PROPERTIES OF ORGANIC PHOSPHORUS INCORPORATED INTO MODEL HUMIC POLYMERS

CHRISTINE SUSAN WITZ BALLENGER, Purdue University

Abstract

To evaluate characteristics of organic P in soil humic materials, model humic polymers were prepared in the presence of low molecular weight organic P compounds and their non-phosphorylated analogues by chemical oxidation of a mixture of polyphenolic residues. Compounds added included serine, phosphoserine, ethanolamine, and phosphoethanolamine. Characterization of model humic polymers by elemental and functional group analyses indicated the following: organic C, 50.6 to 56.8; organic N, 0.70 to 1.65%; organic P, 0.254 to 0.942%; total acidity, 7.86 to 11.87 meq/g; carboxyl, 1.42 to 2.00 meq/g; total hydroxyl, 6.79 to 10.0 meq/g; ash, 6.4 to 13.9% and E(,4)/E(,6) ratio, 5.34 to 6.19. These values are within ranges reported for soil humic substances. Only compounds containing free amino groups were incorporated into model humic polymers. Incorporation of phosphorylated amino compounds (phosphoserine and phosphoethanolamine) decreased the organic N content of model humic polymers by approximately 69% compared with incorporation of non-phosphorylated amino compounds (serine and ethanolamine). The majority of C (83.2%), N (79.8%) and P (75.3%) present in model humic polymer containing phosphoethanolamine (HAFA-PE) was found in the humic acid fraction (acid insoluble, alkaline soluble). Gel filtration of HAFA-PE showed the presence of a high molecular weight (> 100,000) component containing 0.5% of the polymer and two medium molecular weight (10,000 to 50,000) components constituting 74.8% of the polymer. Average molecular weights of the latter components were 20,000 and 15,000. The majority of organic P in HAFA-PE was associated with medium molecular weight fractions (79.2%) whereas 16.8% of the P was associated with low ( < 10,000) molecular weight fractions. Attempts to demonstrate the presence of organic P functional groups contained in HAFA-PE by potentiometric titration and infrared spectroscopic techniques were limited by relatively small amounts of organic P incorporated into model humic polymers. A study was also conducted to evaluate stability of simple organophosphate esters, phosphoserine (FPS) and phosphoethanolamine (FPE), incorporated into model humic polymers (HAFA-PS and HAFA-PE, respectively). Model humic polymer P was resistant to hydrolysis with 1 N HCl and 1 N NaOH. Stability of FPS and FPE to hydrolysis with 6 N HCl was increased by 6.5 and 17.0%, respectively, through incorporation into model humic polymers. Model humic polymer P was markedly stabilized towards hydrolysis with acid and alkaline phosphomonesterases. Only 1.3 and 10.6% of P in HAFA-PS and HAFA-PE, respectively, was hydrolyzed by alkaline phosphatase whereas 0 and 3.3% of P in HAFA-PS and HAFA-PE, respectively, was hydrolyzed by acid phosphatase. Intimate mixing of FPS or FPE with preformed model humic polymers decreased the amount of P hydrolyzed by alkaline phosphatase by 95 and 39% for FPS and FPE, respectively, whereas a 47% decrease in the amount of P hydrolyzed with acid phosphatase was noted for both FPS and FPE intimately mixed with model humic polymer. Model humic polymer decreased phosphatase activity in pure enzyme and soil incubation studies. Incorporation of FPE into model humic polymer markedly reduced the amount of P mineralized in soil incubation studies. Independent of environmental conditions imposed during soil incubations (e.g. pH, aeration, temperature), only 30 to 40% of model humic polymer P was decomposed during a 16 week period. In contrast, essentially 100% of FPS and FPE intimately mixed with preformed model humic polymer was decomposed during the initial 7 days of soil incubation. The results suggest that a portion of the unidentified organic P in soil humic substances may arise from incorporation of organic compounds containing amino and phosphate ester functional groups during oxidative polymerization of polyphenols.

Degree

Ph.D.

Subject Area

Agronomy

COinS