Improved Identification of Organic Phosphorus Species in Tropical/ High Latitude Soils and Aquatic Ecosystems by Advanced Liquid and Solid State NMR Approaches

Abstract

Phosphorus (P) availability is limiting plant growth in many of the worlds biomes. Recent predictions suggest that rock P exploitation peaks within 35 years with severe impacts on future global food production1. In the soil much of mineral P is transformed to organic P species which are unavailable for plants. There is a lack of knowledge about the molecular processes controlling the reactivity of organic P species and their bioavailability. Our aim is to develop solution and solid state 31P NMR techniques to identify P species in soils and water; information ideal for correlating different organic P species to plant and soil processes2. However, NMR studies on soil P are complicated by serious line broadening caused by paramagnetic ions. For liquid-state NMR, soil organic P is commonly extracted by a NaOH-EDTA method, leading to co-extraction of heavy metal. In order to avoid paramagnetic line broadening these ions have to be physically removed. We find that sulfide precipitation removes Fe and Mn ions without affecting the P-composition. It dramatically reduces the line widths from over 100 Hz down to 2 Hz on soil extracts and allows 2D 1H, 31P NMR to be applied3. Using 2D 1H-31P NMR resolves the highly crowded spectral region where abundant monoester P appears. By exploiting 2D 1H-31P correlations in the NMR spectra of soil extracts, we can identify individual organic P species (including unknowns) by a combination of P and H chemical shifts and coupling constants3,4. Studies using passive sampling with ion exchange resin analyzed with solid state 31P MAS NMR have the potential to analyse the reservoir of organic P-species in aquatic systems e.g. streams.