Environmental Engineering Reference
In-Depth Information
dose 2 (2N), sludge dose 3 (4N), and sludge dose 4 (8N). The dose of sludge was
based on an N concentration equivalent to that recommended for maize. Other doses
were two, four and eight times the base dose. The sludge was applied in April 1999,
December 1999, and October 2000. After 132 days of incubation, the sludge dose
8N was detrimental to the soil microlora, as a result of the soil microbial biomass
and dehydrogenase activity being reduced at this amendment dose.
Microbial populations and enzymatic activities in sewage-sludge-amended soils
were studied at two application rates (5 and 1% dry wt.) by Hattori ( 1988 ) to eluci-
date the role of soil microorganisms in decomposing sewage sludge. The authors
found that organic C and N mineralization rates rapidly increased bacterial number
and proteinase activity in the soil and reached a maximum within the irst 3 days,
declining rapidly thereafter. The actinomycetes and fungi counts reached their max-
imum after 2 or 3 weeks of incubation and thereafter remained at the same level.
The amino-acid N content found in 6N HCl extracts of sludge-amended soil
decreased markedly. The proteinase-producing bacteria contributed signiicantly to
the rapid degradation observed during the early days of the sludge amendment
experiment, whereas actinomycetes and fungi contributed to a gradual degradation
during the end phase. The sludge amendment enhanced soil microbial biomass by
8-28% (at the sludge amendment rate of 0.75% dry wt.), and the enhancement was
greatest in the clay-loam, and the least in the sandy-loam soil (Dar 1996 ). The activ-
ities of three soil enzymes (i.e., dehydrogenase, alkaline phosphatase, and arginine
ammoniication) were enhanced by 18-25%, 9-23% and 8-12%, respectively, as
compared to activities in unamended soils. The increase was greater in sandy loam
than in loam, or clay loam soils.
Soil fertility may increase from additions of sewage sludge, although sludges
may also be important causes of soil pollution. Some metals present in sludge, e.g.,
Cu, Ni, and Zn, are essential micronutrients for plants and microorganisms (Alloway
1995 ). However, at higher concentrations, even these micronutrients may be toxic.
Adverse effects of sludge metals on soil microorganisms pose a potential threat to
soil quality, particularly through the disruption of nutrient cycling. In most studies
in which soils have been amended with sewage sludge, reductions in microbial bio-
mass (Leita et al. 1995 ; Fließbach et al. 1994 ) and enzymatic activity (Kuperman
and Carriero 1997 ) were found, when soils were contaminated with heavy metals.
However, the inluence of heavy metals on soil respiration is less well known. Some
researchers have reported signiicantly lower CO 2 evolution in metal-contaminated
soils (Doelman and Haanstra 1984 ; Freedman and Hutchinson 1980 ; Hattori 1992 ,
Kuperman and Carreiro 1997 ). Others have reported the opposite (e.g., Leita et al.
1995 ; Fließbach et al. 1994 ; Bardgett and Saggar 1994 ). Additionally, a range of
studies have indicated that respiration responses to metal inputs may vary with the
time that has elapsed since application (e.g., Doelman and Haanstra 1984 ). Sludge
applied to soils often contains a variety of metals. Responses of microbes to such
metal combinations may be synergistic, antagonistic, or additive (Chander and
Brookes 1991a ). Because of the complexity of such interactions, it is very difi-
cult to establish a minimum soil concentration for individual metals at which
adverse effects on microorganisms may occur (Brookes 1995 ). Chander and
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