Denitrifier Community Dynamics in Soil Aggregates under Permanent Grassland and Arable Cropping Systems

A better understanding of the spatial distribution of denitrifiers and their activity may lead to an improved understanding of the denitrification process in soil. This study determined the spatial distribution of the total bacterial community (16S rRNA), components of the denitrifier community (cnorB(p); Pseudomonas mandelii and related species; nosZ), and denitrification activity across a range of soil aggregate size fractions (4-8, 1-2, and 0.25-0.5 mm) under permanent grassland (PG) and arable cropping (AC) systems. Aggregate size fraction had no significant effect oil the abundance of the nosZ or cnorBp gene-bearing bacteria in soil from the AC system. The highest abundance of denitrifier bacteria was measured ill the smallest size fraction in the PG system. Respiration did not differ among aggregate size fractions within the PG system; however, respiration was higher for the PG system than the AC system for all aggregate size fractions. For the AC system, higher respiration was measured in the 0.25- to 0.5-mm aggregate fraction than the 4- to 8-mm aggregate fraction. Denitrifying enzyme activity (DEA) was higher in the largest size fraction of the PG system than the AC system; however, DEA did not differ among aggregate size fractions within each management system. Cumulative denitrification during a 72-h incubation was significantly higher in the largest aggregate size fractions under both management systems. The results indicate that the differences among the aggregate size fractions were small in magnitude and that the spatial location of the denitrification activity and the abundance of the denitrifier bacteria were uncoupled across aggregate size fractions in the contrasting management systems.