Phosphorus forms and response to changes in pH in acid-sensitive soils on the Precambrian Shield

Soil acidification may explain declines in total phosphorus (TP) levels that have been observed in surface waters in central Ontario, Canada, but much of the research on phosphorus (P) mobility in pH manipulated soils has been performed at high P concentrations (i.e., >500 M). This study investigated P fractionation in acidic (pH=4.6) soils in south-central Ontario and relationships between soil pH and P sorption at relatively low P concentrations to test whether long-term declines in soil pH could have increased soil P sorption. Soils from three forested catchments that vary naturally in soil pH and outlet stream [TP] (0.1/0.4 M in 2008) had very similar soil P concentrations and distributions (Hedley fractionation). Only hydrochloric-acid extractable P (i.e., apatite) differed amongst catchments and was greatest at the catchment with the highest stream [TP]. The fraction of P present as labile/soluble P did not decline with pH as expected and experiments indicated that P sorption at P concentrations between 4.52 and 452.1 M was insensitive to manipulated solution pH. Soils were, however, able to sorb >90% of P added in sorption experiments at [P] 452.1 M. These results suggest that acidification-induced P sorption in upland soils has not contributed to observed decreases in surface water TP concentrations.