Microbial biomass and microcalorimetric methods in tropical soils

The type of organic matter (OM) plays an essential role in nutrient cycling in agricultural soil systems. Microbial activity in tropical soils was calorimetrically followed as a useful tool in this investigation. Tropical soil samples with different textures: Rhodic eutrudox (R), Typic eutrudox (V) and a Quartzipsamment (Q) from Brazil were amended with 25% cattle manure (E), municipal refuse compost (L), earthworm casts (H), the agrochemical trifluralin (T); (23 μg, equivalent dose of 1.25 kg ha−1) were explored. The microbial activity was determined by calorimetry and simultaneously by fumigation–extraction (microbial biomass carbon, C) to compare both methods. The results for R, Q, and V soils were: (212.04A, 195.99B, 204.47A) for microbial biomass C and (0.692B, 0.714B, 0.784A) for thermal effect with P<0.05, respectively, over a period of incubation of 91 days. The microbial activity of the modified soils decreases in the order: E, H, L and T. Both methods showed a coefficient of correlation r=0.7443 and the statistical probability of occurrence of the event, P<0.0001. From this correlation the utility of both methods for measuring the microbial activity in soils could be deduced.