Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 600

Water management of the soil is an important factor. Most organisms need water
to survive. However, water saturation in soil pores hampers gas exchange (oxygen
supply). This has a negative impact on the majority of the organisms in the soil,
which is aerobic and needs to exchange gas, that is, oxygen and carbon dioxide, with
the atmosphere. As a consequence, both soil structure and water management are
important factors for gas exchange processes and thereby for the proper performance
of the habitat function.
Tillage may improve the soil conditions for ecological activities, for example, by
stimulating oxygen and water supply. However, tillage also has a negative impact on
the soil ecosystem, for example, by physically damaging the tunnels of larger soil-
dwelling organisms such as earthworms and these organisms themselves. Sláviková
and Vadkertiová ( 2003 ), for example, showed that the number of yeasts (from the
fungi kingdom) was about ten times higher in forest soils than in tilled agricultural
soils. For this reason non-till systems are increasingly gaining in application.
In conclusion, a healthy soil that stimulates vitality has a good combination
of structure building possibilities, availability of organic material and other food
sources, gas exchange and water management.
13.5.5.3 Food Supply
Since organisms need food, one extremely important factor is the availability of
organic material. The total soil food web is based on the degradation of detritus,
dead organic material which serves as food for soil organisms. The easily degrad-
able organic fraction will be degraded by higher organisms and bacteria; the more
recalcitrant compounds, including the humus fraction, is slowly degraded, after an
initial attack by fungi that are able to convert such material to useable food for other
organisms. Therefore, both the organic matter content of soil and its quality are
important factors in soil vitality. Detritivores consume dead organic material, while
phytophages feed on living plant roots and plant material. Since predators feed on
other living organisms, they depend on the success of other organisms. Besides car-
bon, organisms need nutrients such as nitrogen, phosphate and sulphur which are
recycled in the element cycles, coupled with the degradation of organic material.
13.5.5.4 Sealing and Compaction
In many parts of the world there is a significant increase in urban areas, in which
sealed surface dominate. Sealing, that is, covering the surface of the soil with
asphalt, concrete, paving stones, and also buildings and greenhouses, generally has
a substantial impact on the soil ecosystem. The adverse effects differ from species
to species and range from negligible to disastrous. Since the biggest problem caused
by sealing is the blocking of the water supply and, depending on the degree of per-
meability (possible joints, cracks), on gas exchange, the ecological effects depend
on the closed sealed surface area in relation to the presence of cracks and unsealed
areas. The effect of sealing on Ecosystem Services is that the processes to be per-
formed by organisms will be hampered by the drying out of the soil, which makes