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.