FIGURE 3.1 Mean k values (rate of organic matter decomposition) after sewage sludge is applied to soils. These k values represent a variety of experimental data. (Data modified from Metzger, L. and Yaron, B., Adv. Soil Sci., 7, 141-162, 1987; and Antoniadis, V. and Alloway, B.J., Water Air Soil Pollut., 132, 201-214, 2001.)
compounds increase remarkably in soil organic carbon after sewage sludge application and, as a result, they induce the formation of hydrophobic aggregates. Thus, Hi/Ho ratio will decrease as sewage sludge application rate increases .
Nevertheless, as sewage sludge application increases, DOC also increases in absolute as well as relative terms compared to the total organic carbon added to soil. Experimental data seem to agree on the fact that humic and fulvic acids in soil-sludge mixtures evolve, increasing their concentrations months after sewage sludge application. Heavy metals are preferentially associated with hydrophilic phases, so one could state that metal availability will tend to decrease with time after the termination of sludge application. However, this trend may be different if DOC increases more than the humic acids, and some evidence suggests that this may take place. Metzger and Yaron  reported that the ratio of humic over fulvic acids was reduced following sludge application, and Han and Thompson  found that, in the surface soil (0 to 5 cm), Hi/Ho ratio increased. These data point to the risk of heavy metals exhibiting enhanced availability even months after the termination of sewage sludge application.
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