Tillage effects on denitrifier communities and nitrous oxide (N2O) emissions were mainly studied during the growing season. There is limited information for the non-growing season, especially in northern countries where winter has prolonged periods with sub-zero temperatures. The abundance and structure of the denitrifier community, denitrification gene expression and N2O emissions in fields under long-term tillage regimes [no-tillage (NT) vs conventional tillage (CT)] were assessed during two consecutive winters. NT exerted a positive effect on nirK and nosZ denitrifier abundance in both winters compared to CT. Moreover, the two contrasting managements had an opposite influence on nirK and nirS RNA/DNA ratios. Tillage management resulted in different denitrifier community structures during both winters. Seasonal changes were observed in the abundance and the structure of denitrifiers. Interestingly, the RNA/DNA ratios were greater in the coldest months for nirK, nirS and nosZ. N2O emissions were not influenced by management but changed over time with two orders of magnitude increase in the coldest month of both winters. In winter of 2009-2010, emissions were mainly as N2O, whereas in 2010-2011, when soil temperatures were milder due to persistent snow cover, most emissions were as dinitrogen. Results indicated that tillage management during the growing season induced differences in denitrifier community structure that persisted during winter. However, management did not affect the active cold-adapted community structure.
Keywords: Denitrifiers; Frozen versus unfrozen soils; Greenhouse gas emission; Tillage.