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TECHNICAL REPORT

Heavy Metals in the Environment

Time and Moisture Effects on Total and Bioavailable Copper in Soil Water Extracts

^a Chemistry Department, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
^b Department of Microbiology, Danish Veterinary Institute, Bülowsvej 27, 1790 Copenhagen V, Denmark

^* Corresponding author (oln{at}kvl.dk).

Received for publication December 20, 2002. Environmental risk assessment of heavy metals in soil frequently involves testing of freshly spiked soils kept under stable humidity conditions, but it has been questioned whether these assessments are representative of the field situation. Furthermore, the poor correspondence that is often found between total metal content and metal toxicity calls for integrated chemical and biological analysis. The aim of this work was to determine time- and moisture-dependent changes in total water-extractable Cu as well as bioavailable Cu in soil water extracts. Measurements of total water-extractable copper ([Cu]_tot) were performed using furnace atomic absorption spectrometry. An in vitro assay employing a Cu-specific Pseudomonas fluorescens reporter strain was used to estimate Cu that was biologically available to the reporter strain. We refer to this copper fraction as "bioavailable," [Cu]_bio. We found a time-dependent decrease in [Cu]_tot and [Cu]_bio during incubation for up to 220 d at field capacity. Hence the [Cu]_bio was reduced to between 32 and 40% of the initial values. Furthermore, the [Cu]_bio to [Cu]_tot ratio correlated positively with the amount of added Cu and tended to increase with time. The moisture content of the soil was important for Cu retention. Dry soil had higher [Cu]_tot concentrations than humid soil, but the [Cu]_bio to [Cu]_tot ratio was lower in the dry soil. Alternating drying and wetting did not lead to a more rapid Cu retention than observed under constant humid conditions. Our observations underline the need for considering both time and moisture effects when interpreting short-term toxicity studies and when making predictions concerning possible long-term effects of Cu in the soil environment.

Abbreviations: [Cu]_bio, bioavailable copper concentration • [Cu]_tot, total copper concentration • DMM, Davis Minimal Medium • DOM, dissolved organic matter

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