abundant organisms in soil and can form the largest component of the soil biomass7. Soil micro- organisms often strongly adhere or adsorb onto soil particles such as sand grains or clay–organic matter complexes. Microhabitats for soil microo rganisms include the surfaces of the soil aggregates, and the complex pore spaces between and inside the aggre- gates7,8. Some pore spaces are inaccessible for micro- organisms owing to size restrictions. The metabolism and the survival of soil microorganisms are strongly influenced by the availability of water and nutrients. In contrast to aquatic habitats, surfaces of soil envi- ronments undergo dramatic cyclic changes in water content, ranging from water saturation to extreme aridity. A fraction of the microbial community dies during each drying-and-wetting cycle9. As a conse- quence, the composition of soil microbial communi- ties fluctuates. However, how microbial populations are altered depending on changes in the water content and other environmental factors such as pH, avail- ability of oxygen or temperature has not been studied intensively.