Dehydrogenase activity as an indicator of different microbial responses to pesticide-treated soils

A laboratory study was conducted to assess the impacts of diazinon, linuron or a fungicidal preparation of mancozeb supplemented with dimethomorph on dehydrogenase activity (DHA) in loamy sand (LS) and sandy loam (SL) soils with different features. The pesticides were applied at the maximum predicted environmental concentrations (PEC) under field conditions and at 5 or 100 times the PEC. More distinct effects were observed in LS than SL soils at all sampling times. For PEC, a significant decrease in DHA was found in LS soil treated with diazinon or the fungicidal preparation on day 1 and during the incubation period, respectively. However, DHA did not decrease in SL soil treated with the pesticides at this dosage. For the higher concentrations, decreased DHA was ascertained in LS soil treated with diazinon and the fungicidal preparation at all sampling times, whereas for linuron this effect was evident on days 14 and 28. By contrast, only 100-fold PEC significantly decreased DHA in SL soil amended with diazinon or linuron, compared with the fungicidal preparation that decreased DHA at five-fold PEC. Our results indicate that DHA was particularly sensitive to the fungicidal preparation in both soils, whereas linuron was less harmful than diazinon.