The Effect of Alfalfa on Subsurface Discharge and Nutrient Losses Mediated by Precipitation and Antecedent Moisture Conditions

ABSTRACT This study examines the effect of alfalfa ( Medicago sativa L .) on nitrogen (N) and phosphorus (P) loads in subsurface (tile) drainage across storm events using edge‐of‐field monitoring data from two paired‐field sites (A and B) with a before‐after‐control‐impact (BACI) experimental design, located in the northwest region of Ohio, United States. A k‐medians cluster analysis was used to classify 462 storm events at Site A and 684 storm events at Site B based on precipitation amount and antecedent moisture conditions (AMC), defined as the cumulative 7‐day precipitation prior to a storm event. Patterns of nutrient loss in tile drainage were compared between fields with alfalfa and fields with cash and cover crops using a difference‐in‐differences analysis across three identified storm event types: Dry storm events, wet storm events, and large storm events. Compared to the cash and cover crop rotations, alfalfa had the following effects on discharge and water quality: little to no reduction in subsurface discharge across all storm events at both sites; significant reduction of subsurface nitrate and total N loads across all storm event types at Site A (~200%–800% lower), but not at Site B; ~45% reduction of subsurface dissolved reactive P during large events at both sites; and 11% and 110% reduction of total P loads during large events at Sites A and B, respectively. The impact of alfalfa during large storm events is important given that most nutrient export occurs during these events.