Abstract
Growth in the dairy industry in Idaho has created a surplus of manure that may be a useful source of fertilizer for crops. The purpose of this project was to determine the effects of manure amendments on phosphorus (P) speciation in the calcareous soils of southern Idaho. Sequential extraction, nuclear magnetic resonance (31P-NMR) spectroscopy, and x-ray absorbance near-edge spectroscopy (XANES) analysis were used to analyze changes in the soil caused from manure amendments over a three-year period. Results show an increase in total P concentration over three years with annual manure amendments of 17 Mg/ha and 52 Mg/ha (dry weight). NMR and XANES linear combination fitting results show all soil samples collected were comprised of majority apatite (54-74%), along with adsorbed phosphorus (25-35%) and organic phosphorus (monoesters and diesters) (0-19%). Adsorption isotherms were also prepared to examine the impact of manure on P adsorption to calcite. Results showed that percent P adsorbed dropped significantly in the presence of manure. XANES analysis also found distinct differences in the P minerals formed in the presence of manure, suggesting manure affects P solubility and mineralization. Increases in total soil phosphorus, as well as overall solubility, may affect phosphorus loading to nearby surface waters if not carefully monitored. The results also show the benefits of using multiple analytical methods to provide an in-depth profile of the phosphorus species present.