DENITRIFIERS AND DENITRIFYING ACTIVITY IN SIZE FRACTIONS OF A MOLLISOL UNDER PERMANENT PASTURE AND CONTINUOUS CULTIVATION

The variability of the denitrification process in soils was investigat ed through the distribution of denitrifiers and potential denitrifying activity in the various microenvironments of a mollisol sampled under permanent pasture (PP) and under continuous cultivation (CC). Five si ze-separates (<2, 2-20, 20-50, 50-250, and 250-2000 mum) were obtained by mild crushing in water and physical fractionation (sieving and sed imentation) of the paired soils. Soils and size fractions were analyse d for organic C, total N, and microbial biomass C content and total an d denitrifying bacterial counts (most probable number). The indirect e numeration of denitrifying organisms was improved using microbial biom ass C data. Denitrifying enzyme activity (DEA) and dentrifying specifi c activity (DSA:DEA per cell) were also determined on unfractionated s oils and on fractions. All the data values were higher in the unfracti onated PP soil than in the unfractionated CC soil. The <2 mum fraction s of both soils contained a moderate density of denitrifiers, lower th an in the unfractionated soil, but with high specific activity; three times (PP soil) to five times (CC soil) as much as the value found in the unfractionated soils. The 20-2 mum fraction contained microaggrega tes and exhibited the highest microbial biomass C and organic N conten t and a density of denitrifiers greater than in the unfractionated soi l, with a moderate specific denitrifying activity. Despite these contr asting characteristics, the two finest fractions of both soils had a s imilar potential for denitrification and provided about 90% of the tot al potential denitrification of each unfractionated soil. The particip ation of the particulate organic matter (POM) located in the >250 mum fraction to the total potential denitrification was negligible due to the very small number of denitrifying organisms associated with this f raction. Moreover, a high DSA was found only in the POM of the soil un der permanent pasture and not in the soil under continuous cultivation .