Linking enzyme activity to gene expression in soil is a challenging task. Detection of an enzyme activity in soil only indicates the presence of a protein capable of performing a certain biochemical reaction regardless of its origin. The enzyme maybe intra- or extracellular, and newly synthesised or be part of the soil enzyme pool absorbed to the soil matrix composed of clay minerals, organic matter and humic substances (see Chaps. 4 and 5). Enzymes can be of microbial, plant or animal origin. Even if it is possible to determine the microbial origin of an enzyme, there are usually many microbes capable of producing the same enzyme. In addition, there are oftenmultiplegenescodingforenzymesabletocatalysethesameorsimilar reactions, leading to some functional redundancy, increasing the capacity of the producing organism to adapt and deal with a variety of environmental conditions (Naessens and Vandamme 2003).

The first challenge is the extraction of protein from soil, either as a crude or relatively purified enzyme extract. Several procedures for extraction of enzymes from soil have been described and differ in composition of the extraction buffer, pH and temperature; methods usually involve some kind of mechanical treatment for a variable period of time if intracellular enzymes are to be assayed. Once extracted, different purification processes can be applied. Tabatabai and Fu (1992) and later Nannipieri et al. (1996) summarised many different approaches to the isolation and purification of enzymes from soil, and in this volume the matter is discussed in Chaps. 4 and 5.

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