Land Application of Sewage Sludge: Physicochemical and Microbial Response - Reviews of Environmental Contamination and Toxicology - page 50

3.3 Biological Properties

Species used to monitor the health of an environment or ecosystem are referred to
as “biological indicators.” Biological indicators may constitute any biological
species or group thereof, that have a function or population marker that can be used
to determine ecosystem or environmental integrity. Biological indicators are often
employed to represent some aspect of living soil components, and such indicators
usually respond more rapidly than do physical and chemical indicators to changing
soil conditions (Anderson and Gray 1990 ; Powlson 1994 ; Pascual et al. 2000 ).
Moreover, biological indicators are useful as sensitive tools for detecting changes in
soil conditions that may occur.

3.3.1 Soil Microbial Biomass and Enzymes

Of total soil microbial biomass, soil fungi often comprise at least 75-95% and
together with bacteria are responsible for about 90% of the total energy lux of
organic matter decomposition in soil (Paul and Clark 1996 ). Among the key fertility
parameters and biological properties of soils, special emphases are given to soil
enzyme activity.
Soil enzymatic measurements are used to provide a biological index of soil fertil-
ity, and soil enzyme activity is used as an indicator for many soil biological pro-
cesses. Soil enzymatic activities have often been used to establish indices of soil
fertility, since they relect the effects of cultivation, soil properties, and pedological
amendments (Skujins 1978 ; Ceccanti et al. 1993 ). Soil enzymes are constantly
being synthesized, accumulated, inactivated, and/or degraded and, therefore, play
vital agricultural and nutrient cycling roles (Tabatabai 1994 ; Dick 1997 ). The heavy
metals present in sewage sludge may indirectly affect soil enzymatic activities
(Kandeler et al. 2000 ). According to Fließbach et al. ( 1994 ), the effect of sewage
sludge on biological activity can be used as a soil pollution indicator. Amending
soils with sewage sludge increased soil microbial activity, soil respiration, and soil
enzymatic activities (Banerjee et al. 1997 ). However, when incubations were longer
and heavy metal availability was higher, reduced soil enzyme activities were
reported (Fließbach et al. 1994 ).
Urease (urea amidohydrolase) is the enzyme that catalyzes the hydrolysis of urea
to CO 2 and NH 4 ions, by acting on C-N nonpeptide bonds in linear amides (Antonious
2009 ). Urease is an important soil enzyme that also mediates the conversion of
organic N to inorganic N by hydrolysis of urea to ammonia (Byrnes and Freney
1995 ). Invertase ( b -D-fructofuranosidase) is a ubiquitous enzyme in soils (Gianfreda
et al. 1995 ). The activities of urease and invertase are important in soil for releasing
simple carbon and nitrogen sources that contribute to the growth and multiplication
of soil microorganisms. According to Garcia et al. ( 1993 ), sewage sludge contains
high amounts of enzymatic substrates. These easily available substrates stimulate
microbial growth and enzyme production. Suhadolc et al. ( 2004 ), pointed out that