Water Quality Remediation Faces Unprecedented Challenges from “Legacy Phosphorus”

T he past three decades have witnessed a dramatic increase in recognition of the role of diffuse phosphorus (P) pollution in eutrophication of surface waters.Agricultural conservation and nutrient management programs have been very successful at reducing P losses in runoff at the edge-offield, but there has often been disappointingly little improvement in downstream water quality and ecology. 1 Growing evidence indicates that a major reason for this is the chronic release of P from "legacy P" stores, which have accumulated in watersheds and water bodies. [2]3][4] As a result, we face unprecedented challenges in meeting water quality targets, given that P legacies from past land management may continue to impair future water quality, over time scales of decades, and perhaps longer (Figure 1).Agricultural strategies across Europe and the U.S. are currently undergoing fundamental reevaluation, with reform of the Common Agricultural Policy and revising how conservation programs operate in the new Farm Bill, respectively.This is focusing scrutiny on the costeffectiveness of conservation measures and compliance standards, and raises the following questions concerning legacy P: