Muddied Waters: The Use of “Residual” And “Legacy” Phosphorus

Phosphorus (P) inputs to the biosphere have quadrupled in less than a century due to intensification of rock phosphate mining and the use of P fertilizers for crop production. Accumulation of P in soils can increase P transfers across the soil-water continuum that impair aquatic ecosystem function and water resource quality for society. However, what this accumulated P is called, and subsequent connotations of magnitude versus mechanism at pedon versus watershed scale, varies in the literature. We argue that the two commonly used terms of "residual" and "legacy" P, though often used interchangeably, hold distinct meanings and connotations. Tracing the historical origins and trajectories of these terms reveals that "residual P" refers to the magnitude of fertilizer P that remains in the soil after crop harvest, whereas "legacy P" refers to the mechanism of P transfer across the watershed and its long-term impacts on water quality. The use of "legacy P" in many cases refers to the residuality of anthropogenic P inputs, and thus should be "residual P". We recommend that the term "residual P" be used when referring to the accumulation of P in soils under agricultural management from past inputs, and the term "legacy P" be used when referring to the transfer of P within watersheds. The intentional and thus consistent use of residual versus legacy P stands to provide important nuance in the environmental sciences and overlapping fields of agronomy and biogeochemistry.