Water Supply and Quality - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 157

picture of the interactions between pollutants,
the soil matrix and soil microorganisms.
Soil organic carbon affects the pollutant fate,
not only as a sorbent matrix (i.e. via K d ) but
also indirectly through its profound effects
on soil microorganisms, and hence the bio-
degradation term in Eqn 12.1.

organic carbon is a key factor in shaping this
capability. A diverse array of catabolic en-
zymes has been developed through the evo-
lutionary adaptation of soil microorganisms
to the diversity of complex natural organic
molecules present in soil organic matter,
and many of these microbial enzymes are
perfectly suited to metabolizing structurally
related xenobiotic compounds (Singer et al .,
2003). Furthermore, existing genes coding
for enzymes involved in natural organic mat-
ter biodegradation may provide suitable tem-
plates for the evolution of novel genes involved
in xenobiotic compound biodegradation.
For instance, the microbial biodegradation
of xenobiotic s -triazine ring compounds, a
class which includes many pesticides, dyes
and explosives, is related to the metabolism
of pyrimidine and purine rings, which are
structural components of essential biomol-
ecules such as deoxyribonucleic acid (DNA)
and ribonucleic acid (RNA), adenosine tri-
phosphate (ATP) and nicotinamide adenine
dinucleotide (NAD) (Wackett et al ., 2002).

The Role of Soil Organic
Carbon in the Biodegradation
of Organic Pollutants

If water purification relied on pollutant
sorption by organic matter in soil only, it
could not be effective. No filter lasts forever,
and no matter how strongly pollutants adsorb
to soil organic matter, they will not be re-
tained indefinitely and will eventually break
through organic carbon-rich soil horizons
and leach into surface water and ground-
water. Hence, the pollutant breakdown by
soil organisms is a critical complementary
component of long-term sustainable water
purification in soil. Soil organic carbon ef-
fects on the organic pollutant biodegradation
process can be synergistic or antagonistic.
Many soil microorganisms subsist on plant
organic matter such as leaf litter or root ex-
udates and live in the root zone, which is
also the organic matter-rich topsoil layer.
Much of the soil organic carbon is humified
and poorly biodegradable, but soil organic
carbon content nevertheless correlates with
the abundance and activity of soil microbes
(Schnürer et al ., 1985; Ritz et al ., 2004), and
microbial carbon may contribute several per
cent of total organic carbon in soil (Ananyeva
et al ., 2008). To what extent natural organic
carbon mineralization may stimulate directly
or indirectly the biodegradation of xenobiotic
organic compounds depends on which mech-
anism controls the microbial breakdown of
pollutants.

Co-metabolism versus substrate-
substrate inhibition

Co-metabolism refers to the ability of micro-
organisms to biodegrade a xenobiotic com-
pound together with co-substrates that may
occur naturally and at much higher con-
centration in soil. Co-metabolism can be
explained by non-specific enzymes which
transform a wide range of different organic
compounds. For instance, extracellularly ex-
creted peroxidases and laccases involved in
the oxidation of lignin also enable lignino-
lytic fungi to biodegrade polycyclic aromatic
hydrocarbons (Bamforth and Singleton, 2005).
Monooxygenases and dioxygenases play an
important role in the removal of both natural
and xenobiotic organic compounds (Bamforth
and Singleton, 2005; Meynet et al ., 2012). Co-
metabolism enables the biodegradation of
xenobiotic organic compounds present at very
low concentrations, since microorganisms may
use the co-substrate(s) to meet their carbon
and energy needs. Since soil organic matter
is a rich source of potential co-substrates,

Metabolic capability

Capability to biodegrade xenobiotic organic
compounds is indispensable for their break-
down by soil microbial communities. Soil