We have considered a wide range of subjects, including the biochemical turnover of specific elements and organic components in soil; the properties and function of soil enzymes; and the structure, distribution, and pathways of metabolism of organic compounds, occurring either naturally in soil or indtroduced as a result of human activities. Topics exemplifying each of these aspects of soil biochemistry are reviewed in this volume. Their selection was justified either because relevant established information was scattered and no recent comprehensive overview of the subject was available, or because new discoveries were accruing rapidly from investigations, which often had been initiated after consideration of environmental impact issues, or because recent advances in methodology have permitted new and exciting approaches to be made to obtain a yet more detailed understanding of factors influencing biochemical activities in the soil microenvironment. We have made no attempt to achieve a single theme for this book but have anticipated that the subject matter of various chapters would be linked closely. Numerous examples occur. Thus the turnover of the very resistant and labile organic pools in soils are discussed in separate reviews. Similarly, polyphenol oxidases and related enzymes, long considered important in the formation of resistant humic compounds, also are involved in the transformation in soil of some pesticides and hydrocarbons. Heavy metals, inhibitory in high concentrations to microbial growth and activities, are essential participants in the mechanism of action of some enzymes, e.g., Mo in N2 fixation and NO3® reduction. The enzyme urease has been demonstrated to contain two atoms of Ni per molecule and the metal may participate in the catalysis. In classical biochemistry, urease has received less attention than may other enzymes despite its historical preeminence as the first enzyme to be crystallized. By contrast, in soil biochemistry, urease has been studied far more closely than any other enzyme, partly because of its major involvement in urea fertilizer losses. Thus practical reasons alone would justify studies of the structure(s) and properties of soil urease in order to devise suitable inhibitors to control rates of urea hydrolysis in soils. Previous volumes in this series have reviewed factors which influence the biochemical activities of the soil microflora and microfauna. A feature of the present volume is a discussion of new methodologies, first, for measuring the soil biomass, which in the future will lead to more detailed and precise determinations of nutrient and energy flow through this important soil component, and second, for establishing the location in soils, in situ, of live and dead organisms and many of their products, including active intra- and extracellular enzymes.
