Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 590

(phytophatology). Soil-borne pathogens belong to several different phyla such as
bacteria, fungi or nematodes. They reside in the soil for brief or extended periods,
and survive on plant residues or as resting organisms until root exudates reach them
and allow them to grow. They may escape competition with other microorganisms
by penetrating the roots. They either remain inside the plants until host death, or
move outside the plants to infect other parts of the root or other roots (Haas and
Défago 2005 ).
It is generally acknowledged that soil-borne diseases result from a reduction in
the Biodiversity of soil organisms. Plant diseases can effectively be treated with
chemicals, that is, fungicides, nematocides and bactericides. These chemicals, how-
ever, may have negative site effects such as affecting useful organisms and leaching
into the groundwater.
By the time disease symptoms appear, disease pathogens are inside the plant and
generally beyond control. Therefore, it is important to prevent the penetration of
pathogens. Healthy soils are to a large extent able to prevent infection of plants, even
though pathogens are present. These soils are called natural suppressive soils .The
level of disease suppression is typically related to the level of total microbiological
activity in a soil (e.g., Sullivan 2004 ).
Some fungi are able to trap harmful plant parasitic nematodes physically. The
thick mycorrhizae network, for example, physically obstructs the penetration of
root-feeding nematodes (Sullivan 2004 ). Furthermore, bacteria and fungi are cru-
cial in natural disease control. Haas and Défago ( 2005 ), for example, demonstrated
that some pseudomonades produce antifungal antibiotics, elicit induced systemic
resistance in the host plant or interfere specifically with fungal pathogenicity factors
during root colonization.
Fungi are not just active as pathogens. The first antibiotic isolated for human use
was penicillin, obtained from the soil fungus Penicillium chrysogenum .
Beneficial organisms can be added directly to prevent plant diseases, but a com-
mon and sustainable procedure in plant disease control is also to stimulate the
activities of microorganisms that are antagonistic to plant-parasitic nematodes by
applying organic soil amendments to the soil (e.g., Akhtar and Malik 2000 ).
13.4.3.7 Energy-Related Ecosystem Services
It is a well known fact, also to the general public, that the presence of fossil energy
sources such as coal, oil and gas in soil is the result of former biological life, often
dating back several millions of years. From this perspective, the 'supply of fossil
energy sources' is a special Ecosystem Service (if it can be called that), since the
rate of fossil energy consumption by humans is orders of magnitudes higher than
the production by the soil ecosystem. In other words: fossil energy supply could be
considered as an Ecosystem Service provided in the past and available for a limited
period of time.
Over the last few decades, the mitigation of the negative consequences of the
use of fossil energy sources and the possibilities for using clean energy sources
have made enormous gains in public interest. From this perspective, soil can make