Phosphorus Legacy: Overcoming the Effects of Past Management Practices to Mitigate Future Water Quality Impairment

TLDR

Water‑quality responses to conservation measures are often slower and smaller than expected because legacy phosphorus from past land‑management practices masks reductions in edge‑of‑field losses, making it essential to account for these legacy sinks to correctly apportion sources and design effective watershed remediation. This study investigates the watershed‑scale drivers of legacy phosphorus, focusing on the physical cascades and biogeochemical spirals that transport P from soils to rivers and lakes. The authors analyze how surface and subsurface flow pathways create physical cascades and biogeochemical spirals that sustain legacy P, using case studies to illustrate the processes and timescales involved. The case studies reveal that legacy phosphorus continues to contribute to receiving waters over long timescales, undermining restoration efforts, and suggest management strategies to mitigate these legacies in future conservation programs.

Abstract

The water quality response to implementation of conservation measures across watersheds has been slower and smaller than expected. This has led many to question the efficacy of these measures and to call for stricter land and nutrient management strategies. In many cases, this limited response has been due to the legacies of past management activities, where sinks and stores of P along the land-freshwater continuum mask the effects of reductions in edge-of-field losses of P. Accounting for legacy P along this continuum is important to correctly apportion sources and to develop successful watershed remediation. In this study, we examined the drivers of legacy P at the watershed scale, specifically in relation to the physical cascades and biogeochemical spirals of P along the continuum from soils to rivers and lakes and via surface and subsurface flow pathways. Terrestrial P legacies encompass prior nutrient and land management activities that have built up soil P to levels that exceed crop requirements and modified the connectivity between terrestrial P sources and fluvial transport. River and lake P legacies encompass a range of processes that control retention and remobilization of P, and these are linked to water and sediment residence times. We provide case studies that highlight the major processes and varying timescales across which legacy P continues to contribute P to receiving waters and undermine restoration efforts, and we discuss how these P legacies could be managed in future conservation programs.