EFFECT OF MANURING PRACTICES AND INCREASED COPPER CONCENTRATIONS ON SOIL MICROBIAL-POPULATIONS

Addition of piggery manure to soils over 5 yr (manured soils) increased the amount of Cu extracted by EDTA and diethylenetriaminepentaacetic acid (DTPA) compared to selected reference soils. DTPA-extractable Cu ranged from about 3 to 10 mu g Cu g(-1) soil in the manured soils and from about 1 to 2 mu g Cu g(-1) soil in the reference soils. Although the soil Cu concentrations in the manured soils were many times smaller than currently permitted, the increase in DTPA- or EDTA-extractable Cu was correlated with a 10- to a 1000-fold increase in the number of aerobic Cu-resistant bacteria. Although the Cu was mainly concentrated in the plough layer (15 cm) of the manured soils, Cu-resistant bacteria were also detected down to 110 cm soil depth. In contrast, no such differences were found between the total number of colony forming units, total microbial biomass or the degree of Cu-resistance of fungi and anaerobic bacteria between manured and reference soils. It is suggested that the degree of Cu-resistance of the aerobic soil bacteria may provide a sensitive measure of Cu bioavailability in soil. In general, bacteria were more sensitive to Cu than fungi. All of the 42 Cu-resistant bacterial strains investigated were oxidase-positive and 50% of the strains were pigmented. In contrast, only 20% of the 37 Cu-sensitive bacterial strains investigated were oxidase-positive and none were pigmented. Cu-resistant bacteria exhibited more resistance to several antibiotics and heavy metals than Cu-sensitive bacteria.