forms [12]. During field application, the ammonia contained in inorganic fertilizers
and animal manures is rapidly hydrolysed to the ammonium (NH 4
+
) ion, ammonium
compounds or nitrate (NO 3
−
). This serves as a direct pathway for crop nutrient
uptake, while organic amendments and SOM breakdown require depolymerization
by extracellular enzymes and microbes to enhance N availability [21].
5 Soil Characteristics Influencing Enzymatic Activity
Changes in soil’s physio-chemical characteristics, mostly as a result of land-use
change, vegetation type and microbial status of the soil, produce a strong effect on
enzymes, proving their high sensitivity to such parameters. Each soil enzyme has a
specific range of pH for optimum activity at which enzymes are more stable. On the
contrary, as pH deviates from optimal values, enzyme activity is reduced until
becoming inactive at extremely high or low pH values where irreversible denaturation occurs. Changes in the soil concentration of H
+ ions (protons) have a strong
influence on enzyme dynamics, influencing substrate degradation and acting as a
co-factor in nutrients ionization and solubility properties [22]. Although this is
generally true, the influence of pH in enzymatic activity is enzyme-specific and the
degree of pH sensitivity is variable. For example, optimum pH for b-glucosidase is
6.0 while for urease it is 7.0 [23].
Drying and rewetting cycles in soil also affect enzyme activity to variable
degrees. For example, b-glucosidase activity is reduced in dry soils showing that
lower soil moisture content greatly reduces the activity of extracellular enzymes
produced by microbes associated with the breakdown of SOM. The release of
glucose during SOM decomposition influences the growth of soil microorganisms
since glucose is the preferred carbon source for many of them [12]. b-glucosidase
activity serves as one of the best predictors for evaluating the effect of crop management practices on soil health and soil quality changes. Dry conditions promote
the adsorption of enzymes onto mineral surfaces where their activity is reduced as
well as their access to SOM substrate. Besides reducing b-glucosidase activity, this
can also reduce other enzymes rate of degradation and therefore their contribution to
SOM breakdown following the next rainfall [19]. As soils dry, solutes in the water
are concentrated, leading to increasingly negative osmotic potential between the
inside and outside of microbes. As a response to this stress, many microbes will
accumulate electrolytes and organic solutes to balance osmotic and matric potentials,
which in turn may slow down the activity of enzymes within the organisms [24].
Generally, enzyme activity increases with increasing temperature, doubling the
reaction rate about every 10 °C. However, beyond the enzyme-specific threshold
enzymatic activity decreases drastically and becomes inactivated at high temperature. Moreover, the temperature sensitivity of the different enzymes, and thus the
dynamics of nitrogen and carbon mineralization, are not the same and may be
influenced by climate change in different ways [25]. As for pH, the sensitivity of
enzymes to temperature varies with enzyme type and source.
Nitrogen Footprints and the Role …
141
and animal manures is rapidly hydrolysed to the ammonium (NH 4
+
) ion, ammonium
compounds or nitrate (NO 3
−
). This serves as a direct pathway for crop nutrient
uptake, while organic amendments and SOM breakdown require depolymerization
by extracellular enzymes and microbes to enhance N availability [21].
5 Soil Characteristics Influencing Enzymatic Activity
Changes in soil’s physio-chemical characteristics, mostly as a result of land-use
change, vegetation type and microbial status of the soil, produce a strong effect on
enzymes, proving their high sensitivity to such parameters. Each soil enzyme has a
specific range of pH for optimum activity at which enzymes are more stable. On the
contrary, as pH deviates from optimal values, enzyme activity is reduced until
becoming inactive at extremely high or low pH values where irreversible denaturation occurs. Changes in the soil concentration of H
+ ions (protons) have a strong
influence on enzyme dynamics, influencing substrate degradation and acting as a
co-factor in nutrients ionization and solubility properties [22]. Although this is
generally true, the influence of pH in enzymatic activity is enzyme-specific and the
degree of pH sensitivity is variable. For example, optimum pH for b-glucosidase is
6.0 while for urease it is 7.0 [23].
Drying and rewetting cycles in soil also affect enzyme activity to variable
degrees. For example, b-glucosidase activity is reduced in dry soils showing that
lower soil moisture content greatly reduces the activity of extracellular enzymes
produced by microbes associated with the breakdown of SOM. The release of
glucose during SOM decomposition influences the growth of soil microorganisms
since glucose is the preferred carbon source for many of them [12]. b-glucosidase
activity serves as one of the best predictors for evaluating the effect of crop management practices on soil health and soil quality changes. Dry conditions promote
the adsorption of enzymes onto mineral surfaces where their activity is reduced as
well as their access to SOM substrate. Besides reducing b-glucosidase activity, this
can also reduce other enzymes rate of degradation and therefore their contribution to
SOM breakdown following the next rainfall [19]. As soils dry, solutes in the water
are concentrated, leading to increasingly negative osmotic potential between the
inside and outside of microbes. As a response to this stress, many microbes will
accumulate electrolytes and organic solutes to balance osmotic and matric potentials,
which in turn may slow down the activity of enzymes within the organisms [24].
Generally, enzyme activity increases with increasing temperature, doubling the
reaction rate about every 10 °C. However, beyond the enzyme-specific threshold
enzymatic activity decreases drastically and becomes inactivated at high temperature. Moreover, the temperature sensitivity of the different enzymes, and thus the
dynamics of nitrogen and carbon mineralization, are not the same and may be
influenced by climate change in different ways [25]. As for pH, the sensitivity of
enzymes to temperature varies with enzyme type and source.
Nitrogen Footprints and the Role …
141