Quantifying phosphorus loads from legacy-phosphorus fields

Abstract

Agriculture is a major contributor to eutrophication across the world. Such is the case for Lake Erie where P loads support recurrent harmful cyanobacterial blooms. To mitigate these blooms, targets have been set for total and dissolved reactive P (TP/DRP) loads during the annual and March-July periods. To accelerate progress towards meeting these goals, a targeted management approach aimed at the fields with the greatest P loads has been suggested. A public–private partnership recruited legacy-P fields to quantify P loads in runoff by leveraging the positive relationship between soil test P (STP) and discharge P concentrations. Legacy-P fields were previously defined as having one or more zones with STP >100 ppm (two-fold above agronomic needs); all others were termed agronomic-P fields. Edge-of-field monitoring was performed during 2021 on 11 legacy-P and 13 agronomic-P fields (total n = 24). Legacy-P fields had 2-fold greater March-July DRP loads but only 0.5-fold greater annual TP loads, though both were statistically nonsignificant effects (g≈0.5). Effects were smaller (g≈0.2) for March-July TP and annual DRP. Over half of the legacy-P fields lacked meaningful (<1% of total) surface loads with little to no observed runoff. Further analysis revealed that legacy-P fields had a stronger effect on subsurface discharge, with a statistically significant effect (g = 0.81, p = 0.05) on the March-July DRP concentrations. Statistical significance in this study was obscured due to a large degree of variance and limited sample size, however, the differences reported here could have practical importance to managers working to address eutrophication across Lake Erie and other watersheds.