Forms and bioavailability of phosphorus associated with natural organic matter

Natural organic matter （NOM） is an important ingredient in soil which can improve physical, chemical, and biological properties of soils and nutrient supplies. In this study, we investigated the spectral features and potential availability of phosphorus （P） in the IHSS Elliott Soil humic acid standard （EHa）, Elliott soil fulvic acid standard Ⅱ （EFa）, Waskish peat humic acid reference （WHa）, and Waskish peat fulvic acid reference （WFa） by fluorescence spectroscopy, FT-IR, solution 31P NMR, 3-phytase incubation and UV irradiation. We observed more similar spectral features between EHa and EFa as well as between WHa and WFa than between the two humic acids or two fulvic acids themselves. Phosphorus in WHa and WFa was mainly present in the orthophosphate form. However, only about 5% was water soluble. After treatment by both UV irradiation and enzymatic hydrolysis, soluble orthophosphate increased to 17% of WHa P, and 10%o of WFa P. Thus, it appears that a large portion of P in Waskish peat humic substances was not labile for plant uptake. On the other hand, both orthophosphate and organic phosphate were present in EHa and EFa. Treatment by both UV irradiation and enzymatic hydrolysis increased soluble orthophosphate to 67% of EHa P and 52% of EFa P, indicating that more P in Elliott soil humic substances was potentially bioavailable. Our results demonstrated that source （soil vs. peat） was a more important factor than organic matter fraction （humic acid vs. fulvic acid） with respect to the forms and lability of P in these humic substances.

Forms and bioavailability of phosphorus associated with natural organic matter