The spatial distribution, magnitude and speciation of legacy P are challenging to quantify, making it a common blind spot in P biogeochemistry that has implications for nutrient management. Illinois has undergone drastic acceleration P fluxes driven by agricultural intensification in less than two centuries, making it a case study on legacy P. Drawing on agronomic P balances, soil testing trends, long-term field experiments, and historical soil archives, we will demonstrate a multifaceted approach that stands to provide insight on where, how much, and what type of legacy P exists at varying spatial scales (10 cm to 1000 km). Combining multiple approaches to the spatial distribution and magnitude of legacy P identifies shortcomings of single-approach methods and demonstrates their complementarity. Quantifying native soil P stocks and utilizing chronosequence approaches provides much-needed baseline measurements, which cannot be obtained by P balances, to better quantify legacy P magnitudes and speciation. Observed P speciation by XANES and sequential fractionations in soils with net P surpluses as well as deficits is consistent with hypothesized transformations of historical P inputs. Finally, we will discuss implications for management of the soil-water P cycle. Untangling the anthropogenic and edaphoclimatic interactions can help determine where and how much legacy P persists, and thus how humans may direct the anthropogenic P biogeochemical cycle.
