Toward a transdisciplinary and unifying definition of legacy phosphorus.

Legacy phosphorus (P) is a concept advanced by Dr. Andrew Sharpley and colleagues that was originally applied to the persistence of anthropogenic signatures in watersheds, and it has since been adopted in a diversity of settings to help guide the science and management of P. Following Sharpley's example to develop consensus-based science, we considered contrasting perspectives on legacy P and defined legacy P as those stores within the environment that arise from historic human activity excluding "natural" or "background" geogenic sources. Legacy P is not restricted to one system or setting; it may reside in soils, sediments, biota, and water bodies. Legacy P has been estimated by fluxes (inputs minus outputs of P to a system) or, equivalently, by mass stocks (total minus geogenic). Because the origin of P in the environment cannot currently be directly quantified, we recommend that researchers report "total P" to track wider watershed P stocks and fluxes of P that include legacy P. We recognize that the definition of legacy P will continue to evolve as we continue to work toward consensus. Ultimately, the final definition of legacy P has consequences for the implementation and success of regulatory and voluntary strategies for legacy P management in agricultural systems. We support continued progress toward a consensus-backed, research-grounded definition for legacy P that is widely applicable yet useful for guiding management and policy.