Nitrogen dynamics and nitrogen: Phosphorus stoichiometry in cold region agricultural streams.

Cold agricultural regions are getting warmer and experiencing shifts in precipitation patterns, which affect hydrological transport of nutrients through reduced snowpack and higher annual proportions of summer rainfall. Previous work has demonstrated that the timing of phosphorus concentrations is regionally coherent in streams of the northern Great Plains, suggesting a common climatic driver. There has been less investigation into patterns of stream nitrogen, despite its importance for water quality. Using high-frequency water quality data collected over 6 years from three southern Manitoba agricultural streams, the goal of this research was to investigate seasonal patterns in nitrogen and phosphorus concentrations and the resultant impacts of these patterns on nitrogen:phosphorus stoichiometry. In the spring, high concentrations of inorganic nitrogen were associated with snowmelt runoff, while summer nitrogen was dominated by organic forms; inorganic nitrogen concentrations remained consistently low in the summer, suggesting increased biological nitrogen transformation and nitrogen removal. Relationships between nitrogen concentration and discharge showed generally weak model fits (r2 values for significant relationships ranging from 0.33 to 0.48), and the strength and direction of model fits differed among streams, seasons, and forms of N. Dissolved organic nitrogen concentrations were strongly associated with dissolved organic carbon. Nitrogen to phosphorus ratios varied among streams but were significantly lower during summer storm events (p < 0.0001). These results suggest that climate-driven shifts in temperature and precipitation may negatively impact downstream water quality in this region. This article is protected by copyright. All rights reserved.