106
S. A. El.-M. M. Abd El-Azeem
3.3 Impact of Soil Moisture Content
Water is a critical factor in global change. Egypt has been suffering from severe water
scarcity in recent years, and according to the United Nations classification, Egypt
falls into the category of high-water stress countries. The status of soil water can
be described as soil water content and potential. Soil water content indicates how
much water is present in soil (water balance), whereas soil water potential relates
to the energy level by which the water is held in the soil (water movement). Thus,
soil matric potential may be considered an important measure of water availability, whereas soil moisture content may appear to be a more useful parameter when
water availability is not limiting for microbial activity [62]. Soil moisture content
is changed based on time, space, a form of wet-dry cycles, flooding and drought
[52, 14]. The general interaction between soil moisture and oxygen (O 2 ) concentrations is intuitively clear for predominate aerobic or anaerobic conditions due to
regulating O 2 diffusion. Therefore, the relationship between soil moisture and soil
microbial activity is more greatly variable and complicated than that of temperature.
For instance, high soil moisture content leads to decrease the diffusion of O 2 in water
in which the activity of aerobic microbes is declined, but these conditions led to an
increase the activity of anaerobic microbes [14]. The maximum of aerobic microbial
activity occurred at moisture levels between 50 and 70% of water-holding capacity.
At high moisture content (matric potential > −0.01 MPa) decreases rates of organic
matter decomposition, due to a low oxygen supply, while low soil moisture decreases
microbial activity by reducing diffusion of soluble substrates, microbial mobility and
intracellular water potential [19].
Soil microbial activity is sensitive to changes in soil moisture, but the mechanisms suppressing microbial activity under dry conditions continue to be debated.
Soil microbes are substrate limited because solute diffusivity is halted due to the
breaking of water film continuity under dry conditions. Therefore, the rates of soil
microbial respiration were declined with decreasing soil moisture content to the point
that the effective soil microbial activity is halt. This “water stress” moisture threshold
corresponds to water potential −15 MPa in disturbed soil samples, regardless of the
climate of origin of the soil and its texture [13]. It has been recently indicated that
the precipitation affecting microbial communities and vary with moisture content.
Precipitation led to a decrease in soil bacterial diversity in the Mediterranean region
compared with arid sites. Using phospholipid fatty acids, real-time quantitative PCR
and terminal restriction fragment length polymorphism fingerprint analyses, Bachar
et al. [63] indicated that although soil bacterial abundance decreases with precipitation, bacterial diversity is independent of precipitation gradient. Moreover, the
composition of the bacterial community was found to be distinct to each site along
the rainfall gradient. Specifically, in Egyptian agricultural soil, the effect of soil water
content on the degradation of cellulose has not been well investigated, and few studies have been conducted on the cellulolytic activity of soil fungi. However, Badran
and Abdel-Rahiem [64] found that the highest population of cellulose-decomposing
S. A. El.-M. M. Abd El-Azeem
3.3 Impact of Soil Moisture Content
Water is a critical factor in global change. Egypt has been suffering from severe water
scarcity in recent years, and according to the United Nations classification, Egypt
falls into the category of high-water stress countries. The status of soil water can
be described as soil water content and potential. Soil water content indicates how
much water is present in soil (water balance), whereas soil water potential relates
to the energy level by which the water is held in the soil (water movement). Thus,
soil matric potential may be considered an important measure of water availability, whereas soil moisture content may appear to be a more useful parameter when
water availability is not limiting for microbial activity [62]. Soil moisture content
is changed based on time, space, a form of wet-dry cycles, flooding and drought
[52, 14]. The general interaction between soil moisture and oxygen (O 2 ) concentrations is intuitively clear for predominate aerobic or anaerobic conditions due to
regulating O 2 diffusion. Therefore, the relationship between soil moisture and soil
microbial activity is more greatly variable and complicated than that of temperature.
For instance, high soil moisture content leads to decrease the diffusion of O 2 in water
in which the activity of aerobic microbes is declined, but these conditions led to an
increase the activity of anaerobic microbes [14]. The maximum of aerobic microbial
activity occurred at moisture levels between 50 and 70% of water-holding capacity.
At high moisture content (matric potential > −0.01 MPa) decreases rates of organic
matter decomposition, due to a low oxygen supply, while low soil moisture decreases
microbial activity by reducing diffusion of soluble substrates, microbial mobility and
intracellular water potential [19].
Soil microbial activity is sensitive to changes in soil moisture, but the mechanisms suppressing microbial activity under dry conditions continue to be debated.
Soil microbes are substrate limited because solute diffusivity is halted due to the
breaking of water film continuity under dry conditions. Therefore, the rates of soil
microbial respiration were declined with decreasing soil moisture content to the point
that the effective soil microbial activity is halt. This “water stress” moisture threshold
corresponds to water potential −15 MPa in disturbed soil samples, regardless of the
climate of origin of the soil and its texture [13]. It has been recently indicated that
the precipitation affecting microbial communities and vary with moisture content.
Precipitation led to a decrease in soil bacterial diversity in the Mediterranean region
compared with arid sites. Using phospholipid fatty acids, real-time quantitative PCR
and terminal restriction fragment length polymorphism fingerprint analyses, Bachar
et al. [63] indicated that although soil bacterial abundance decreases with precipitation, bacterial diversity is independent of precipitation gradient. Moreover, the
composition of the bacterial community was found to be distinct to each site along
the rainfall gradient. Specifically, in Egyptian agricultural soil, the effect of soil water
content on the degradation of cellulose has not been well investigated, and few studies have been conducted on the cellulolytic activity of soil fungi. However, Badran
and Abdel-Rahiem [64] found that the highest population of cellulose-decomposing