Soils as Driver and Victim of Climate Change in Egypt
133
Global warming may not have a direct effect on the ecological composition
because soil fauna and flora have a relatively broad range of optimum temperature. However, changes in ecosystems and migration of vegetation zones are likely
in some areas as a result of increased temperature and changes in rainfall. Some soil
flora and fauna may be seriously affected by such changes because their migration
rates are likely to be too small [122]. Warming by 5 °C in a temperate forest, for
example, altered the relative abundance of soil bacteria and increased the bacterial
to the fungal ratio of the community [123]. Further, drought amplifies the differential temperature sensitivity of fungal and bacterial groups [124]. Even with small
changes in soil moisture availability (<30% reduction in water holding capacity), soil
fungal communities may shift from one dominant member to another while bacterial
communities remain constant. The activity of some microorganisms increased in the
warmer soil, resulting in faster degradation of carbohydrates and other labile components. However, the soil fungi numbers and activity may increase in the warmer
soil, with a corresponding rise in abundance of lignin-derived compounds (reflecting
decomposition by the fungi) [125]. SOM loss from 5 to 2% over a 60-year period at
Rothamsted resulted in a 90% decrease in microbial biomass, but no significant effect
on microbial diversity [126] or substrate utilization [127]. Smaller fungal biomass
[128] and fungal-to-bacterial biomass ratios [129] have been found in soils of low
SOM content compared with undisturbed and botanically rich grassland soils in the
UK. A further significant impact of climate change on soil fauna and flora is through
enhanced CO 2 levels in the atmosphere leading to enhanced plant growth and in
turn, increased allocation of carbon below ground. The microbial population and its
activity under this regime would lead to higher rates of nitrogen fixation, nitrogen
immobilisation and denitrification, increased mycorrhizal associations, increased soil
aggregation and increased weathering of minerals [122].
4.1.4 Soil Compaction and Crusting
Soil structure is an important property which indicates how the soil particles
combine. Soil structure is responsible for the movement of gases, water, pollutants/contaminants, seepage, nutrients, maintenance of water quality, building foundations, soil fauna and the emergence of crops. The nature and quality of the structure
are strongly influenced by the amount and quality of organic matter present. A decline
in soil organic matter levels lead to a decrease in soil aggregate stability, infiltration
rates and increase in susceptibility to compaction, run-off furthermore susceptibility
to erosion (similar to the observations of [113]). In some areas, there could be an
increase in flash flooding as a result of increased cracking and change in structure.
(a) Increase Soil Compaction
Soil organic matter is undoubtedly the most important soil component as it improves
soil quality through the influences in soil structure, water holding capacity, soil
stability, and oxygen-holding capacity. Soil organic matter is highly susceptible to
changes in land use and management, soil temperature and moisture. In the last
133
Global warming may not have a direct effect on the ecological composition
because soil fauna and flora have a relatively broad range of optimum temperature. However, changes in ecosystems and migration of vegetation zones are likely
in some areas as a result of increased temperature and changes in rainfall. Some soil
flora and fauna may be seriously affected by such changes because their migration
rates are likely to be too small [122]. Warming by 5 °C in a temperate forest, for
example, altered the relative abundance of soil bacteria and increased the bacterial
to the fungal ratio of the community [123]. Further, drought amplifies the differential temperature sensitivity of fungal and bacterial groups [124]. Even with small
changes in soil moisture availability (<30% reduction in water holding capacity), soil
fungal communities may shift from one dominant member to another while bacterial
communities remain constant. The activity of some microorganisms increased in the
warmer soil, resulting in faster degradation of carbohydrates and other labile components. However, the soil fungi numbers and activity may increase in the warmer
soil, with a corresponding rise in abundance of lignin-derived compounds (reflecting
decomposition by the fungi) [125]. SOM loss from 5 to 2% over a 60-year period at
Rothamsted resulted in a 90% decrease in microbial biomass, but no significant effect
on microbial diversity [126] or substrate utilization [127]. Smaller fungal biomass
[128] and fungal-to-bacterial biomass ratios [129] have been found in soils of low
SOM content compared with undisturbed and botanically rich grassland soils in the
UK. A further significant impact of climate change on soil fauna and flora is through
enhanced CO 2 levels in the atmosphere leading to enhanced plant growth and in
turn, increased allocation of carbon below ground. The microbial population and its
activity under this regime would lead to higher rates of nitrogen fixation, nitrogen
immobilisation and denitrification, increased mycorrhizal associations, increased soil
aggregation and increased weathering of minerals [122].
4.1.4 Soil Compaction and Crusting
Soil structure is an important property which indicates how the soil particles
combine. Soil structure is responsible for the movement of gases, water, pollutants/contaminants, seepage, nutrients, maintenance of water quality, building foundations, soil fauna and the emergence of crops. The nature and quality of the structure
are strongly influenced by the amount and quality of organic matter present. A decline
in soil organic matter levels lead to a decrease in soil aggregate stability, infiltration
rates and increase in susceptibility to compaction, run-off furthermore susceptibility
to erosion (similar to the observations of [113]). In some areas, there could be an
increase in flash flooding as a result of increased cracking and change in structure.
(a) Increase Soil Compaction
Soil organic matter is undoubtedly the most important soil component as it improves
soil quality through the influences in soil structure, water holding capacity, soil
stability, and oxygen-holding capacity. Soil organic matter is highly susceptible to
changes in land use and management, soil temperature and moisture. In the last