A potential reason for the limited success of phosphorus reduction efforts is the mobilization of legacy phosphorus accumulated from historical fertilizer applications. The new study reports the potential contribution of legacy phosphorus, defined as phosphorus from anthropogenic inputs before 1992, to total river phosphorus export for 143 river sites in the continental United States. The authors compared river phosphorus export to watershed agricultural phosphorus balance during 1992–2012. At more than two-thirds of the sites, phosphorus inputs from fertilizer and manure exceeded crop uptake and harvest removal, resulting in a surplus of phosphorus.

Legacy phosphorus contributions to river export could be clearly identified in 49 watersheds but could not be distinguished from large contemporary inputs in the remaining 94 watersheds, indicating that legacy contributions were likely small compared to contemporary phosphorus inputs.

The data reveal some success stories, however.  At 43 sites, about one-quarter of those studied, a reduction in the phosphorus surplus resulted in a decrease in phosphorus stream load.  Results also show that water-quality responses to changes in agricultural management are highly variable. Therefore, the most effective management actions will be system-specific and account for both the long-term effects of total historical phosphorus storage and reductions in contemporary surpluses.

The study, published in Proceedings of the National Academy of Sciences, provides the first comprehensive analysis of how loads of phosphorus in rivers respond to changes in agricultural management practices while also accounting for effects of legacy phosphorus.