Phosphorus Stratification and Edge of Field Phosphorus Losses in the Western Lake Erie Basin.

Harmful and nuisance algal blooms resulting from excess phosphorus (P) have placed agriculture in the spotlight of the water quality debate. Sixty-eight site years of P loading data (surface runoff and tile flow) from 36 fields in Ohio were used to investigate phosphorus stratification in the Western Lake Erie Basin. The sites were categorized by tri state (Michigan, Indiana and Ohio) fertilizer recommendations and by a unit-less ‘P stratification’ value calculated by dividing the Mehlich-3 phosphorus (M3P) concentration in the top 0-5 cm of soil by the M3P concentration in the lower 5-20 cm. The fields were arbitrarily categorized as low (0-1), medium (1-3), and high (3+) P stratification based on their unit-less P stratification value. The majority of the fields (30) could be classified as “medium” stratification with 2 fields categorized in ‘low’ and ‘high’ stratification respectively. Next, regression analysis was conducted for surface runoff and tile flow versus soil test P (STP) at different depths (0-2.5 cm, 2.5-5, 5-10, and 10-20). The strongest relationship (best linear regression fit) corresponded between DRP and TP concentrations in surface runoff and M3P STP in the top three soil sections, followed by DRP concentrations in tile drainage and M3P STP concentrations in the top 0-2.5 cm of soil. Our findings provide further evidence of the relationship between P concentrations measured in surface runoff and STP concentrations, but there is also evidence of a relationship with DRP concentrations measured in tile flow. Additional change point analysis indicated that STP above 39 M3P increased the risk of DRP losses from tile flow.