Water Supply and Quality - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 158

it facilitates the co-metabolic biodegradation
of xenobiotic organic compounds. Indeed, the
addition of organic carbon-rich substrates to
contaminated soils is used as a bioremedi-
ation strategy to stimulate microbial activity
and facilitate the co-metabolic removal of
pollutants (Wagner and Zablotowicz, 1997).
Some versatile degraders of xenobiotic
organic compounds, for instance Pseudo-
monas putida strains, are known to regulate
the expression of their degradation path-
ways in response to substrate availability
(Rojo, 2010), and also to maintain the carbon-
nitrogen balance (Amador et al ., 2010). For
instance, expression of the genes of the al-
kane degradation pathway encoded in the
P. putida octane (OCT) plasmid are subject to
negative and dominant global control, depend-
ing on the carbon source used and on the
physiological status of the cell (Dinamarca
et al ., 2002). Effective regulation of metabolic
pathways in response to carbon substrate
availability and carbon-nitrogen ratios will
increase the competitiveness of the micro-
organisms that can grow on their preferred
substrate. Substrate-substrate inhibition
refers to the situation where expression of
the metabolic pathway leading to the break-
down of one substrate, for instance a xeno-
biotic compound, is inhibited in the pres-
ence of another, more preferred substrate,
for instance more readily biodegradable
natural organic compounds. This provides
a potential mechanism by which the pres-
ence of soil organic matter could impede
the biodegradation of xenobiotic compounds
in soil.

metabolism of persistent, heavily chlorin-
ated pesticides, and many nitro-aromatic
compounds are susceptible to nitro reduc-
tion under anaerobic conditions. Inorganic
pollutant speciation will also depend on the
prevailing redox conditions. Because it is
the most abundant organic substrate in soil,
the decomposition of natural organic matter
will set the soil redox conditions. Especially
in very wet conditions and waterlogged
soils, the available oxygen in soil pore water
may be consumed rapidly by natural or-
ganic matter decomposition, which results
in anaerobic conditions and the accumu-
lation of partially decomposed natural or-
ganic matter in organic carbon-rich soils
such as peat.

Nutrient availability

An adequate supply of essential nutrients is
important for the biodegradation of xeno-
biotic organic compounds in soil (Atlas and
Philp, 2005). Microbial degradation of soil
organic matter releases nutrients into soil
solution, which can then potentially support
the growth of xenobiotic compound-degrading
bacteria. Soil organic matter decomposition
releases nitrogen, phosphorus, potassium,
calcium, magnesium and other essential
growth elements assimilated (Tian et al .,
1992). Organic matter also contributes to the
cation exchange capacity of soils, and thus
it helps with the retention of positively
charged inorganic nutrients released by the
decomposition of biomolecules. Soil organic
matter decomposition also releases chelat-
ing organic chemicals into soil solution,
which may enhance the solubility and bio-
availability of essential micronutrients such
as copper, iron, manganese and zinc, which
is particularly important in alkaline soils.
Dissolved organic matter may also prevent
phosphate precipitation.

Redox conditions

Redox conditions define the thermodynamic
landscape of xenobiotic compound biodeg-
radation in soils. While most xenobiotic
compounds are biodegraded most rapidly by
aerobic microorganisms using oxygen as their
ultimate electron acceptor, some important
classes of xenobiotic compounds are trans-
formed more effectively under anaerobic
conditions. For instance, reductive dechlor-
ination is often an important first step in the

Xenobiotic compound availability

As outlined above, soil organic matter is an
important sorbent matrix for xenobiotic