Denitrification Rates Under Typical Dairy Rotations in Atlantic Canada.

High denitrification rates from agricultural land under the cool, humid climate of Atlantic Canada constitute a significant nitrogen (N) loss pathway that contributes to poor utilization of applied N. Higher rates have been reported when inorganic fertilizer N is substituted with an organic N source such as livestock manure. This six year study on a typical tile-drained soil examined the effect of environmental parameters and three principle land management practices: manure application, tillage and crop rotation on growing season (GS) denitrification rates. Treatments were a perennial hayfield rotation (PH), a corn-soybean-wheat rotation with spring tillage (CSW-T) and without tillage (CSW-NT), using liquid dairy manure as principle N source. Mean GS DDR’s ranged between 0.9 and 27.0 g N ha^-1 d^-1 across all treatments and growing seasons.  There was no significant difference in mean DDR’s between CSW-T and NT rotations in any of the six years with the exception of the 2011 growing season, when CSW-T was significantly higher than CSW-NT. However, when year effects were combined, T or NT in the CSW rotation did not give rise to a significant difference in mean GS DDR’s over the study period. Mean DDR’s of the PH rotation were generally lower than those of the CSW rotations and significantly lower than CSW-NT in 2007 and 2011 and CSW-T in 2008 and 2011. When the overall means for the six-year study period were considered, the mean GS DDR of the PH rotation (4.4 g N ha^-1 d^-1) was significantly lower than that of  CSW-T and CSW-NT (8.1 and 7.6 g N ha^-1 d^-1 respectively). When treatments and growing seasons data were combined, a positive relationship between water-filled pore space (WFPS) and DDR indicated a threshold of approximately 40 % WFPS for onset of significant denitrification. Similarly, the relationship between soil nitrate levels and denitrification rates in the population data set demonstrated that NO₃^- began to limit DDR when the soil levels dropped below 2 - 5 mg N kg^-1. Environmental parameters were more often the drivers of denitrification in all three crop rotations than were land management practices. The study suggests a primary effect of WFPS on denitrification rates, a secondary effect of O₂ consumption as reflected by soil respiration, and that soil nitrate is limiting only at relatively low concentrations. The findings also point towards the importance of tile drainage as a land management practice under Atlantic Canada's cool humid climate.