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Soil biological activity has important implications for soil carbon (C) sequestration. However, very little is known about the environmental factors, particularly the effect of soil mineralogy on availability of C for soil microorganisms. In this study, we have investigated the influences of soil type (clay mineralogy) on C mineralization and its effects on biological activity at different levels of soil organic matter stability. Two soils an allophanic, derived from recent volcanic ash and a kaolinitic, resulting from metamorphic parent materials were physically fractioned in to light (LF, coarse sand 250-2000 µm), intermediate (IF, fine sand53-250 µm) and mineral (MF, silt and clay < 53 µm) fractions. Several biological and biochemical analyses at Ah horizons of mineral soil and physical fractions were conducted: soil respiration, enzymatic activities, carbohydrates and microbial biomass, amongst others soil variables. The results indicated that the bulk soil and physical fractions had a significant impact on cumulative C mineralized after 30 days of incubation and soil enzyme activities. More than 76% of total C-CO 2 variation was explained by stepwise multiple regression analysis including factors such as soil enzymes (β-glucosidase, dehydrogenase and phosphatase) and inorganic P. Soil ATP extraction was a good indicator of microbial activity, because of a positive and significant correlation among ATP and i) C-CO 2 and ii) metabolic quotient (soil respiration rate divided by microbial biomass). We also found an inverse and significant relationship between Al pyrophosphate (Al bound to SOM) and the C-CO 2 in volcanic soil, whereas the same correlation did not occur in kaolinitic soil. Our results confirmed a greater stabilization capacityof MF in allophanic than in kaolinitic soils due to the amorphous minerals clay materials.

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