Effects of metsulfuron-methyl on microbial biomass and populations in soils

Effects of the herbicide metsulfuron-methyl on soil microorganisms and their activities in two soils were evaluated under laboratory conditions. Measurements included their populations, soil respiration, and microbial biomass. In the clay soil, bacterial populations decreased with increasing concentration of metsulfuron-methyl during the first 9 days of incubation but exceeded that of the control soil from day 27 onward. In the sandy loam soil, the herbicide reduced bacterial populations during the first 3 days after application, but these increased to the level of untreated controls after 9 days' incubation. Fungal populations in both soils increased with increasing metsulfuron-methyl concentrations, especially in the sandy loam soil. CO2 evolution was stimulated in both soils in the presence of the herbicide initially, but decreased during days 3 to 9 of the incubation period before increasing again afterward. The presence of metsulfuron-methyl in the soil increased microbial biomass, except in sandy loam soil at the first day of incubation.