Phytic acid (inositol hexaphosphoric acid, IP6) has long been recognized as the predominant organic P form in soil and animal manure. Whereas many studies have investigated the wet chemistry of IP6, there is little information on the characterization of solid metal IP6 compounds. This information is essential for further understanding and assessing the chemical behavior of IP6 in diverse soil-plant-water ecosystems. As the first step in full characterization, we synthesized eight metal phytate compounds and investigated their structural features using Fourier transform infrared spectroscopy (FT-IR). The absorption features from 900 to 1200 cm(-1) in FT-IR could be used to identify these phytates as: (i) light divalent metal (Ca and Mg) compounds with a sharp band and a broad band, (ii) heavy divalent metal (Cu and Mn) compounds with splitting broad bands, and (iii) trivalent metal (Al and Fe) compounds with a broad band and a shoulder band. Three different types of chemical structures of metal-phytate compounds were presented based on the FT-IR information. We further demonstrated that metal orthophosphates possessed different FT-IR spectral characteristics from their IP6 counterparts. The unique spectral features of metal phytates from 1000 to 700 cm(-1) could be used to distinguish phytate compounds from metal phosphate compounds. Thus, FT-IR analysis after fine tuning could provide an analytical tool to investigate the basic metal phytate chemistry in molecular levels, such as the competitive interactions between phosphate and phytate with a specific metal ion, and the conversion (or hydrolysis) of metal phytate to metal phosphate under various conditions.

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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 31 P 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.

In characterizing organic phosphorus (P o ) by phosphatase hydrolysis, the quantity of hydrolyzable P o is represented by the difference in orthophosphate [i.e., inorganic P (P i )] determined after and prior to enzymatic incubation. Therefore, precise determination of P i is of major importance for accurate application of the enzymatic hydrolysis approach. The strong acid conditions required for conventional molybdenum blue methods interferes with P i determination due to rapid hydrolysis of labile P o and precipitation of enzymes (proteins). The molybdenum blue method of Dick and Tabatabai in 1977 reduced errors pertaining to nonenzymatic hydrolysis of P o . This study revisited the method, finding that the absorption coefficient at 850 nm was 45 -49% higher than at 700 nm, and linear up to at least 80 nmol P i in 1-mL assay solution. Therefore, adaptation of the readings at 850 nm improved the sensitivities of P i determination by about 45%. Enzyme precipitation during P i determination was prevented by addition of 2% sodium dodecyl sulfate (SDS) before color-forming reagents were added. This method modification provides increased sensitivity for P i determination, thereby improving the accuracy of P o analysis by phosphatase hydrolysis.

The role of sediment-bound organic phosphorus (Po) on lake eutrophication was studied using sequential extraction and enzymatic hydrolysis by collecting sediments from Dianchi Lake, China. Bioavailable Po species including labile monoester P, diester P, and phytate-like P were identified in the sequential extractions by H2O, NaHCO3, and NaOH. For the H2O-Po, 36.7% (average) was labile monoester P, 14.8% was diester P, and 69.9% was phytate-like P. In NaHCO3-Po, 19.9% was labile monoester P, 17.5% was diester P, and 58.8% was phytate-like P. For NaOH-Po, 25.6% was labile monoester P, 7.9% was diester P, and 35.9% was phytate-like P. Labile monoester P was active to support growth of algae to form blooms. Diester P mainly distributed in labile H2O and NaHCO3 fractions was readily available to cyanobacteria. Phytate-like P represents a major portion of the Po in the NaOH fractions, also in the more labile H2O and NaHCO3 fractions. Based on results of sequential extraction of Po and enzymatic hydrolysis, lability and bioavailability was in decreasing order as follows: H2O-Po > NaHCO3-Po > NaOH-Po, and bioavailable Po accounted for only 12.1-27.2% of total Po in sediments. These results suggest that the biogeochemical cycle of bioavailable Po might play an important role in maintaining the eutrophic status of lakes.

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Knowledge of the P forms in poultry litter (PL) and their transformations in soil will help improve our understanding of the long‐term role of P in eutrophication. In this study, samples of PL and pasture soils with and without 20 yr of PL application were sequentially extracted to separate P into H2O, 0.5 mol L−1 NaHCO3, 0.1 mol L−1 NaOH, and 1 mol L−1 HCl fractions. After appropriate dilution and pH adjustment, the fractions were incubated in the presence of orthophosphate‐releasing enzymes. Cross‐examination of the solution 31P nuclear magnetic resonance spectra of the enzymatically treated and untreated fractions revealed that the peaks of organic P (Po) species of the enzymatically treated fractions became very weak or disappeared, confirming enzymatic hydrolysis of Po in the untreated fractions. Although the majority of P in the NaOH and HCl fractions of PL was in organic forms, these stable Po forms could be subjected to enzymatic hydrolysis after being applied to soil, an occurrence that was supported by the soil P data. Compared with soil without litter applied, 20 yr of PL application increased the pools of both labile and stable inorganic P in the soil; however, repeated application of PL did not lead to a significant accumulation of hydrolyzable Po in NaOH and HCl fractions, indicating that the stable Po must have been converted to other forms. The transformation of stable PL Po observed in this study could be an important mechanism for maintaining a balance between labile and immobile P in soils.