Time-Dependent Phosphorus Extractability in Calcium- And Iron-Treated High-Phosphorus Soils

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. An enzymatic P fractionation study was conducted to elucidate mechanisms of stabilization and relative P extractability as affected by Ca and Fe amendments in a Burch loam (Aridic Haploxeroll) and a Thurmont gravelly loam (Oxyaquic Hapludult). Water-extractable dissolved and complexed P forms (i.e., bioactive P) and Mehlich 3 P were determined at 2-week intervals during soil incubation at 26 °C. The Fe additive reduced water-extractable P by more than 90% when applied at a rate of 0.18 mol kg−1. However, potentially bioactive P can increase with time and the use of Mehlich 3 P may not appropriately reflect this potential to contribute P to the impairment of aquatic ecosystems. Mehlich 3 P was reduced and after that remained unchanged up to 16 weeks in Fe-treated soils. The phytase-hydrolyzable P (PHP) method, however, showed that the additives' effect was transitory; increasing organic PHP was susceptible to enzymatic hydrolysis over time to revert back to initial PHP levels. Diffusion-limited and ligand exchange processes also increased desorption of previously water-insoluble inorganic EDTA-extractable P. Calcium carbonate negated the environmental benefit of adding the Fe additive to both soils. The temporary suppression might resolve a short term elevated soluble P condition; however, the Fe additive did not reduce complexed EDTA-extractable P and PHP solubilization, and was ineffective in mitigating the long-term risks of bioactive P losses from P-enriched soils.