Long term effects of heavy metal enriched sewage sludge disposal in agriculture on soil biota.

摘要

Long term effects of heavy metal pollution caused by decennial sewage sludge disposal (12 t ha-1 yr1) from 1980 to 1990 on soil biota of an experimental field in Northern Germany were investigated 16 years after the last sludge application. The plots still exist to date as permanent pasture. The consequences of the soil treatment were shown by means of heavy metal concentrations of soil and earthworms. Additional effects on soil micro-organisms by enzyme activities were investigated. The disposal of heavy metal enriched sewage sludge caused increased soil heavy metal concentrations compared to mineral fertilised control plots. The increased soil heavy metal concentration was transferred to soil organisms. The earthworm biomass from the sewage sludge treated plots bore increased heavy metal concentrations. Cadmium showed the highest transfer factor from soil to earthworms. The amount of transfer into biomass is specific for each element and depends on heavy metal soil concentration. Despite the higher heavy metal concentration of earthworm biomass, the number of earthworms was increased in soil of the sewage sludge treated plots which is attributed to the enhanced soil organic matter (SOM) concentration. The number of earthworms did not correlate significantly with SOM but the weight of earthworms did. The microbial activities of dehydrogenase and alkaline phosphatase enzymes were reduced in soil of the sewage sludge treated plots compared to the control plots. The enzyme activities of cellulase and protease however were enhanced by the application of heavy metal enriched sewage sludge compared to the control plots. One the one hand sewage sludge addition enhanced soil organic matter concentration; on the other hand its microbial decomposition was retarded by toxic effects of heavy metals due to reduced enzyme activities.