Technical Abstract: Elevated concentrations of P in surface soils increases the risk of P transport by erosion. Particulate nutrient transport from agricultural fields contributes to water quality degradation; however, insufficient data exist about potential bioactive P transport from erosion. Traditional measures of soil P such as total P or soil test P (STP) do not adequately assess the risk of water quality degradation due to particulate P because these measures do not necessarily reflect soil P bioactivity. To better understand the risk of water quality degradation due to sediment associated P, we quantified the bioactive P fractions in five aggregate size classes from conventional no-till (NT) and chisel till (CT) systems as well as a tilled organic system. Cropping system did not affect the concentration of bioactive P fractions in whole soils. However, aggregate size did affect bioactive P fraction concentrations. In general, macroaggregates had greater concentrations of bioactive P than did microaggregates across all cropping systems; the <0.05 mm size class had the lowest concentrations of bioactive P. To better understand the dynamics of these bioactive P fractions over time, we conducted an incubation study on the aggregates of the NT system and quantified the bioactive P changes over a 56 d period. Water extractable P and EDTA-P concentrations did not change significantly during the incubation. However, EDTA-PHP concentrations increased up to two-fold during the 56 d incubation. Using soil bioactive P fraction measurements from easily erodible aggregate sizes should aid in water quality degradation risk assessments. However our research demonstrates the need to quantify the dynamics of bioactive P fractions in order to better predict water quality degradation from soil erosion.