Soil Microbial Communities Associated with Nitrous Oxide Fluxes Are Affected by Seasonal Variation and Tillage System.

In a 5-year study at our field site in Ontario, Canada, spring thaw accounted for up to 90% of the annual N₂O emission.  It was also shown that improved soil structure in no-till (NT) systems at this site led to reduced N₂O during spring-thaw compared to conventional tillage (CT), suggesting that NT might be an appropriate system in cold climates.  The current study examined whether changes in community structure of ammonia-oxidizers and denitrifiers was associated with these differences in measured N₂O fluxes.  A long-term field study was established in Elora, Ontario in 2000, and managed using CT or NT. Soil samples were taken for microbial community analysis at six time points throughout one year (2005/06). The diversity of the ammonia-oxidizing and denitrifying communities were assessed by PCR-denaturing gradient gel electrophoresis (DGGE) using primer pairs targeting the amoA, nirS and nirK genes. Intact soil cores were also sampled and taken to the lab to study the active communities of denitrifying bacteria using reverse-transcriptase PCR-DGGE during a simulated thaw event.  Results from the field study indicated that seasonal variation had the largest influence on the diversity of ammonia-oxidizer and denitrifier populations.  Where the diversity was the lowest in February, in frozen soils, and rapidly increased in March, immediately after a spring thaw associated N₂O flux event.  Tillage practice also affected the structure of the microbial community, however, this was dependent on sampling date.  The lab study confirmed that a soil thaw releasing N₂O was associated with increased diversity of active denitrifiers.