Soil enzymes and biological activity at different levels of organic matter stability

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-CO2variation 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-CO2 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-CO2 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.