Possible Causes of Soluble Phosphorus Pollution in Lake Erie Watershed and in Ohio.

Since 1995, Lake Erie soluble reactive P (SRP) has raised 2.5 fold from 0.04 mg L^-1 to above 0.1 mg L^-1.  Possible causes include higher intensity, increased number, and longer duration precipitation events; accelerated surface runoff and subsurface drainage; highly soluble P  fertilizer; more corn and soybean acres due to higher crop prices; bigger  farms; and increased commercial surface applications. Producers are not over applying P because fertilizer sales and average soil test values have declined. A no-till misconception is that soil macropores created by earthworms/root channels are a major path for SRP in subsurface drains.  Census data indicates that less than 6% of Lake Erie’s soils are in long-term no-till corn, although 65-70% of soybeans are no-tilled, indicating rotational tillage is a dominant practice.  Producers started using vertical tillage in the 1990’s to chop corn stalks and to prepare soybean stubble as a stale seed bed for corn.  With increased P surface applications in late fall/early winter, more soluble P fertilizer, increased drainage, and increased corn and soybean acres; P saturation is occurring in the top 7 cm of soil, exasperated by vertical tillage and a compacted soil layer (1.7 to 1.9 g cm^-3  <7 cm).   Soils around Lake Erie basin high in ferric iron (Fe³⁺) are converting to ferrous iron (Fe²⁺), resulting in the release of SRP in surface runoff or preferential flow and higher SRP in subsurface drains due to decreased water infiltration and saturated anaerobic soil conditions from annual cycles of higher and intense precipitation events and melting of snow followed by drying of soil. Freezing-thawing processes in late winter/early spring may contribute to SRP release from decomposing crop residue especially soybeans which is high in P.  These dominant factors explain why Lake Erie SRP has risen while total P in soil-water systems have decreased.