Soil Enzymes: Importance, State and Kinds

After reading this article you will learn about:- 1. Introduction to Soil Enzymes 2. Importance of Soil Enzymes 3. State 4. Kinds 5. Classification 6. Myrosinase Activity 7. Quantitative Assay of Enzymatic Activity.

Introduction to Soil Enzymes:

Soil enzymes increase the reaction rate at which plant residues decompose and release plant available nutrients. The substance acted upon by a soil enzyme is called the substrate. For example, glucosidase (soil enzyme) cleaves glucose from glucoside (substrate), a compound common in plants. Enzymes are specific to a substrate and have active sites that bind with the substrate to form a temporary complex.

The enzymatic reaction releases a product, which can be a nutrient contained in the substrate. Sources of soil enzymes include living and dead microbes, plant roots and residues, and soil animals. Enzymes stabilized in the soil matrix accumulate or form complexes with organic matter (humus), clay and humus-clay complexes, but are no longer associated with viable cells.

It is thought that 40 to 60% of enzyme activity can come from stabilized enzymes, so activity does not necessarily correlate highly with microbial biomass or respiration. Therefore, enzyme activity is the cumulative effect of long term microbial activity and activity of the viable population at sampling.

However, an example of an enzyme that only reflects activity of viable cells is dehydrogenase, which in theory can only occur in viable cells and not in stabilized soil complexes.

Importance of Soil Enzymes:

Enzymes respond to soil management changes long before other soil quality indicator changes are detectable. Soil enzymes play an important role in organic matter decomposition and nutrient cycling (see Table 18.7). Some enzymes only facilitate the breakdown of organic matter (e.g. hydrolase, glucosidase), while others are involved in nutrient mineralization (e.g. amidase, urease, phosphatase, sulfates).

With the exception of phosphatase activity, there is no strong evidence that directly relates enzyme activity to nutrient availability or crop production. The relationship may be indirect considering nutrient mineralization to plant available forms is accomplished with the contribution of enzyme activity.

1. Release of nutrients in soil by means of organic matter degradation

2. Identification of soils.

3. Identification of microbial activity.

4. Importance of soil enzymes as sensitive indicators of ecological change.

Specific problems that might be caused by poor function:

Absence or suppression of soil enzymes prevents or reduces processes that can affect plant nutrition. Poor enzyme activity (e.g. pesticide degrading enzymes) can result in an accumulation of chemicals that are harmful to the environment; some of these chemicals may further inhibit soil enzyme activity.

What to do:

Organic amendment applications, crop rotation, and cover crops have been found to enhance enzyme activity. The positive effect of pasture is associated with the input of animal manure and less soil disturbance. Agricultural methods that modify soil pH (e.g. liming) can also change enzyme activity.

i. Distributed or dissolved in soil solution.

ii. Adsorbed on soil clay surface.

iii. Adsorbed on soil organic polymers (humus).

iv. Partition in soil organic polymers (humus).

v. Polymerized to soil organic polymer (humus).

vi. Distributed in interlayers of clays.

vii. Distributed in cytoplasm of living organisms.

viii. Distributed in the periplasmic slime of living organisms.

ix. Distributed in residues of organisms (plant and animals).

State of Enzymes in Soil:

Role of Clays:

(a) Most activity associated with clays.

(b) Increases resistance to proteolysis and microbial attack.

(c) Increases the temperature of inactivation.

Role of Organic Matter:

(a) Humus material provides stability to soil nitrogen compounds.

(b) Enzymes attached to insoluble organic matrices exhibit pH and temperature changes.

(c) Inability to purify soil enzymes free of soil organic matter (bound to O.M.)

Role of O.M.—Clay Complex:

(a) Lignin + bentonite (clay) protect enzymes against proteolitic attack, but not bentonite alone.

(b) Enzymes are bound to organic matter which is then bound to clay.

Kinds of Enzymes:

Constitutive:

Always present in nearly constant amounts in a cell (not affected by addition of any particular substrate…genes always expressed) (pyrophosphatase).

Inducible:

Present only in trace amounts or not at all, but quickly increases in concentration when its substrate is present. (Amidase).

Both enzymes are present in the soil.

Enzyme Classification:

1. Oxidoreductases—Oxidation reduction reaction (Dehydrogenase, Catalase, Peroxidase).

2. Transferases—The transfer of group of atoms from donor to an acceptor molecule. (Aminotransferases, Rhodonese).

3. Hydrolases—Hydrolitic cleavage of bonds. (Phosphatase, Cellulase, Urease).

4. Lyases—Cleavage of bonds other than hydrolysis or oxidation. (Aldolase).

5. Isomerases—Isomarization reaction.

6. Ligases—Formation of bonds by the cleavage of ATP. (Acetyl—CoA carboxylase)

Myrosinase Activity in Soil:

Sinigrin + myrosinase → glucose + SO₄^2- + iso-thiocyanates

Quantitative Assay of Enzymatic Activity:

The following points to be considered:

1. The overall stoichiometry of the reaction catalysed.

2. Whether the enzyme requires the addition of cofactors such as metal ions or coenzymes.

3. Its dependence on substrate and cofactor concentrations.

4. Its optimum pH.

5. A temperature zone in which it is stable and has high activity.

6. A simple analytical procedure to measure the disappearance of substrate or appearance of product.

A model for binding urease to hydrophobic HDTMA smectite. The dark sites of the enzymes are hydrophobic areas.