Irrigation-Induced Changes in Phosphorus Fractions of Caribou Sandy Loam Soil Under Different Potato Cropping Systems

Sequential fractionation is a common method used in evaluating the impacts of soil management practices on soil phosphorus (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 sodium bicarbonate (pH 8.5), 0.1 M sodium hydroxide (NaOH), and 1 M hydrochloric acid (HCl) in Caribou sandy loam soil samples from 10 potato fields under different 3-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, but had no significant changes of organic P fractions. The impact of crop rotation was mainly reflected by 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 rainfed fields, whereas higher levels of HCl-extractable P were observed in soils under irrigated management. Our data indicate that irrigation may eventually decrease P availability and runoff potential in these potato soils over the long term because of the partial transfer of P in the sink from the active NaOH fraction to more stable HCl and residual fractions. Whereas information and knowledge derived from this study may shed some light on the transformation mechanism of soil P fractions for sustainable agricultural production, more field data from short- and long-term experiments are needed to confirm our observations.