PARENT SESSION
Oral Session 124: Soil Ecology V: Nutrient Dynamics.
Presiding: E Thiffault
Thursday, August 5, 1:30 PM to 5:00 PM, Meeting Room D 139.
Community composition and activities of denitrifying bacteria from adjacent agricultural soil, riparian soil, and creek sediment in Oregon, USA.
Rich, Jeremy*,1, Myrold, David2, 1 Princeton University, Princeton, NJ2 Oregon State University, Corvallis, OR
ABSTRACT- We examined denitrifying bacteria from wet soils and creek sediment in an agroecosystem in Oregon, USA that received inputs of nitrogen (N) fertilizer. Our objective was to determine variation in denitrifying community composition and activities across three adjacent habitats: a fertilized agricultural field planted to perennial ryegrass, a naturally vegetated riparian area, and creek sediment. Using acetylene inhibition, denitrifying enzyme and N2O-reductase activities were determined in short-term incubations of anaerobic slurries. A key gene in the denitrification pathway, N2O reductase (nosZ), served as a marker for denitrifiers. Mean denitrifying enzyme activity (DEA) was similar among habitats, ranging from 0.5 to 1.8 
g N per g dry soil per h. However, the ratio of N2O production, without acetylene, to DEA was substantially higher in riparian soil (0.64 ± 0.02; mean ± standard error, n = 12) than in agricultural soil (0.19 ± 0.02) or creek sediment (0.32 ± 0.03). Mean N2O-reductase activity ranged from 0.5 to 3.2 g N per g dry soil per h, with greater activity in agricultural soil than in riparian soil. Denitrifying community composition differed significantly among habitats based on nosZ T-RFLPs (Terminal-Restriction Fragment Length Polymorphisms). The creek sediment community was unique. Communities in the agricultural and riparian soil were more closely related but distinct. A number of unique nosZ genotypes were detected in creek sediment. Sequences of nosZ obtained from riparian soil were closely related to nosZ from Bradyrhizobium japonicum. Although nosZ distribution and N2O-reductase activity differed among habitats, relationships between activity and community composition appeared uncoupled across the agroecosystem.
Key words: diversity, nitrous oxide, microbial, denitrification