Do fields with elevated soil test phosphorus disproportionately contribute to Western Lake Erie Basin dissolved phosphorus loading?

Crop production fields with elevated soil test phosphorus (STP) concentrations are often purported as the primary contributor to dissolved reactive phosphorus (DRP) loads in the Western Lake Erie Basin (WLEB). Annual DRP loading from 41 fields categorized into low/medium (<75 mg kg⁻¹), high (75–150 mg kg⁻¹), and very high (>300 mg kg⁻¹) STP were upscaled using three scenarios (i.e., 5%, 10%, and 20% of fields contained STP > 75 mg kg⁻¹). An estimated 34% (±10%) of DRP loading originated from elevated STP fields when assuming 10% of the WLEB had STP > 75 mg kg⁻¹. Assuming 5% of the WLEB had STP > 75 mg kg⁻¹, the contribution reduced to 17% (±5%). Elevated STP fields disproportionately contribute to WLEB DRP loads but are not the dominant source leading to harmful and nuisance algal blooms. Targeted upland (e.g., nutrient management) and edge-of-field (e.g., P removal structures) practices are needed across the broad spectrum of STP concentrations to meet loading goals.

Core Ideas

• Elevated soil test phosphorus (STP: >75 mg kg⁻¹) disproportionately influenced dissolved reactive phosphorus (DRP) loads.
• A majority of the DRP load in the Western Lake Erie Basin (WLEB) originated in fields with STP <75 mg kg⁻¹.
• The low occurrence of very high STP (>300 mg kg⁻¹) fields limited their contribution to WLEB DRP loads.
• Targeted upland and edge-of-field conservation practices are required across a broad spectrum of STP.