Relationships amongst organic matter content, heavy metal concentrations, earthworm activity, and soil microfabric on a sewage sludge disposal site

Abstract

From 1973 to 1980, sewage sludge precipitated by three chemical treatment processes (aluminum sulphate, ferric chloride and calcium hydroxide) was applied to replicated plots on a silt loam soil at four different rates. The sludge was contaminated by heavy metals associated with industrial activity in the metropolitan areas from where it was collected. The grass on the plots has been mown regularly from 1973 onwards. Soil and earthworm populations were subjected to analysis in 1989 and 1990. Soil pits were dug and sampled to a depth of 50 cm in four plots receiving the highest rates of the three different types of sewage treatment processes (and a control plot treated through the same period with ammonium nitrate) to determine heavy metal contents and soil organic matter distributions with depth. The earthworm fauna of the plots was almost entirely represented by populations of Lumbricus terrestris L. that were analyzed for abundance and biomass following collection using the formalin expulsion method. Earthworm biomass was higher in sludge treatments than in control plots. Cadmium concentrations in earthworm tissues were correlated with soil Cd concentrations Only worms from Al-sludge treatments had elevated tissue concentrations of Cd. Cadmium distributions in sludgetreated soils were correlated with organic matter distributions in the soil profile (for individual profiles). Intact blocks of soil were removed from soil pits for chemical analysis of the soil microfabric by PIXE (proton induced X-ray emission) and electron micro-probe (EMP). Concentrations of several trace metals on the inner walls of the earthworm channels correlated with those added to the soil via the sludge. Water flow through earthworm burrows lined with fecal material, which had higher metal concentrations than the adjacent soil matrix, could move soluble forms of the elements, thus accelerating leaching of metals into the aquifer from surface-applied sludge.