Denitrification, Nitrous Oxide Emissions and Denitrifier Abundance in Cold and Freezing Soils Under Different Agricultural Management Practices.

Agricultural management practices change soil organic carbon (C) and nitrogen (N) concentrations.  Limited information is available on how agricultural management practices may influence the dynamics of denitrifier community, N₂O emissions and denitrification during the winter period.  Our objective was to study the effect of agricultural management practices on denitrifier community abundance, denitrification and N₂O emissions during winter in Eastern Canada (New Brunswick (NB) and Québec (QC)).   Crop residues with contrasting C:N ratios (barley vs red clover) were applied at the NB site, while various N sources (mineral N, dairy cattle or poultry manures) were compared at the QC site. The sites were sampled (0-7.5 cm) five times overwinter between November and April of 2009.  Average soil temperature at a depth of 5 cm ranged from -0.8 to 3.5^oC in NB and from -0.4 to 1.3^oC in the QC site.  There was no significant difference in denitrification and N₂O emissions between treatments at any sites.  The highest denitrification and N₂O emission rates were measured in February, with 6.69x10⁴ and 4.30x10⁴ mg N₂O-N ha^-1 d^-1, respectively; rates at the other dates were lower with an average across treatments of 468 and 84 mg N₂O-N ha^-1 d^-1, respectively. Similar dynamics were found at the QC site as denitrification and N₂O emission rates increased by an average of 467 and 203-fold, respectively, in January and February compared to other sampling dates. These preliminary results suggest that agricultural practices did not influence denitrification and N₂O emissions during the winter period at the selected sites.  However, denitrification and N₂O emissions were stimulated during freezing periods. Results on denitrifier community abundance and their possible relation with ecosystem N processes are pending.