Predicting Accumulation and Potential Edge-of-Field Loss of Phosphorus to Surface Water from Diverse Ecosystems

Phosphorus (P) is an important nutrient essential for agricultural production, but it is highly reactive, leading to its soil accumulation and making it susceptible to environmental impact footprints. The goal of our study was to determine the critical threshold values of both soluble reactive P (SRP) and oxalate-extracted P (Ox-P) to predict soil P accumulation and its susceptibility to edge-of-field loss. Composite soils were collected from geo-referenced ecosystems within the Lake Erie drainage basin under agriculture in northwestern Ohio, USA. Soils were analyzed for SRP, Ox-P, Fe, and Al concentrations to calculate P sorption capacity, P saturation ratio, degree of P saturation (DPS), and P storage capacity (SPSC). A threshold P saturation ratio of 0.12 (~ 24% DPS), corresponding to 2.4 mg SRP/kg (equivalent Ox-P), was determined to calculate SPSC for predicting the risk of SRP accumulation. A significant relationship between the SPSC and SRP suggested that soils under all the agroecosystems had accumulated SRP compared to the forest. Surface soils (0–10 cm depth) under tilled, chemically fertilized, and organically managed corn (Zea mays)-soybean (Glycine max (L) Merr.) rotations, including those treated with chicken and dairy manure, exhibited excessive SRP accumulation, making them susceptible to edge-of-field losses. While the soils at 10–20 cm depth were acting as transitional, the deeper soils (20–30 cm depth) still acted as a net sink. When accounting for bulk density to calculate SPSC stocks, it showed that surface soils across the agroecosystems were saturated with 148 to 240 kg SRP/ha and were susceptible to edge-of-field loss to the water systems. In conclusion, we suggest that SPSC could be used as an early indicator to predict the risk of SRP accumulation and its potential edge-of-field loss to Lake Erie from agroecosystems.