The toxic effect of the herbicide picloram on the microbial population of a
typical Brazilian red Latosol soil was studied in a series of microcalorim
etric experiments. The activity of the soil was stimulated by the addition
of 6.0 mg of glucose and 6.0 mg of ammonium sulfate, under 34.8% controlled
moisture, to a 1.50 mg soil sample, at 298.15 +/- 0.02 K. The net thermal
effect due to the addition of the picloram to the soil was determined by in
terpreting the power-time curves, which were recorded on the microcalorimet
er. The total thermal effect evolved by the microorganisms was affected by
the increasing doses of herbicide, and varied from 0 to 10.00 mug g(-1). An
increase in picloram exposure caused a decrease of the original thermal ef
fect, reaching a null value above 20.89 mug of herbicide per gram of soil.
The decreases of the thermal effect evolved by microorganisms and the incre
ase of the lag phase period are associated with the death of the microbial
population. The effects caused by picloram application in this typical Braz
ilian soil resulted in a strong effect on the soil microbial communities.