Phosphorus speciation and sorption-desorption characteristics in heavily manured soils

Managing heavily manured soils for decreased P loss to waters requires improved understanding of the chemical and sorption–desorption characteristics of P in these soils. We used soils from agricultural fields receiving 8 yr of dairy, poultry, swine manure or spent mushroom compost for the determination of P functional groups in NaOH-EDTA extracts by solution 31P nuclear magnetic resonance (NMR) spectroscopy, degree of P saturation (DPS), and P sorption–desorption isotherms. The 31P NMR results show that inorganic orthophosphate was the primary form of P in manure treated (79–93% of total extract P) and untreated soils (33–71%). Pyrophosphate and phosphate monoesters were identified in all soils, whereas phosphate diesters were present in small proportions (<3%) in only a few soils.

Polyphosphate, a more condensed form of inorganic P, was present in seven out of nine manured soils (9–47 mg P kg–1, <2%) but absent in untreated soils. Concentrations of inositol hexakisphosphate (IHP), mostly myo-IHP plus some scyllo-IHP, were similar in manured soils (52–116 mg P kg–1, 2–8%) and untreated soils (43–137 mg P kg–1, 6–22%), suggesting a lack of IHP accumulation despite long-term manure applications, including poultry manures that are typically rich in IHP. Most of the treated soils had DPS  80 to 90% compared with 11 to 33% for the untreated samples. Results from P sorption isotherms showed that potential P release was 3 to 30 times greater from treated than untreated soils. The lack of IHP accumulation in soils receiving long-term manure applications implies that manure-derived IHP may not be biologically and environmentally benign.