Soil enzymatic response to addition of heavy metals with organic residues

Changes in organic C, available P, available heavy metal contents and enzym atic activities induced by addition of two heavy metal rich organic residue s [a municipal solid waste compost (MWC) and a non-composted paper sludge ( PS)] were determined in two different soils during a 280-day incubation exp eriment. The addition of the organic materials caused a rapid and significa nt increase in the organic C and enzymatic activities in both soils, this i ncrease was specially noticeable in soils treated with MWC. In general, enz ymatic activities in amended soils tended to decrease with the time. Organi c materials also increased heavy metal contents in soil. However, the prese nce of available soil heavy metals due to the addition of the organic mater ials at doses of 50,000 kg ha(-1) did not negatively affect dehydrogenase, beta -glucosidase or urease activities in the soils. There were significant and negative correlations between heavy metals and phosphatase activity in the soils at the beginning of the incubation. This negative correlation wa s probably due to the decrease in the enzyme activity in soils treated with PS in which high levels of available P were also found. It is difficult, t herefore, to attribute an inhibition of the enzyme activity to the presence of these heavy metals because a high available P concentration in soils al so depresses phosphatase activity.