soil enzymes

2. ASSIGNMENT -2
SOIL ENZYMES, CLASSIFICATION AND IMPORTANCE
INTRODUCTION
Soil enzymes are the key players in biochemical process of organic matter recycling
in the soil system and their activ- ities are closely related to soil organic matter , soil
physical properties, and microbial activity and/or biomass. Depending on their
location, enzymes can be extracellular or intracellular. Intracellular enzymes are
found in cell’s cytoplasm or bound to the cell walls of living and metabol- ically
active cells, viable but non-proliferating cells (such as spores) and dead cells.
Extracellular enzymes released into the soil and are “permanently” immobilized on
clay and humic colloids via ionic interactions, covalent bonds, hydrogen bonding,
entrapment, and other mechanisms.
Soil enzymes are necessary catalysts for decomposition of SOM and nutrient cycling
and, strongly influence energy transformation, environmental quality, and agro-
nomic productivity. However, mechanical tillage, mono- culture, and residues
removal adversely impact enzymatic activity and availability of plant nutrients. In
general, enzymatic activity decreases with an increase in soil depth. Further, soil
enzymes provide early detection of changes in soil health because they respond to
soil management changes and environmental factors much
sooner than other soil quality parameters. Moreover, avail- ability of well-
documented assays for a large number of soil enzyme activities makes them the
preferred tool for asses- sing soil health. However, it is necessary to understand the
relationship between different enzyme pools and biotic and abiotic factors to predict
the potential impact of soil man- agement and environmental changes on ecosystem
func- tions and productivity

3. There are 2 types of enzymes:
Constitutive
Always present in nearly constant amounts in a cell (not
affected by addition of any particular substrate…genes always
expressed.) (pyro-phosphatase).
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.

4. Enzyme Classification Oxidoreductases –
Oxidation reduction reaction (Dehydrogenase, Catalase,
Peroxidase)
Transferases – The transfer of group of atoms
from donor to an acceptor molecule. (Aminotransferases,
Rhodonese)
Hydrolases – Hydrolytic cleavage of bonds.
(Phosphatase, Cellulose, Urease)
Lysates – Cleavage of bonds other than
hydrolysis or oxidation.
Isomerases – Isomerisation reaction.
Ligases – Formation of bonds by the cleavage
of ATP. (Acetyl-CoA carboxylase)

5. 4
Major soil enzymes and their functions.
Enzyme Source Reaction catalyzed End product
Soil function
indicated
Factors influencing
enzyme activity
α-Amylase Starch hydrolysis C-cycling Management practices,
type of vegetation,
environment, and soil
types.
Plants, animals, and
microorganisms
Mainly plants
Microorganisms
Glucose and/or
oligosaccharides
Maltose
Transfer of H to
NAD or NADP
(electron
transport
system)
β-Amylase
Dehydrogenase
Starch hydrolysis
Oxidation of organic
compounds
C-cycling,
microbial
oxidative
activity
Soil water content,
temperature, pesticides,
trace elements,
management practices,
pollution, etc.
Endo-1, 4-β-
glucanase
Exo-1, 4-β-
glucanase
β-glucosidase
Microorganisms,
protozoa, and
termites
Cellulose endohydrolysis Oligosaccharides C-cycling Temperature, pH, water,
O2 contents, quality and
location of organic matter,
mineral elements, and
fungicides.
Cellulose cleavage at ends Glucose and
cellobiose
Glucose (sugar)
Cellobiose hydrolysis
Phenol oxidase Plants and
microorganisms
Urease Microorganisms,
plants, and some
invertebrates
Lignin hydrolysis C compounds
(humic
substances)
Urea hydrolysis Ammonia
(NH3) and CO2
C-cycling Soil pH, mean annual
precipitation and
temperature, SOM content,
management practices, N
enrichment, etc.
N-cycling Cropping history, organic
matter content, soil depth,
management practices,
heavy metals, temperature,
pH, etc.
Organic matter content,
pH, management
practices, pollution, crop
species, and varieties.
Alkaline
phosphatase
Acid
phosphatase
Arylsulfatase
Mainly bacteria Hydrolysis of esters and
anhydrides of phosphoric
acid
Phosphate
(PO4)
P-cycling
Plants, fungi, and
bacteria
Microorganisms,
plants, and animals
Protease Microorganisms and
plants
Plants and
microorganisms
Hydrolysis of sulfate esters Sulfate (SO−2) S-cycling Heavy metal pollution,
pH, organic matter
content and composition,
and availability of organic
sulfate esters
N mineralization Plant available N N-cycling Humic acid concentration,
availability of C and N,
etc.
Chitinase Degradation and hydrolysis
of chitin
Carbohydrate
s and
inorganic
nitrogen
C- and
N-
cycling
Availability of N, soil
depth, atmospheric
CO2 levels, etc.

6. Response of soil enzyme activity under different
ecosystems/practices.
Enzymes of
C-cycling
Enzymes
Total
organic
Ecosystem/Practice Phosphatase Dehydrogenase N-cycling carbon References
Forest vs. pasture vs.
agricultural
Highest
(β-glucosidase)
in
forest, lowest in
agricultural soil
— Highest
in forest
Kizilkaya
and
Dengiz[20]
Highest (alkaline
phosphatase) in
forest, lowest in
agricultural soil
High
Conservational vs.
conventional tillage
Organic residue with RDF
(maize residue in rice and
wheat cultivation)
Organic vs. unamanded (in
bell pepper)
High
(β-glucosidase)
High (invertase)
High High
Highest (urease) in
pasture lowest in
agricultural soil
High urease and
protease
High urease and —
protease
Roldan
et al.[21]
Tao
et al.[17]
High alkaline
phosphatase
High
High
(β-
glucosidase)
High acid
phosphatas
e
High High urease High Gopinath
et al.[22]
Zhang et
al.[23]
Rehabilitated (stabilized soils
(over a 50 yr period) on
moving sand dunes
High (α- and
β-
glucosidase)
High High High protease High
Degraded vs. native vegetation Low (cellulose) — Low — Low Araújo
et al.[24]
Mined (coal mine soil vs.
forest soil)
— — Low — Low Kumar
et al.[25]
Polluted — Low acid and alkaline
phosphatase
—

7. QUANTITATIVE ASSAY OF ENZYMATIC ACTIVITY
1. The overall stoichiometry of the reaction catalysed.
2.Whether the enzyme requires the addition of
cofactors such as metal ions or coenzymes.
1. Its dependence on substrate and cofactor
concentrations.
1. 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 the appearance of
product.
Usually measure enzyme activity at substrate
concentrations
above saturation level, where the reaction rate is at a maximum

8. State of Enzymes in Soil
1. Role of Clay
2. Most activity associated with clays.
3. Increases resistance to proteolysis and microbial attack
4. Increases the temperature of inactivation
5. Role of Organic Matter
6. Humus material provides stability to soil nitrogen compounds
7. Enzymes attached to insoluble organic matrices exhibit pH and temperature
changes.
8. Inability to purify soil enzymes free of
9. Role of Clay and Organic Matter Complexes
10.Lignin + bentonite (clay) protect enzymes against proteolytic attack, but not
bentonite alone.
11.Enzymes are bound to organic matter, which is then bound to clay.