Divergence Between Long‐Term and Event‐Scale Nitrate Export Patterns

Abstract

Abstract The mechanisms driving catchment nitrogen storage and release operate at multiple spatiotemporal scales. Consequently, analyses grounded in different observational timescales can yield discrepant interpretations of underlying mechanisms. To assess the consistency of nitrate export patterns between event‐ and inter‐annual scales, we evaluated multiple years of high‐frequency observations of nitrate concentrations (C) and discharge (Q) including 3,480 discrete discharge events from 28 dominantly agricultural catchments. We observed consistent and often drastic divergence between long‐term and median event‐specific C‐Q patterns. Most catchments showed long‐term enrichment patterns (positive C‐Q slope), but events were, on average, more chemostatic (close‐to‐zero C‐Q slopes). C‐Q slope variability was high for small events but decreased with event magnitude, approaching chemostatic patterns during the largest storms, yielding compelling evidence against nitrate source limitation. We conclude that nitrate export patterns observed at different temporal scales and event magnitudes are controlled by different processes, therefore embedding complementary information.

Keywords

• C‐Q relationships
• nitrate
• nitrogen
• catchment
• time scales
• high‐frequency data