Long-Term Effects of Legacy Copper Contamination On Microbial Activity and Soil Physical Properties.

Soils heavily contaminated with copper (Cu) are considered unsuitable for agricultural use due to adverse impacts on microbial activity, soil physical properties, and direct toxicity to crops. This study investigated effects of Cu pollution from timber preservation activities between 1911 and 1924 on soil micro-organisms and subsequent effects on physical properties of a sandy loam soil. Tillage operations over the last 70 years have caused spreading of the initially localized contamination and have created a Cu concentration gradient from 20 to 3800 mg kg^-1 across an agricultural field in Hygum, Denmark. Soil samples obtained from the fallow field were used to determine total microbial activity using fluorescein diacetate and dehydrogenase assays. The physical properties measured included water-dispersible clay, bulk density, air permeability and air-filled porosity. Significant differences in microbial activity (for both assays) were observed at Cu concentrations >500 mg kg^-1. Although, unfavorable changes in all physical properties were obvious for Cu concentrations >500 mg kg^-1, significant increases in bulk density and water dispersible-clay, together with decreases in total porosity, air-filled porosity and air permeability occurred for Cu concentrations >900 mg kg^-1. There was significant negative correlation between microbial activity and the susceptibility of clay dispersion by water. These results suggest that a threshold level for Cu exists (~500 mg kg^-1 for this soil type) beyond which microbial activity decreases and soil structure becomes more compact with reduced permeability to air and water.