Page 100 - Plant-Based Remediation Processes

5 Impact of Metal/Metalloid-Contaminated Areas on Plant Growth 89

on different biologically active compounds (vitamins, enzymes, phytohormones,
etc.), extending their activity in soil (Bednarek et al. 2004).
Plant growth can be significantly repressed by mineral and organic xenobiotics
finding their way to surface soil layers as a result of increasing anthropogenic
pressure (Mocek and Mocek-Pło ´ciniak 2011). These compounds include, primar-
ily, heavy metals and polycyclic aromatic hydrocarbons (PAHs). By accumulating
in plants, xenobiotics cause changes in their metabolism and induce mutagenic
and carcinogenic changes in the next links of the trophic chain, namely among
animals and humans. The inhibiting influence of the above-mentioned xenobiotics
was also reported in both under- and aboveground parts of plants (Klimkowicz-
Pawlas 2009; Smreczak and Maliszewska-Kordybach 2003). At small quantities
of the above-mentioned pollutants (less than 100 mg kg 1 soil), a stimulating
effect of PAHs on plant growth and development was sometimes observed
(Klimkowicz-Pawlas 2009).
Soil, as a living formation, constitutes a natural environment for many different
organisms. Micro- and mesoflora as well as micro- and mesofauna play a special
role in making essential nutrients available to plants. Soil microorganisms perform
almost all processes of biochemical nature which play a decisive role in processes of
mineralisation and humification of residues of plant and animal origin. Huge
quantities of microorganisms, ranging from 0.5 to 5 million bacteria and from
1,000 to 15,000 fungi, may be found in 1 g of soil (Smyk 1999). The total mass
of such microorganisms inhabiting 1 ha of land can range from 1.5 to 15 mg. These
enormous amounts of microbes, on the one hand, release nutrients indispensable for
higher plants but, on the other hand, compete with them for the very same food. This
refers, in particular, to nitrogen and to a lesser degree also to phosphorus, potassium,
and calcium (Buckman and Brady 1969). Also the competition between
microorganisms and plants is sometimes viewed negatively. The important trace
elements indispensable as catalysers of plant physiological processes are usually
taken up faster by microorganisms, which frequently results in shortages of these
elements. Nevertheless, these inconveniences are more than compensated for by the
advantageous impact of microorganisms on higher plants through processes of
organic matter decomposition, transformation of mineral compounds, nitrogen
fixation, etc. Measurements of soil enzyme activity are employed more and more
frequently to assess the intensity of transformations taking place in soil under the
influence of microorganisms. Dehydrogenases, phosphatases, ureases, and
proteases turned out to be particularly useful in this regard (Bielin ´ska 2001;
Bielin ´ska and Mocek-Pło ´ciniak 2009). Soil enzymatic indices have become good
indicators of soil biological condition or even an indirect way of determination of its
fertility (Januszek 1999; Bielin ´ska 2001). From the mesofauna group, earthworms
exert a stimulating effect on plant growth and development. In 1 year, they can
‘process’ through their bodies about 35 mg of soil dry matter per hectare. This
means that in approximately 70 years, the entire arable layer on an area of 1 ha
passes through their organisms, resulting in enrichment of soil with numerous
enzymes and addition of many nutrients, primarily nitrogen, essential for plant
development and growth (Buckman and Brady 1969).