Influence of increasing soil copper concentration on the susceptibility of phosphomonoesterase and urease to heat disturbance

Long-term exposure to elevated copper (Cu) concentrations may affect the ability of soil microbes to withstand additional transient disturbances, such as heat. Bulk surface soil samples collected from three south east Australian locations were spiked with a series of Cu concentrations ranging from 0 to 1,000 mg/kg. To determine the effect of increasing soil Cu concentrations on phosphomonoesterase and urease activity following a simulated heat disturbance, aliquots of each of the Cu-treated soils were exposed to 60 °C for 24 h (perturbed group), and phosphomonoesterase and urease activity measured after 1, 2 and 7 days. Without heat disturbance, the Cu concentration causing significant inhibition of enzyme activity ranged from 50 to >1,000 mg/kg added Cu for phosphomonoesterase activity and 500 to > 1,000 mg/kg added Cu for urease activity. The Cu pre-exposure concentration increased the susceptibility of phosphomonoesterase activity but not urease activity to the heat disturbance. However, this did not follow a clear dose-response relationship and the effects were not lasting. The response differed according to soil type, with decreased resistance to heat found at concentrations ranging from 100 mg/kg added Cu in the silty loam soil to >1,000 mg/kg added Cu in the clay loam soil. The results from this study suggest that, at the concentrations of Cu typically reported in agricultural soils, Cu stress is unlikely to cause lasting alterations to the susceptibility of soil enzymes to a single moderate heat disturbance.