Responses of indigenous microorganisms to a fungicidal mixture of mancozeb and dimethomorph added to sandy soils

The objective of this study was to assess changes in microbiological characteristics of loamy sand (LS) and sandy loam (SL) soils in response to a mixture of two fungicides, mancozeb (MB) and dimethomorph (DT), applied at three dosages of 15, 75 and 1500 mg kg−1 soil. Microbial activity, assessed by substrate-induced respiration, was either stimulated or unchanged by the first two dosages of MB + DT in both soils throughout 28 days. However, MB + DT at the concentration of 1500 mg kg−1 soil significantly decreased the respiration in LS soil. All MB + DT concentrations increased the total numbers of culturable bacteria, ammonification rate but did not change the numbers of nitrate-reducing bacteria in both soils. In comparison to acid and alkaline phosphatases and urease, dehydrogenase was most sensitive to MB + DT and its activity declined in both soils. Decreases in nitrate concentrations, the numbers of fungi, nitrifying and N2-fixing bacteria were ascertained in both soils during the incubation time. Furthermore, slow growing K-strategists generally dominated in both soils after the treatment with MB + DT. Overall, the fungicidal preparation released at higher concentrations may disturb some non-target microbial groups inhabiting sandy soils. Consequently, to diminish an ecological hazard, careful application of MB + DT should be followed.