Inferring Legacy P Sources from Plot and Hillslope Runoff Data.

Differentiating between various sources of phosphorus in runoff is key to developing successful strategies that minimize off-site losses of phosphorus from agriculture. Even as new techniques emerge to precisely discriminate between sources of phosphorus in agricultural landscapes, there is value in examining other techniques used to infer phosphorus sources. We review plot, field and hillslope studies in the Valley and Ridge and Coastal Plain Provinces of the Chesapeake Bay Region, with an eye to differentiating between “legacy phosphorus” and other, more recent sources of phosphorus in runoff. Hillslope, runoff plot, monolith lysimeter and small watershed monitoring all provide different insights into the role of legacy phosphorus. Often, signatures of legacy and recent phosphorus sources can be inferred with traditional techniques, such as end-member mixing analysis. However, the value of these techniques often comes with the questions that they raise, helping to generate new hypotheses on processes, pathways and sources of legacy phosphorus.  We reflect upon the advantages and shortcomings of traditional approaches to identifying legacy phosphorus sources.