Novel Approaches for Identifying Phosphorus Species in Terrestrial and Aquatic Ecosystems with 31P NMR

Abstract

Phosphorus (P) limits plant growth especially where agriculture is performed on older soils. Predictions indicate that rock-P mining peaks within 35 years with severe impacts on global food production1. Several studies have been made to completely understand fate and cycle of P in soils but knowledge in this field remains limited. Therefore we aim in our physicochemical/ecological research to elucidate overall speciation and sorption mechanisms of organic P in environmental matrices ranging from boreal to African soils and even water systems. 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. NMR studies on soil P have consequently suffered from serious line broadening caused by paramagnetic ions. We developed a new approach to avoid paramagnetic line broadening by physically removing them. using sulfide precipitation which reduces and removes Fe and Mn ions without affecting P-composition, we could see a dramatic reduction in NMR line widths from over 100 Hz down to 2 Hz on soil extracts. This resolution allowed application of 2D 1H, 31P-NMR to resolve the crowded spectral region where abundant monoester P appears.3 By exploiting 2D 1H-31P correlations in the NMR spectra of soil extracts we were able to identify unambiguously individual organic P species by combining 31P and 1H chemical shifts and coupling constants, an approach even suitable for a structural characterization of unknown P-components3, 4. Applications on boreal4 and tropical soil are underway, where tropical soils from Burkina Faso can be studied in greater detail using the new protocols. Studies using passive sampling with ion-exchange resin analyzed with solid state 31P-MAS NMR have the potential to analyze organic P-species in aquatic systems.