Nh

D-Alanine

Lysozyme-sensitive bond

H Meso-diaminopimelic acid NH

FIGURE 12.14 The structure of one of the repeating units of peptidoglycan found in gramnegative bacteria.

molecules joined through phosphoid-ester linkages. Most contain D-alanine. More than 100 different types of peptidoglycans have been identified.

soil organic matter

The study of SOM began two centuries ago because of the recognition that it influences soil quality and plant growth. SOM contributes directly to plant and microbial growth through its effects on the chemical, physical, and biological properties of soil. Soil organic matter is the primary source of the essential nutrients N, P, and S. The association of SOM with secondary minerals such as clay and amorphous oxides to form organomineral complexes creates soil structure through the formation of aggregates. Aggregate formation enhances soil physical structure by organizing soil mineral grains and promoting aeration and water infiltration and storage. Good soil structure promotes plant growth and creates a labyrinth of habitats for microbial activity. The formation and decay of SOM is an essential process regulating atmospheric trace gases, particularly the production of CO2 and nitrous oxide and consumption of methane. Another important consequence of the interactions of SOM with minerals is the creation of hydrophilic and hydro-phobic domains that affect the adsorption of pesticides and other toxic compounds. Environmental toxicologists often refer to these domains as rubbery and glassy C, respectively, to explain the degree of adsorption and sorption of various chemical compounds in soil.

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