McNeill, Laura Mary

Eutrophication is an ongoing global problem and agriculture is an important non-point source of nutrient loading. Specifically, nitrogen (N) and phosphorus (P) losses from agricultural landscapes continue to drive water quality issues. In southern Ontario, agriculture has intensified in recent decades, with major expansions of cash crop production and extensive tile drainage (TD). Through intensive monitoring of 12 tile outlets draining operational fields under the conventional corn-soybean-wheat rotation, this study examined differences in measured and volume-weighted total P, total N, and nitrate-N concentrations and loads over 28 months (October 2020- April 2023) amongst crop covers and between growing (GS; May – September) and non-growing seasons (NGS; October – April). Nitrogen concentrations (i.e., TN and NO3-N) in TD eluent were consistently high both between seasons and were found to be significantly highest from winter wheat (WW) fields in the NGS, and corn fields in the GS. Volume-weighted TP concentrations were not significantly different either amongst crop covers or between seasons, although TP losses tended to be highest from the cover crop (CC) fields in the NGS. Differences in N and P losses between years and amongst crop covers were attributed to differences in legacy soil nutrients, the establishment and decomposition of over-winter cover crops, and physical soil properties. The results of this study can inform agricultural management by addressing the urgent need for improved information around the relationship between agricultural practices and nutrient losses, especially in the NGS.

Author Keywords: Best management practices, Crop rotation, Over-winter cover crops, Seasonality, Tile drainage, Water quality