Water and Nitrate Loss Partitioning through Surface Runoff and Tile Drainage from a Long-Term (56 year) Fertilization and Crop Rotation Study on a Clay Loam Soil.

Loss of nitrate derived from nitrogen (N) fertilizers applied to field crops constitutes both an economic loss to the producers and a source of pollution to groundwater and surface waterways. Surface runoff and tile drainage (TD) are two pathways in which nitrate can be lost from the system.  The magnitude of N loss from either pathway is influenced by crop type, fertilizer regime, crop rotation, tillage and soil physical parameters. Monitoring of surface and tile nitrate levels was initiated in 2008 on a long-term experiment established in Southwestern Ontario in 1959 which consists of crop rotations including continuous corn (CC), continuous bluegrass and 4-year corn rotation (RC) involving corn-oat-alfalfa-alfalfa. In addition each rotation treatment has either received fertilized annually or has never received fertilizer over the 56-year period. Continuous monitoring between 2008 and 2015 showed a drastic difference in flow partitioning between fertilized CC and fertilized RC, with TD accounting for 21% of water loss in CC, whereas 75% of the water loss was through TD in RC. Each treatment that received fertilization displayed a greater proportion of water being lost through TD than treatments that were unfertilized. Nitrate loss partitioning followed a similar manner, highlighted in the 2008-2009 season which had 32% of the nitrate loss from TD in the CC treatment as compared to 90% nitrate loss via TD in RC. Significantly greater losses of nitrate occurred in the RC in the 2008-2009 season at 40 kg N ha^-1 as compared to the CC at 22 kg N ha^-1 due in part to the plowdown of the alfalfa in the RC treatment prior to planting. Our results highlight the importance of monitoring surface and tile nitrate losses, especially on poorly drained soils and they demonstrate the impacts of fertilization and crop rotation on flow partitioning and nitrate loss.