Linuron effects on microbiological characteristics of sandy soils as determined in a pot study

The aim of this study was to find what dosages of linuron introduced into soil can be tolerated by microorganisms, and whether a dosage equal to 100-times higher than the predicted environmental concentration (PEC) is harmful. A pot study was performed to determine the effects of the herbicide at PEC (4 mg kg(-1)), 5 times PEC (20 mg kg(-1)), and 100 times PEC (400 mg kg(-1)) on substrate-induced respiration (SIR), four enzyme activities (dehydrogenase, acid and alkaline phosphatases, and urease), ammonification and nitrification rates, plate counts of total bacteria, fungi, N-2-fixing bacteria, nitrifiers and denitrifiers, and distribution of r- and K-strategists in sandy soils of different texture throughout 28 days of incubation. Linuron increased SIR and ammonification, especially at 100 times PEC. In contrast, a decrease in nitrate concentrations was detected in both soils treated with the highest dosage of linuron. Although some changes in microbial numbers were ascertained, they were transient (i.e. total bacteria, N-2-fixing bacteria and nitrifiers) or not significant (i.e. total fungi and denitrifiers). Among the enzymes tested, dehydrogenase was the most sensitive to linuron, showing decreased activity for all treatments and two higher dosages in the loamy sand (LS) and sandy loam (SL) soils, respectively. The addition of 100 times PEC of linuron to LS resulted in the domination of slow-growing K-strategists on days 1 and 14. However, r-strategists dominated on both days 1 and 28 in SL. Our results indicate that linuron may disturb indigenous soil microorganisms, especially when released at high concentrations.