XANES Speciation of Phosphorus in Organically Amended and Fertilized Vertisol and Mollisol

Knowledge of the dominant P species present in the soil following the application of organic amendments and fertilizer is important in understanding the fate of P in the environment. This study was performed to identify P species in two calcareous soils, an Osborne series (Typic Humicryert) and a Lakeland series (Typic Calciudoll), treated with organic amendments (biosolids and hog and dairy cattle manure) and fertilizer (monoammonium phosphate, MAP). Phosphorus 1s x-ray absorption near-edge structure (XANES) spectroscopy was used to speciate P compounds in these amended soils. The result showed that “adsorbed P” was the dominant P species in both soils. For the Osborne soil, the unamended soil (control) and those amended with biosolids and MAP contained an appreciable amount of hydroxyapatite (HAP), the most thermodynamically favored Ca phosphate. In addition, soils amended with biosolids or hog or dairy manures contain β-tricalcium phosphate (TRICAL), a more soluble form of Ca phosphate than HAP. The amended Lakeland soils contained a variety of species in addition to the dominant “adsorbed P.” While TRICAL was found in all the amended soils except in that amended with hog manure, HAP was present in appreciable amount only in the unamended soil. Overall, the adsorbed P, most likely through inner-sphere complexation, in these amended soils may not be readily available as a source of P into the environment. In addition, the HAP and TRICAL will have limited solubility, and thus, are probably only a very minor source of P in the environment.