Evaluation of the factors affecting direct polarization solid state ³¹P-NMR spectroscopy of bulk soils

³¹P-NMR spectroscopy on bulk soils is a powerful tool for the identification of the different phosphorus forms in soils and for the evaluation of the dynamics of soil P. Up to now the majority of the papers dealt with liquid state ³¹P-NMR spectroscopy on soluble soil organic substances. Only few papers were addressed to the study of the different phosphorus forms directly in bulk soils. In the present paper, some organic and inorganic phosphates of known structures, which are likely to be present in soil systems, were studied by direct polarization (DP) magic angle spinning (MAS) ³¹P-NMR spectroscopy in order to understand the electronic factors responsible for chemical shifts of the phosphorus (P) nucleus and to serve as guidelines to assign P resonances in soil spectra. Number of hydrating water molecules, type of counter-cation, degree of covalence, and spatial conformation of P in phosphate structures were found to affect signal positions in ³¹P-NMR spectra. Both hydrating water and increase in degree of covalence of the X-O-P bonds (X=H, Na) enhanced the electronic density (ED) around P, thereby producing up-field shifts in ³¹P-NMR spectra. The exchange of the Na⁺ counter-cation with NH₄⁺ resulted in an increase of the cation potential (PC) that is a measure of the cation polarizing power, and induced a down-field shift of P signals, due to a corresponding reduction in ED around the P nucleus. Both NMR down- and up-field shifts were observed in organic phosphates, and were dependent on the spatial orientation of the phosphate groups that may have been fixed anisotropically in the solid state. Based on the factors that influence P chemical shifts for standard phosphates, attempts to assign ³¹P-NMR signals in the spectra of five different unperturbed bulk soils were made.