Practices to reduce phosphorus (P) source bioavailability are gaining acceptance in the management of excessive P in manure and P-enriched soils although the environmental behavior of the immobilized P is largely unknown. Water-extractable dissolved and complexed P forms (i.e. bioactive P) and Mehlich-III P were determined during a 16-week soil incubation to elucidate mechanisms of stabilization and P extractability as affected by Ca and Fe-amendments in an Aridic Haploxeroll and an Oxyaquic Hapludult. The Fe additive reduced WEP by over 90% when applied at a rate of 0.18 mole kg^-1. However, potentially bioactive P can increase with time and that the use of Mehlich-III P may not appropriately reflect this potential to contribute P to the impairment of surface waters. Mehlich-III P levels in Fe-treated soils were reduced, after that remained unchanged up to 16 weeks. The ligand-based phytase-hydrolyzable P (PHP) fractionation method, however, revealed that the additives' effect was transitory; increasing previously insoluble inorganic EDTA-extractable P (EEP) was extractable and organic PHP was increasingly exchangeable and susceptible to enzymatic dephosphorylation over time to revert back to initial levels. Calcium carbonate amended at a liming rate to raise soil pH to near neutrality negated the environmental benefit of applying iron-rich P-immobilizing additives to both soils. The temporary suppression might resolve a short-term elevated soluble P condition, however, was not effective in mitigating the long-term risks of bioactive P losses from P-enriched soils.