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9.2.2.2 Soil–Biological Interactions
The biologically active portion of a soil profile is generally to the depth of plant
roots—which may be only a few centimeters for lawn grass or several meters for
prairie plants in the Midwestern USA. Climate and its temperature and precipitation
therefore influence soil formation because of both the associated weathering and the
vegetation types that will grow. Moderate conditions—not too cold or hot and not
too dry or wet—tend to create the most biologically productive soils.
Topographic position in the landscape, in which gravity works on both materials
and water to collect in low areas, may result in concentrated areas of organic materials that have fallen or been transported to those areas as well as soil that is saturated
by water. The presence of water slows the rate at which those organic materials
decompose both because of the lower temperatures typically present in water and
also because of the relatively anaerobic saturated conditions which result in less
efficient decomposition processes. In warmer climates biological activity is
increased, increasing the rate at which organic material decomposes; hence the
lower organic content of tropical soils.
Microorganisms that occupy the soil such as bacteria play an essential role not
only in decomposition of matter into its component parts, making nutrients available to other organisms, but also in symbiotic interactions with plants and other
microorganisms such as fungi to enhance nutrient utilization, increase carbon storage and other functions just becoming known to science.
Anthropogenic processes alter the way in which natural processes function, by
changing soil structure and reducing soil organic matter. Under moderate climatic
conditions with natural grassland or forest ecosystem processes at work, it may take
500 years to generate a 2.5-cm thick layer of topsoil rich in organic matter and nutrients. An unvegetated cultivated field during a heavy rain—because it lacks the plant
roots and natural pedolithic structure that would otherwise stabilize and retain the
soil—can lose that much soil in a couple of hours.
9.2.2.3 The Land–Water Interface
Water comes in contact with soil not only from precipitation and the global water
cycle but also from surface water such as rivers, streams, and lakes, and groundwater that emerges naturally from springs and via human pumping. The underlying
geology influences both groundwater and surface water chemistry, which may
supply helpful nutrients such as calcium, magnesium, and iron, and also elements
toxic to many organisms at high concentrations such as arsenic and sulfur.
Because groundwater may emerge as natural springs in surface water bodies,
both its chemistry and temperature influence the organisms that can occupy those
ecosystems. In particular, aquatic organisms such as certain insect and fish species
that require higher dissolved oxygen content to survive may only be found in rivers
fed by cold water springs because cold water is capable of holding more oxygen
than warm water. The presence of particular benthic macroinvertebrate species is
9 Ecosystems and Ecosystem Services
9.2.2.2 Soil–Biological Interactions
The biologically active portion of a soil profile is generally to the depth of plant
roots—which may be only a few centimeters for lawn grass or several meters for
prairie plants in the Midwestern USA. Climate and its temperature and precipitation
therefore influence soil formation because of both the associated weathering and the
vegetation types that will grow. Moderate conditions—not too cold or hot and not
too dry or wet—tend to create the most biologically productive soils.
Topographic position in the landscape, in which gravity works on both materials
and water to collect in low areas, may result in concentrated areas of organic materials that have fallen or been transported to those areas as well as soil that is saturated
by water. The presence of water slows the rate at which those organic materials
decompose both because of the lower temperatures typically present in water and
also because of the relatively anaerobic saturated conditions which result in less
efficient decomposition processes. In warmer climates biological activity is
increased, increasing the rate at which organic material decomposes; hence the
lower organic content of tropical soils.
Microorganisms that occupy the soil such as bacteria play an essential role not
only in decomposition of matter into its component parts, making nutrients available to other organisms, but also in symbiotic interactions with plants and other
microorganisms such as fungi to enhance nutrient utilization, increase carbon storage and other functions just becoming known to science.
Anthropogenic processes alter the way in which natural processes function, by
changing soil structure and reducing soil organic matter. Under moderate climatic
conditions with natural grassland or forest ecosystem processes at work, it may take
500 years to generate a 2.5-cm thick layer of topsoil rich in organic matter and nutrients. An unvegetated cultivated field during a heavy rain—because it lacks the plant
roots and natural pedolithic structure that would otherwise stabilize and retain the
soil—can lose that much soil in a couple of hours.
9.2.2.3 The Land–Water Interface
Water comes in contact with soil not only from precipitation and the global water
cycle but also from surface water such as rivers, streams, and lakes, and groundwater that emerges naturally from springs and via human pumping. The underlying
geology influences both groundwater and surface water chemistry, which may
supply helpful nutrients such as calcium, magnesium, and iron, and also elements
toxic to many organisms at high concentrations such as arsenic and sulfur.
Because groundwater may emerge as natural springs in surface water bodies,
both its chemistry and temperature influence the organisms that can occupy those
ecosystems. In particular, aquatic organisms such as certain insect and fish species
that require higher dissolved oxygen content to survive may only be found in rivers
fed by cold water springs because cold water is capable of holding more oxygen
than warm water. The presence of particular benthic macroinvertebrate species is
9 Ecosystems and Ecosystem Services