The effect of bacterial and fungal activities on organic matter degradation in Brazilian soils was studied by a microcalorimetric method. Bacteria and fungi isolated from tropical soils and added to: Rhodic, eutrudox (R), Typic eutrudox (V) and Quartzipsamment (Q) soils amended separately with moisture (control) (A) and 25% of cattle manure (E), municipal refuse compost (L), earthworm casts (H) or 23 mug of trifluralin (T) were investigated. The number of colony forming units in soil suspension was quantified by microscopy and inoculated in respective soil. All processes were measured at intervals of 7 days over a period of 35 days. The exothermic thermal effect (muJ) per cm(3) of bacteria or fungi per gram of dry soil, respectively, for each substrate was: [(9 +/- 1), (4 +/- 1)] RA; [(478 +/- 24), (105 +/- 5)] RE; [(121 +/- 6), (71 +/- 4)] RL; [(121 +/- 6), (71 +/- 4)] RH; [(8 +/-1); (3 +/- 1)] RT; [(10 +/- 1), (9 +/- 1] VA; [(347 +/- 17), (261 +/-13)] VE; [(71 +/- 4), (28 +/- 1)] VL; [(22 +/- 1), (33 +/- 2)] VH; [(7 +/- 1), (10 +/- 1)] VT; [(19 +/- 1), (12 +/- 1)] QA; [(1301 +/- 65), (46 +/- 2)] QE; [(89 +/- 4), (9 +/- 1)] QL; [(130 +/- 7), (11 +/- 1)] QH; [(32 +/- 2), (8 +/- 1)] QT. The calorimetric values are higher for bacteria than for fungi. In general, the results showed higher activities in the soil amended with cattle manure than with other additives. (C) 2004 Elsevier B.V. All rights reserved.