ABSTRACT

Soil enzymes are either actively secreted by living microorganisms and plant roots or released after the death of soil biota by cell lysis. One class of enzymes, the hydrolases, have a very important role in the biogeochemical cycles of major elements (C, N, P, and S) since their substrates in soil are mainly in a polymerized form. Usually microorganisms (and plant roots) cannot take up macromolecules directly from the external medium. There are few exceptions to this, such as the uptake of fragments of deoxyribonucleic acid (DNA) or plasmids, that lead to transformation in bacteria. In general, the membrane transport systems are specific and recognize the universal biological monomers, such as amino acids, sugars, and nucleotides; low number oligomers, such as cellobiose or maltose; or mineral ionic groups that can be released by enzymatic hydrolysis of organic molecules, such as orthophosphate and sulfate. Extracellular enzymes perform three main functions in soil: (1) they reach substrates in pores with dimensions roughly 100 times smaller than those of bacteria; (2) they hydrolyze these substrates and make them soluble and consequently able to diffuse back to the microorganisms or plant roots; and at the same time (3) they transform polymers into monomers or oligomers that can be recognized and taken up by membrane transport systems to undergo intracellular metabolism.