A laboratory incubation experiment was set up to determine the effects of atrazine herbicide on the size and activity of the soil microbial biomass. This experiment was of a factorial design (0, 5, and 50 μg g−1soil of non-labelled atrazine and 6.6×103Bq g−1soil of14C-labelled atrazine) x (0, 20, and 100 μg g−1soil of urea-N) x (pasture or arable soil with a previous history of atrazine application). Microbial biomass, measured by substrate-induced respiration and the fumigation-incubation method, basal respiration, incorporation of14C into the microbial biomass, degradation of atrazine, and14C remaining in soil were monitored over 81 days. The amount of microbial biomass was unaffected by atrazine although atrazine caused a significant enhancement of CO2release in the non-fumigated controls. Generally, the amounts of atrazine incorporated into the microbial biomass were negligible, indicating that microbial incorporation of C from atrazine is not an important mechanism of herbicide breakdown. Depending on the type of soil and the rate of atrazine application, 18–65 of atrazine was degraded by the end of the experiment. Although the pasture soil had twice the amount of microbial biomass as the arable soil, and the addition of urea approximately doubled the microbial biomass, this did not significantly enhance the degradation of atrazine. This suggests that degradation of atrazine is largely independent of the size of the microbial biomass and suggests that other factors (e.g., solubility, chemical hydrolysis) regulate atrazine breakdown. A separate experiment conducted to determine total amounts of14C-labelled atrazine converted into CO2by pasture and arable soils showed that less than 25 of the added14C-labelled atrazine was oxidised to14CO2during a 15-week period. The rate of degradation was significantly greater in the arable soil at 24, compared to 18 in the pasture soil. This indicates that soil microbes with previous exposure to atrazine can degrade the applied atrazine at a f
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