Technical Abstract: Sequential fractionation is a common method used in evaluating the impacts of soil management practices on soil P distribution. However, to our knowledge, this method has not been used in investigating the effects of irrigation on the changes in soil P fractions. In this work, we measured sequentially-extracted P by deionized H2O, 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH and 1 M HCl in Caribou sandy loam soil samples from 10 potato fields under different three-year crop rotations both with and without irrigation. As inorganic fertilizer was applied to these fields, irrigation and rotation management practices mainly affected the distribution of inorganic P fractions, with little significant changes of organic P fractions. The impact of crop rotation was mainly expressed on H2O extractable P. Irrigation had greater influence on stable or recalcitrant P in NaOH, HCl, and residual fractions. Higher levels of NaOH extractable inorganic P were observed in soil from irrigated fields, while higher levels of HCl extractable P were observed in soils under rainfed management. Our data indicate that irrigation may eventually decrease P availability and runoff potential in these potato soils over the long term due to the partial transfer of P in the sink from the functional NaOH fraction to more stable HCl and residual fractions. Whereas information and knowledge derived from this study may shed light on the dynamics of soil P fractions for sustainable agricultural production, more field data from short- and long- term experiments are needed to confirm our observations.