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 investigated 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 size-separates (<2, 2–20, 20–50, 50–250, and 250–2000 μm) were obtained by mild crushing in water and physical fractionation (sieving and sedimentation) of the paired soils. Soils and size fractions were analysed for organic C, total N, and microbial biomass C content and total and denitrifying bacterial counts (most probable number). The indirect enumeration of denitrifying organisms was improved using microbial biomass C data. Denitrifying enzyme activity (DEA) and denitrifying specific activity (DSA:DEA per cell) were also determined on unfractionated soils and on fractions. All the data values were higher in the unfractionated PP soil than in the unfractionated CC soil. The <2 μm fractions of both soils contained a moderate density of denitrifiers, lower than 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 μm fraction contained microaggregates and exhibited the highest microbial biomass C and organic N content and a density of denitrifiers greater than in the unfractionated soil, with a moderate specific denitrifying activity. Despite these contrasting characteristics, the two finest fractions of both soils had a similar potential for denitrification and provided about 90% of the total potential denitrification of each unfractionated soil. The participation of the particulate organic matter (POM) located in the > 250 μm fraction to the total potential denitrification was negligible due to the very small number of denitrifying organisms associated with this fraction. Moreover, a high DSA was found only in the POM of the soil under permanent pasture and not in the soil under continuous cultivation.