Tile Drainage Losses of Nitrogen and Phosphorus from Manure-Fed Dairy Rotations in Atlantic Canada.

Nitrogen (N) and phosphorus (P) are plant essential macro-elements that are required for crop production at application rates which frequently give rise to environmental concerns. Poor N utilization efficiency in cropping systems is a leading cause of nitrate N contamination of surface and ground waters under Atlantic Canada’s cool humid climate. In addition, while surface losses of phosphorus (P) are known to occur on soils receiving repeated applications of manure, there is little information available on soluble P loading of tile drainage systems in this region. The effect of tillage practices in a perennial hayfield (PH) and corn-soybean-spring wheat (CSW) rotations on growing season (GS) and non-growing season (NGS) tile drainage volumes and mean, flow-weighted nitrate and ortho-P concentrations and loads was investigated over a six year period. Spring applied liquid dairy manure (LDM) was used as the principle N source. Treatments included (i) PH rotation established and re-established with tillage (T) practices; (ii) CSW-T rotation; (iii) CSW-NT (no-till). Non-GS drainage volumes were significantly higher when compared with GS, averaging 425 and 225 mm over the study period, respectively. Non-GS nitrate concentrations were generally 1.5 to 3.0 mg NO₃-N L^-1 lower when compared with their GS counterparts. When rotations were compared, nitrate concentrations under PH were significantly lower (factor of ~ 2.5) when compared with CSW. No consistent differences or trends between GS and NGS nitrate concentrations of the two levels of tillage in the CSW rotation were observed. Nitrogen loading losses ranged between 2.1 and 58.5 kg N ha^-1 and 4.9 and 63.0 kg N ha^-1 for GS and NGS, respectively. Non-GS loads were approximately 3 times higher than their GS counterparts. The lower N utilization efficiency of the CSW rotation was reflected in the threefold higher loading losses when compared with PH. An exception was the re-establishment year for the latter, when this trend was reversed. There were no consistent differences between mean GS and non-GS ortho-P concentrations  which ranged between 10 – 80 µg PO₄-P L^-1. When rotations were compared, ortho-P concentrations under PH were not significantly lower than concentrations under CSW-T and NT. There were also no differences or trends between GS and NGS ortho-P concentrations for the two levels of tillage in the CSW rotation. Ortho-P loading losses ranged between 0.05 and 0.35 kg PO₄-P ha^-1. Non-GS loads were approximately twofold greater than those of GS. There were also no consistent or significant differences in PO4-P loading between PH and CSW rotations or between CSW-T and CSW-NT over the six year study period. These observations demonstrate that N loading losses to tile drainage in this region are greatly influenced by land management practices such as manure application, tillage and crop rotation, but that loading losses for any given season do not necessarily reflect the biological response time of the manure-soil system. Conversely, these land management practices do not appear to have a large impact on ortho-P concentrations and loading of tile drainage systems.