THE ORGANIC PHOSPHORUS COMPOSITION OF AN IRISH GRASSLAND SOIL

Abstract

An understanding of the chemical nature of P in soil is essential to the sustainability of agriculture in Ireland. This study used ³¹P nuclear magnetic resonance (NMR) spectroscopy and wet-chemical analysis to characterise NaOH—EDTA-extractable P in a grassland soil in south-east Ireland. Soil samples of 10cm depth were collected over five sampling days between September and December 2003. Total soil P (Pt) ranged between 723 and 819mg P kg⁻¹ soil, while Morgan's P (Pm) ranged between 3.4 and 12.5mg P l⁻¹ soil. Recovery rates of P extracted in the NaOH—EDTA solution were high, ranging from 75 to 88% of soil Pt. Four distinct classes of P compound were detected in the ³¹P NMR spectra: inorganic orthophosphate (δ = 6.72 to 6.85ppm, with a mean of 6.79ppm), orthophosphate monoesters (δ = 3.83 to 6.11ppm), orthophosphate diesters (δ = —0.32 to 0.54ppm) and pyrophosphates (δ = —3.44 to —3.13ppm, with a mean of —3.26ppm). Distinct signals at 5.64, 5.39, 5.02 and 4.55ppm in the spectra showed the dominance of myo-inositol hexakisphosphate in the orthophosphate monoester region. Orthophosphate diesters were subclassified as phospholipids (δ = 0 and 1.88ppm) and DNA (δ = 0.09 to —0.10ppm). Over the three-month sampling period the mean percentages of the four main P functional classes—inorganic orthophosphate, orthophosphate monoesters, orthophosphate diesters and pyrophosphates—in the soil samples were approximately 52, 42, 3 and 3%, respectively. The ratio of orthophosphate monoesters to orthophosphate diesters ranged from 10 to 19. Further research is required to assess the potential bioavailability of organic P (Po) in Irish grassland soils in order to understand its role in the transfer of P from agricultural soil to our water bodies.