108
S. A. El.-M. M. Abd El-Azeem
soil microbial counts and their activities on natural or synthetic saline soils. In recent
decades, increased soil salinity has been a critical problem in the agricultural soils of
Egypt due to increasing the salinization of soils and ground waters [70]. Increasing
soil salinity led to an increase in soil osmotic potential in water and nutrient uptake
by plants, poor aeration and a decrease in water permeability. These conditions
may affect soil microbial activities such as soil enzyme activities, reduce microbial
community structure and biochemical functioning in soil [15]. Also, increased soil
salinity has a negative impact on the sustainability of beneficial soil microorganisms
associated with plant rhizosphere. These negative impact can be attributed to adverse
effects of toxic salts and lose soil aggregates and soil structure that leading unsuitable
habitat or niches for soil microbial growth [71].
Soil enzymes play an essential role in soil biochemical transformation and nutrients cycles that related to soil fertility and quality. They play a fundamental role
in the decomposition of organic residues, the formation of organic matter and in
intercellular metabolic reactions in the soil. Amongst the several soil enzymes, βglucosidase, urease, invertase, phosphatases, arylsulfatase and dehydrogenase are
necessary for nutrient transformations for different plants. Under controlled conditions, soil salinity had a negative effect on soil enzyme activities, but the degree of
inhibition varied according to soil microbial community, the enzyme analyzed, and
the nature and the amount of soil added [72]. The inhibition of enzyme activity in
saline soils can be due to the osmotic hydration of the microbial cell that releases
intracellular enzymes that become vulnerable to the attack by soil proteases. The
salting-out effect modifies the ionic conformation of the protein-enzyme active site,
and specific ionic toxicity causes a nutritional imbalance for microbial growth and
subsequent enzyme synthesis [72, 15]. Some researchers in Egypt studied the effect
of salinity on soil enzyme activities using clayey soil under laboratory conditions.
The authors showed that the activity of urease and invertase significantly decreased
with an increasing concentration of NaCl during the incubation period. Additionally,
they also observed that the effect of salinity on the activity of nitrate reductase was
mainly inhibitory in the majority of the treatments [15].
Other researchers, for instance Wong et al. [73] also indicated that the effect
of soil salinity on soil microbial biomass and soil respiration. For example, soil
microbial biomass was highest (459–565 mg kg
−1 soil) in the high salinity treatments (30 dS m
−1 ) and lowest (158–172 mg kg
−1 soil) in the low salinity treatments
(0.5 dS m
−1 ). On the other hand, soil respiration rate was highest (56–80 mg CO 2 -C
kg
−1 soil) in the low salinity treatments and lowest (1–5 mg CO 2 -C kg
−1 soil) in the
mid salinity treatments (0.5 dS m
−1 ) [73].
Several investigators reported that fungi are more sensitive to osmotic stress than
bacteria or actinobacteria [72, 74]. There is a significant decrease in the total count
of fungi and most general fungi and species in some Egyptian soils that salinized
with different concentration of sodium chloride (5–20% NaCl). Similarly, the total
count of bacteria and actinobacteria was remarkably decreased with an increase in
soil salinity level to above 5% NaCl [15]. In fungi, increased soil salinity decreases
spore germination and the growth of hyphae and changes the morphology, resulting
in the formation of spores with thick walls [72].
S. A. El.-M. M. Abd El-Azeem
soil microbial counts and their activities on natural or synthetic saline soils. In recent
decades, increased soil salinity has been a critical problem in the agricultural soils of
Egypt due to increasing the salinization of soils and ground waters [70]. Increasing
soil salinity led to an increase in soil osmotic potential in water and nutrient uptake
by plants, poor aeration and a decrease in water permeability. These conditions
may affect soil microbial activities such as soil enzyme activities, reduce microbial
community structure and biochemical functioning in soil [15]. Also, increased soil
salinity has a negative impact on the sustainability of beneficial soil microorganisms
associated with plant rhizosphere. These negative impact can be attributed to adverse
effects of toxic salts and lose soil aggregates and soil structure that leading unsuitable
habitat or niches for soil microbial growth [71].
Soil enzymes play an essential role in soil biochemical transformation and nutrients cycles that related to soil fertility and quality. They play a fundamental role
in the decomposition of organic residues, the formation of organic matter and in
intercellular metabolic reactions in the soil. Amongst the several soil enzymes, βglucosidase, urease, invertase, phosphatases, arylsulfatase and dehydrogenase are
necessary for nutrient transformations for different plants. Under controlled conditions, soil salinity had a negative effect on soil enzyme activities, but the degree of
inhibition varied according to soil microbial community, the enzyme analyzed, and
the nature and the amount of soil added [72]. The inhibition of enzyme activity in
saline soils can be due to the osmotic hydration of the microbial cell that releases
intracellular enzymes that become vulnerable to the attack by soil proteases. The
salting-out effect modifies the ionic conformation of the protein-enzyme active site,
and specific ionic toxicity causes a nutritional imbalance for microbial growth and
subsequent enzyme synthesis [72, 15]. Some researchers in Egypt studied the effect
of salinity on soil enzyme activities using clayey soil under laboratory conditions.
The authors showed that the activity of urease and invertase significantly decreased
with an increasing concentration of NaCl during the incubation period. Additionally,
they also observed that the effect of salinity on the activity of nitrate reductase was
mainly inhibitory in the majority of the treatments [15].
Other researchers, for instance Wong et al. [73] also indicated that the effect
of soil salinity on soil microbial biomass and soil respiration. For example, soil
microbial biomass was highest (459–565 mg kg
−1 soil) in the high salinity treatments (30 dS m
−1 ) and lowest (158–172 mg kg
−1 soil) in the low salinity treatments
(0.5 dS m
−1 ). On the other hand, soil respiration rate was highest (56–80 mg CO 2 -C
kg
−1 soil) in the low salinity treatments and lowest (1–5 mg CO 2 -C kg
−1 soil) in the
mid salinity treatments (0.5 dS m
−1 ) [73].
Several investigators reported that fungi are more sensitive to osmotic stress than
bacteria or actinobacteria [72, 74]. There is a significant decrease in the total count
of fungi and most general fungi and species in some Egyptian soils that salinized
with different concentration of sodium chloride (5–20% NaCl). Similarly, the total
count of bacteria and actinobacteria was remarkably decreased with an increase in
soil salinity level to above 5% NaCl [15]. In fungi, increased soil salinity decreases
spore germination and the growth of hyphae and changes the morphology, resulting
in the formation of spores with thick walls [72].