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SOIL BIOLOGY & BIOCHEMISTRY

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

^a Agriculture and Agri-Food Canada, Potato Research Centre, Fredericton, NB, Canada E3B 4Z7
^b Univ. of Guelph, Dep. of Environmental Biology, Guelph, ON, Canada N1G 2W1
^c currently at, AgriTECH Park, 90 Research Dr., Bible Hill, NS, Canada B6L 2R2
^d currently at, CSIRO Land and Water, Waite Rd, Glen Osmond, SA 5064, Australia
^e Nova Scotia Agricultural College, Dep. of Environmental Sciences, Truro, NS, Canada B2N 5E3
^f Univ. of Guelph, Dep. of Environmental Biology, Guelph, ON, Canada N1G 2W

^* Corresponding author (bernie.zebarth{at}agr.gc.ca).

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 on the abundance of the nosZ or cnorB_P gene-bearing bacteria in soil from the AC system. The highest abundance of denitrifier bacteria was measured in 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.

Abbreviations: AC, arable cropping • DEA, denitrifying enzyme activity • EOC, extractable organic carbon • PCR, polymerase chain reaction • PG, permanent grassland