Importance of Organic Phosphorus in Floodplain Soils From Northwestern VT as Revealed by Nuclear Magnetic Resonance Spectroscopy and Phosphatase Enzymes.

An improved understanding of organic P cycling and the factors affecting its bioavailability in agricultural soils will ultimately contribute to improved nutrient management strategies for P.  The susceptibility of soil organic P to hydrolysis (e.g., conversion of organic to inorganic P) varies widely depending on the nature of the P compounds, soil factors, and environmental conditions.  The objectives of this study were to: (i) determine the exact forms of P extracted from a range of soils using solution nuclear magnetic resonance spectroscopy (³¹PNMR), and (ii) characterize the potential bioavailability of water-soluble unreactive P with phosphatase enzymes.  Surface horizon samples (0-15 cm) were collected from discrete soil series differing in management (pasture, riparian buffer, or corn silage).  Phosphorus extracted by NaOH-EDTA (dry soils) was analyzed by ³¹PNMR at the Stanford Magnetic Resonance Laboratory.  In addition to ³¹PNMR analysis and standard soil test analysis (Modified Morgan’s), soils were analyzed for: (i) distilled water-extractable reactive and unreactive P (organic and complex inorganic P), (ii) water-extractable P hydrolyzed by the addition of alkaline phosphomonoesterase (largely labile sugar phosphates), and (iii) P hydrolyzed by alkaline phosphomonoesterase and phosphodiesterase (largely DNA and phospholipids).  Results showed that organic P was the dominant average P form in both the water (78%) and in the extracts analyzed by ³¹PNMR (57%).  Approximately 48% of the water-extractable unreactive P was hydrolyzed by the addition of phosphatase enzymes, implying its potential in situ bioavailability.  The ³¹PNMR results revealed an array of organic P compounds that were dominated by orthophosphate monoesters (58%).  Results show the importance of organic P in these soils and underscore the need to incorporate organic P transformations into current agronomic P management guidelines.