TIME DEPENDENT PHOSPHORUS EXTRACTABILITY IN CALCIUM- AND IRON-TREATED HIGH-PHOSPHORUS SOILS

Authors: Dao, Thanh; Codling, Eton; Schwartz, Robert

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/17/2005
Publication Date: 11/1/2005
Citation: Dao, T.H., Codling, E.E., Schwartz, R.C. 2005. Time dependent phosphorus extractability in calcium- and iron-treated high-phosphorus soils. Soil Science. 170:810-821.

Interpretive Summary: Reducing soluble and particulate nutrient discharges from nutrient-laden fields depended heavily upon soil erosion control practices. Additives such as gypsum, alum, and municipal by-products rich in amorphous aluminum or iron compounds such as drinking water treatment residuals or coal-combustion ashes can lower water and plant-extractable phosphorus (P) in manure and P-enriched soils and prevent loss of these forms of P into the environment. Such management practices are gaining acceptance in the management of excessive levels of P in manure and P-enriched soils although the environmental behavior of the immobilized P is largely unknown. We conducted an enzymatic assay and soil P fractionation study to determine the effects of calcium as lime and iron amendments on the extractable P from two high-P soils. The iron additive decreased soil water-extractable P and a soil test P levels in iron-treated soils to maintain the suppression of these P forms for up to 16 weeks. Liming to raise soil pH to near neutrality increased soluble bioactive P. The phytase assay for organic P, however, revealed that the iron additives' effect was transient; organic P becomes increasingly exchangeable over time to revert back to their initial levels. The temporary suppression may resolve a short term elevated soluble P condition; however, the Fe3+ treatment did not reduce the potential solubilization of the inorganic and organic reserves and, in the long term, is ineffective in reducing risks of P losses from P-enriched soils.

Technical Abstract: Practices to reduce soluble P source bioavailability are gaining acceptance in the management of excessive P in manure and P-enriched soils although the environmental behavior of immobilized phosphorus (P) is largely unknown. An enzymatic hydrolysis and soil P fractionation study was conducted to elucidate mechanisms of stabilization and relative P extractability as affected by Ca2+ and Fe3+-amendments in a Burch loam and a Thurmont gravelly loam. Water-extractable dissolved and complexed P forms (i.e. bioactive P) and Mehlich 3 P were determined at 2-wk intervals during soil incubation at 26 oC. The Fe3+ additive reduced soil water-extractable P (WEP) by over 89% when applied at a rate of 0.1 mole kg-1. Mehlich 3 P levels in Fe3+-treated soils averaged 77 and 88% lower than those of the controls, and remained unchanged up to 16 wks. The CaCO3 amended at a liming rate to raise soil pH to near neutrality increased WEP and all other bioactive P fractions. The ligand-based phytase-hydrolyzable P (PHP) fractionation method, however, revealed that the additives' effect was more transitory; increasing organic PHP was exchangeable and susceptible to enzymatic dephosphorylation 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 (EEP). The temporary suppression may resolve a short term elevated soluble P condition; however, the Fe3+ treatment did not reduce complexed EEP and PHP mobilization and, in the long term, is ineffective in mitigating risks of P losses from P-enriched soils.