Soil enzymes. Enzymes in any of the states of occurrence in soils (Ladd et al., 1996).
Numerous processes in soil depend on enzymatic catalysis, including organic matter decomposition, gas emission and sink, pollutants breakdown, energy flow, and nutrient cycling in ecosystem.
Enzymes in soils are associated with different soil constituents. Intracellular enzymes are present in proliferating microorganisms (primarily bacteria and fungi), plant roots, and soil microfauna, or attached to dead cells and cell debris. Numerous extracellular enzymes usually operate at a distance from the parent cell; the capacity to produce and secrete biocatalysts is of survival importance for soil microorganisms and plants. Enzymes in the liquid phase (extracellular as well as released from lysed cells) have generally a short half-life due to rapid denaturation and degradation. However, when complexed with clay minerals or humic colloids, enzymes retain their catalytic properties for a long time, usually at the cost of some loss of the activity. Thus, enzymes may persist and may be active in soil in which no proliferation takes place (accumulated or abiontic enzymes). While some enzymes (e.g., polysaccharidases) as a rule are extracellular, other are considered to act only intracellularly, hereby reflecting the actual activity of the soil biota (e.g., dehydrogenases) (Burns, 1978; Nannipieri et al., 2002; Skujins, 1967).
Many enzymes are measured in soils, predominantly of the classes of hydrolases and oxidoreductases. The assays are generally simple, sensitive, and relatively rapid. The standard methods determine an overall enzyme activity in soil, and combined with molecular techniques can elucidate many aspects of microbial activity in soil (Bastida et al., 2008).
Soil enzymes are sensitive to variations induced by environmental factors and anthropogenic disturbances, are strongly affected by physical conditions, soil type, and vegetation, and respond to changes in soil management more quickly than other soil variables. Numerous studies confirm that soil enzymes correlate with other indices of fertility and biological activity and are useful in making predictions about the soil environment. There is growing interest in the applications of soil enzymes as the early and sensitive indicators of soil ecological stress and soil quality change in agroecosystem and natural or contaminated soils.