Metal Controls on Forest Floor and B Horizon Extractable Phosphorus in Forested Watersheds.

Phosphorus (P) in acidic soils of forested watersheds originates from mineral apatite but is increasingly controlled by secondary aluminum (Al) and iron (Fe) as pedogenesis progresses.  Research is underway to determine the relationship between illuvial secondary Al and Fe phases in these subsoils and P availability.  We studied these relationships at eight watersheds in the US and Europe with varying parent materials, climates, vegetation, and histories of atmospheric acid deposition.  Soil chemical fractionation techniques were used to determine labile-P, Fe-P, Al-P and apatite-P phases.  The Al-P fraction was dominant in the B horizon of all watersheds with Al-P ranging from 68% to 82% of total extractable P.  Evidence across all watersheds suggested that O horizon labile-P was correlated (r²=.54) with B horizon total extractable Altot (Altot=sum of Al in all fractions) despite differing site characteristics.  There was a poor correlation between B horizon Altot and labile-P in the B horizon.  This suggests bioavailable subsoil P is controlled by Al that is expressed through biocycling of P to the surface soils.