Land Application of Sewage Sludge: Physicochemical and Microbial Response - Reviews of Environmental Contamination and Toxicology - page 49

increase in soil organic matter (bounded with soil particles) and pH decreases heavy
metal mobility (Liu et al. 2007 ; McCarthy and Zachara 1989 ; McBride et al. 1999 ).
As a result of the net negative charge of DOM at typical soil pHs, it generally moves
easily through the soil system (Dunnivant et al. 1992 ).
Magdoff and Amadon ( 1980 ) performed both laboratory and ield experiments to
evaluate the contribution that the nitrogen in sewage sludge makes to crops.
Aerobically treated secondary liquid sewage sludge was applied to supply 50, 100,
150, and 200 kg N ha −1 year −1 (as ammonium nitrate) to corn ( Zea mays L. ) and hay
(timothy, Kentucky bluegrass, quack grass, and red and white clover). These forages
were grown on Hadley sandy loam and Nellis loam soils. Under laboratory condi-
tions, more than 54% of the organic N added to the sludge-amended corn soil was
mineralized. Under ield condition, mineralization of organic N from sludge that
had been applied to corn and hay averaged 55% during the irst year of application.
The amount of sludge organic N mineralized appeared to vary according to the per-
cent organic N present in the sludge (Sommers 1977 ).
In soil, trace elements may exist as solid phases, free ions in soil solution, soluble
organic mineral complexes, or adsorbed onto colloidal particles. Addition of sew-
age sludge to soils may affect the potential availability of heavy metals to plants
(Wang et al. 1997 ). The solubility, and consequently the mobility of metals added in
sewage sludge are controlled, in part, by organic matter decomposition and resultant
creation of soluble organic carriers of metals (Chaney and Ryan 1993 ). Trace metal
bioavailability is also dependent upon the form of organic matter present, i.e., solu-
ble (fulvic acid) or insoluble (humic acid) forms (McBride 1995). Insoluble organic
matter inhibits the uptake of metals, which are tightly bound to organic matter, thus
reducing bioavailability. Soluble organic matter, however, increases bioavailability
by forming soluble metal organic complexes (Mc Bride 1995 ). When organic matter
decomposition rates are stable, the level of soluble organic matter present is reduced,
which leads to a reduction in metal bioavailability.
Morera et al. ( 2002 ) studied the bioavailability of Cu, Ni, Pb, and Zn from
municipal sewage sludge to sunlower plants ( Helianthus annus L. ) in four different
types of soils (i.e., Ithnic Haplumbrept (LH), Calcixerollic Xerochrept (Cx1 and
Cx2), and Paralithic Xerorth (Px)). Each of these soils retained different physico-
chemical properties. The purpose of the experiment was to evaluate the inluence of
several sewage sludge application rates (0, 80, 160, and 320 tons/ha dry wt.), and soil
type on the bioavailability of heavy metals, and interaction among these variables.
The acid pH of the LH soil favored the bioavailability of Zn from sewage sludge,
whereas Cu bioavailability was greater in alkaline soils. The high organic matter
content of the acid soil (LH) produced complexes with Cu and thus impaired its
uptake by plants. A contrasting trend occurred with respect to metal concentrations
in acid and alkaline soils. The plant concentrations of Zn, Cu, Pb, and Ni decreased
with increases in sludge application rates for the acid soil (LH), whereas in alkaline
soils (Cx1), Zn and Cu levels increased. There were minor changes in metal concen-
trations of plants grown in Cx2 and Px soils that resulted from increased sludge
doses. The results of this study further suggested that soil type has a larger effect on
metal bioavailability than did sludge application rates.