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Soil Chemistry

Spectral and Chemical Characterization of Phosphates Associated with Humic Substances

^a USDA-ARS, New England Plant, Soil, and Water Lab., Orono, ME 04469
^b Dep. of Plant, Soil, and Environmental Sciences, Univ. of Maine, Orono, ME 04469
^c Geol. & Environ. Sci. Dep., Stanford Univ., Stanford, CA 94305

^* Corresponding author (Zhongqi.He{at}ars.usda.gov)

Because humic substances are involved in many processes in soils and natural waters, characterization of phosphorus (P) associated with humic substances may shed light on the function of natural organic matter in P cycling and nutrition. In this study, we investigated the spectral features and potential availability of P in the International Humic Substance Society (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, Fourier-transform infrared spectroscopy (FT-IR), solution ³¹P nuclear magnetic resonance (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 the P in WHa, and 10% of the P in WFa. 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 the P in EHa and 52% of the P in EFa, 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. This work represents a much more complete characterization of humic substance-bound P than previously reported in the literature, thus providing a comprehensive approach for improved understanding of P cycling in relation to ecosystem function.

Abbreviations: EFa, IHSS Elliott soil standard fulvic acid II • EHa, IHSS Elliott soil standard humic acid • EEM, emission-excitation matrix • EM, emission • EX, excitation • FT-IR, Fourier-transform infrared spectroscopy • IHSS, the International Humic Substances Society • LHa, IHSS Leonardite standard humic acid • NMR, nuclear magnetic resonance spectroscopy • NOM, natural organic matter • PARAFAC, parallel factor analysis • PHa, IHSS Pahokee peat reference humic acid • P_i, soluble inorganic orthophosphate determined by a molybdenum blue method • WFa, IHSS Waskish peat reference fulvic acid • WHa, IHSS Waskish peat reference humic acid

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