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 phospho-rus 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 pres-ent 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 spec-troscopy 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 (XH, Na) enhanced the electronic density (ED) around P, thereby producing up-field shifts in 31P-NMR spectra. The exchange of the Naþ counter-cation with NH4 þ 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 depen-dent 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