INTERACTIONS BETWEEN C-14-LABELED ATRAZINE AND THE SOIL MICROBIAL BIOMASS IN RELATION TO HERBICIDE DEGRADATION

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 mu g g(-1) soil of non-labelled atrazine and 6.6 x 10(3) Bq g(-1) soil of C -14-labelled atrazine) x (0, 20, and 100 mu g g(-1) soil of urea-N) x (pasture or arable soil with a previous history of atrazine applicatio n). Microbial biomass, measured by substrate-induced respiration and t he fumigation-incubation method, basal respiration, incorporation of C -14 into the microbial biomass, degradation of atrazine, and C-14 rema ining in soil were monitored over 81 days. The amount of microbial bio mass was unaffected by atrazine although atrazine caused a significant enhancement of CO2 release in the non-fumigated controls. Generally, the amounts of atrazine incorporated into the microbial biomass were n egligible, 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 experimen t conducted to determine total amounts of C-14-labelled atrazine conve rted into CO2 by pasture and arable soils showed that less than 25% of the added C-14-labelled atrazine was oxidised to (CO2)-C-14 during a 15-week period. The rate of degradation was significantly greater in t he arable soil at 24%, compared to 18% in the pasture soil. This indic ates that soil microbes with previous exposure to atrazine can degrade the applied atrazine at a faster rate.