substrates (e.g. those involved in oxidative degradation of toxic organic pollutants).
DHA mediates the transfer of hydrogen atoms from organic compounds, accompanied by energy generation. DHAs are present in all microorganisms and do not
exist as extracellular enzymes, being specific for different substrates. The DHA
activity in the soil reflects the total oxidative metabolic capacity of microbes, being
considered as a good indicator of soil biological activity [15]. DHA intervenes in
the oxidation of SOM by mediating proton and electron transfer from organic
matter to suitable inorganic acceptors. These enzymes are affected by soil type,
moisture content, and the redox state (aeration) in the soil [12].
The dominant class of extracellular enzymes found in the soils is the hydrolase
class [12]. These hydrolytic enzymes are involved in the breakdown of macromolecules to obtain smaller forms utilisable by plants and microbes but are also
responsible for mediating the removal of inorganic groups or ions to allow the
release of the inorganic available forms [12]. This class includes amidases, amylases, cellulases, glucosidases, phosphoesterases, sulfatases and ureases.
Lyase enzymes act without hydrolysis and mediate the removal of certain
chemical species breaking covalent bonds. These enzymes include ammonia lyases,
decarboxylases and dehydratases. Ammonia lyases are the most important enzymes
in this class as they deaminate amino acids. Enzymes representing other classes are
not so common in the soil environment and less important from the ecological point
of view.
Proteases are important enzymes that act in the mineralization of organic N in
soil playing a vital role in maintaining the ecosystem function and in ensuring N
nutrition for plant growth. Because it is an extracellular enzyme, it is linked to both
organic and inorganic colloidal substances in soil. Protease degrades protein to
release short peptides through hydrolyzation of the peptide bonds. If proteins are
further degraded, amino acids are released and act as N sources for the soil
microbes. Amino acids may be taken up by the microbes or may be further mineralized to release ammonia for plant N nutrition [16–18].
Urease is another enzyme affecting N balance in soils and can be found both in
extra and in intracellular forms in the microorganisms [16]. These enzymes are
released by almost all the soil microbial groups, including fungi, bacteria, algae,
yeast and even some plants roots, and have as main role to hydrolyse urea into
ammonium and carbon dioxide. N dynamics are influenced by urease as it increases
the ammoniacal nitrogen concentration in soils. If not taken up by the plants
ammonium release leads to soil pH increase after urea fertilization, and is also
volatilized as ammonia, hence negatively impacting air quality and reducing the
nitrogen use efficiency (NUE) of fertilizers. The use of polymer-coated urea reduces
the effect of soil urease, hence reducing N losses while increasing NUE (e.g. as now
widely practiced in India—Neem coating).
b-glucosidase activity, one of the most common enzymes in soils [19], plays a
key role in the breakdown of low molecular weight carbohydrates of SOM, which is
strongly related to the carbon cycle in soils and the products of its enzymatic
activity supplies energy to soil microorganisms. Since these enzymes are proteins
and therefore very sensitive to both anthropogenic and natural variable factors,
Nitrogen Footprints and the Role …
139
DHA mediates the transfer of hydrogen atoms from organic compounds, accompanied by energy generation. DHAs are present in all microorganisms and do not
exist as extracellular enzymes, being specific for different substrates. The DHA
activity in the soil reflects the total oxidative metabolic capacity of microbes, being
considered as a good indicator of soil biological activity [15]. DHA intervenes in
the oxidation of SOM by mediating proton and electron transfer from organic
matter to suitable inorganic acceptors. These enzymes are affected by soil type,
moisture content, and the redox state (aeration) in the soil [12].
The dominant class of extracellular enzymes found in the soils is the hydrolase
class [12]. These hydrolytic enzymes are involved in the breakdown of macromolecules to obtain smaller forms utilisable by plants and microbes but are also
responsible for mediating the removal of inorganic groups or ions to allow the
release of the inorganic available forms [12]. This class includes amidases, amylases, cellulases, glucosidases, phosphoesterases, sulfatases and ureases.
Lyase enzymes act without hydrolysis and mediate the removal of certain
chemical species breaking covalent bonds. These enzymes include ammonia lyases,
decarboxylases and dehydratases. Ammonia lyases are the most important enzymes
in this class as they deaminate amino acids. Enzymes representing other classes are
not so common in the soil environment and less important from the ecological point
of view.
Proteases are important enzymes that act in the mineralization of organic N in
soil playing a vital role in maintaining the ecosystem function and in ensuring N
nutrition for plant growth. Because it is an extracellular enzyme, it is linked to both
organic and inorganic colloidal substances in soil. Protease degrades protein to
release short peptides through hydrolyzation of the peptide bonds. If proteins are
further degraded, amino acids are released and act as N sources for the soil
microbes. Amino acids may be taken up by the microbes or may be further mineralized to release ammonia for plant N nutrition [16–18].
Urease is another enzyme affecting N balance in soils and can be found both in
extra and in intracellular forms in the microorganisms [16]. These enzymes are
released by almost all the soil microbial groups, including fungi, bacteria, algae,
yeast and even some plants roots, and have as main role to hydrolyse urea into
ammonium and carbon dioxide. N dynamics are influenced by urease as it increases
the ammoniacal nitrogen concentration in soils. If not taken up by the plants
ammonium release leads to soil pH increase after urea fertilization, and is also
volatilized as ammonia, hence negatively impacting air quality and reducing the
nitrogen use efficiency (NUE) of fertilizers. The use of polymer-coated urea reduces
the effect of soil urease, hence reducing N losses while increasing NUE (e.g. as now
widely practiced in India—Neem coating).
b-glucosidase activity, one of the most common enzymes in soils [19], plays a
key role in the breakdown of low molecular weight carbohydrates of SOM, which is
strongly related to the carbon cycle in soils and the products of its enzymatic
activity supplies energy to soil microorganisms. Since these enzymes are proteins
and therefore very sensitive to both anthropogenic and natural variable factors,
Nitrogen Footprints and the Role …
139