104
S. A. El.-M. M. Abd El-Azeem
to around 30 °C, while increased soil temperature over this degree is suitable for
Clostridium than Azotobacter [46]. Most of the studies reported that soil microorganisms have a different response to elevated temperature or global changes. For
instance, the populations of bacteria (Gram-positive or -negative bacteria) increased
with increased temperature, may be due to declining the availability of substrates.
However, fungi and actinobacteria biomarkers decreased at higher temperatures [47].
It has been indicated that the nitrification activities in the subtropical Egyptian clay
loam soil increased with temperature and showed the maximum rate at 30 °C. The
optimum temperature for the potential activation and population size of ammonium
oxidizing or nitrite oxidizing bacteria, nitrate reducing bacteria and most probable
numbers is 25 or 30 °C and exhibited apparent activation energies between 61 and
202 kJ mol
−1 . However, a few nitrifiers and denitrifiers were also able to grow at 8
or 50 °C [48].
3.2 Impact of Carbon Dioxide
The Egyptian Central Agency for Public Mobilization and Statistics mentioned that
the amount of carbon dioxide (CO 2 ) emissions from the consumption of oil products
and gas in Egypt is 197.1 million tons in the year 2014 and increased to 299 million
tons in the year 2015 [49]. The CO 2 is the most abundant and important well-mixed
greenhouse gases contribute to human-induced climate change and responsible for
74% of global warming over the past decade [50]. Increasing atmospheric CO 2
concentration and simultaneous rises in temperature are influencing the global climate, henceforth affecting growth, development and functioning of plants [51]. The
response of soil microbial activity to changes in atmospheric CO 2 concentrations
is mainly indirectly and can be positive or negative, and consistent overall trends
between sites and studies have not been observed. Moreover, the primary effects of
the increased concentration of atmospheric CO 2 include increased belowground C
allocation (root biomass and root respiration) to the root zone, changed the composition of root exudates, altered C/N ratio and impact on soil nutrients status [52].
For example, Soussana and Hartwig [53] found that increased the concentration of
atmospheric CO 2 led to an increase the C:N ratio of plant residues and exudates,
resulting in temporary N-immobilization and reduce the availability of soil nitrogen.
Soils exposed to elevated CO 2 had higher relative abundances of fungi and higher
activities of a soil carbon degrading enzyme that led to more rapid rates of soil organic
matter degradation than soils exposed to ambient CO 2 . The isotopic composition of
microbial fatty acids confirmed that elevated CO 2 increased microbial utilization
of soil organic matter. These results illustration how elevated CO 2 through altering
soil microbial communities, can cause a potential carbon sink to become a carbon
source [54]. Soil enzymes activity, community structure and microbial total biomass
also have shown highly varied responses to elevated CO 2 . For instance, Niklaus [55]
investigates in situ effects of elevated CO 2 on soil microbial biomass and activity
in a nutrient-poor calcareous soil exposed to long-term CO 2 enrichment. He found
S. A. El.-M. M. Abd El-Azeem
to around 30 °C, while increased soil temperature over this degree is suitable for
Clostridium than Azotobacter [46]. Most of the studies reported that soil microorganisms have a different response to elevated temperature or global changes. For
instance, the populations of bacteria (Gram-positive or -negative bacteria) increased
with increased temperature, may be due to declining the availability of substrates.
However, fungi and actinobacteria biomarkers decreased at higher temperatures [47].
It has been indicated that the nitrification activities in the subtropical Egyptian clay
loam soil increased with temperature and showed the maximum rate at 30 °C. The
optimum temperature for the potential activation and population size of ammonium
oxidizing or nitrite oxidizing bacteria, nitrate reducing bacteria and most probable
numbers is 25 or 30 °C and exhibited apparent activation energies between 61 and
202 kJ mol
−1 . However, a few nitrifiers and denitrifiers were also able to grow at 8
or 50 °C [48].
3.2 Impact of Carbon Dioxide
The Egyptian Central Agency for Public Mobilization and Statistics mentioned that
the amount of carbon dioxide (CO 2 ) emissions from the consumption of oil products
and gas in Egypt is 197.1 million tons in the year 2014 and increased to 299 million
tons in the year 2015 [49]. The CO 2 is the most abundant and important well-mixed
greenhouse gases contribute to human-induced climate change and responsible for
74% of global warming over the past decade [50]. Increasing atmospheric CO 2
concentration and simultaneous rises in temperature are influencing the global climate, henceforth affecting growth, development and functioning of plants [51]. The
response of soil microbial activity to changes in atmospheric CO 2 concentrations
is mainly indirectly and can be positive or negative, and consistent overall trends
between sites and studies have not been observed. Moreover, the primary effects of
the increased concentration of atmospheric CO 2 include increased belowground C
allocation (root biomass and root respiration) to the root zone, changed the composition of root exudates, altered C/N ratio and impact on soil nutrients status [52].
For example, Soussana and Hartwig [53] found that increased the concentration of
atmospheric CO 2 led to an increase the C:N ratio of plant residues and exudates,
resulting in temporary N-immobilization and reduce the availability of soil nitrogen.
Soils exposed to elevated CO 2 had higher relative abundances of fungi and higher
activities of a soil carbon degrading enzyme that led to more rapid rates of soil organic
matter degradation than soils exposed to ambient CO 2 . The isotopic composition of
microbial fatty acids confirmed that elevated CO 2 increased microbial utilization
of soil organic matter. These results illustration how elevated CO 2 through altering
soil microbial communities, can cause a potential carbon sink to become a carbon
source [54]. Soil enzymes activity, community structure and microbial total biomass
also have shown highly varied responses to elevated CO 2 . For instance, Niklaus [55]
investigates in situ effects of elevated CO 2 on soil microbial biomass and activity
in a nutrient-poor calcareous soil exposed to long-term CO 2 enrichment. He found