Phosphorus drawdown management reduced soil phosphorus and edge-of-field phosphorus concentrations

Phosphorus (P) from historic P applications stored in agricultural soils can be drawn down by harvesting crops without fully replacing harvested P with P fertilizer, thereby potentially reducing a source of P loss to surface waterways that impairs water quality. Soil P drawdown research has typically consisted of small plot studies, and the applicability of these findings to the field scale remains uncertain. Additionally, soil P drawdown rate studies typically do not quantify the potential water quality benefits. In this study, soil P drawdown rates and edge-of-field P concentrations were measured for 13 years on two commercially farmed crop fields in Ohio with initial average soil test P (STP) measured by Mehlich-3 of 450 and 530 mg kg−1 (0 to 20 cm depth). Drawdown management resulted in declines in soil P in both fields that were fit with exponential decay functions that estimated soil P drawdown rates at 5% per year for STP and 4% per year for water-extractable P (WEP). Soil P drawdown rates were similar at 0 to 5, 5 to 15, and 15 to 30 cm depths but somewhat greater for the 30 to 50 cm depth. Water quality measurements indicated that annual flow-weighted dissolved reactive P (DRP) and total P (TP) concentrations trended down for both surface runoff and tile drainage water. In each field, one of the four combinations of loss pathway and P form had statistically significant linear trends: DRP declined by 0.07 mg L−1 y−1 in tile drainage from one field, and TP in surface runoff declined by 0.17 mg L−1 y−1 from the other field. These results clearly show that commercial crop production without P fertilization resulted in significant reductions in soil P concentrations that were in turn reflected in edge-of-field P concentration reductions. However, the annual rates of change in both soil and edge-of-field P concentrations were relatively small, indicating that decades would be required to meet soil P and water quality goals through drawdown alone on fields with elevated soil P.