Plant phenolic acids are found in soils and their importance has been implicated in various soil processes, including biochemical weathering of minerals, humus formation, interactions between plants (allelopathy), and nutrient availability to plants. P‐hydroxybenzoic, vanillic, p‐coumaric, and ferulic acids were added to soils at a rate of 5.15 mmol/kg, extracted with Mehlich III extractant immediately and on Days 1, 2, 4, 8, 16, and 32, and quantified by high‐performance liquid chromatography. The recovery of phenolic acids from Cecil (Typic Hapludults, clayey, kaolinitic, thermic), Portsmouth (Typic Umbraquualts, fine loamy, mixed, thermic), and White Store (Vertic Hapludalfs, fine, mixed, thermic) soils varied with soil type, horizon (A1 and B1), time, and type of functional group present on the aromatic ring. There was significant instantaneous sorption of all compounds in all soils; in A1 horizon materials, recovery decreased as soil organic matter increased. There was generally greater sorption of phenolic acids in the A1 horizon materials than in the B1 horizon materials of a particular soil type. Significant declines in recovery occurred with time for all phenolic acids in each type of soil, irrespective of horizon. The sharpest decline in recovery of phenolic acids from soils generally occurred within 2 d after addition of the compounds. The presence of methoxy groups and acrylic side chains on the aromatic ring of phenolic acids increased the “sorption” of these compounds in soils. Sorption of the phenolic acids by the various soils was generally in the order of p‐hydroxybenzoic ≤ vanillic <p‐coumaric < ferulic.
Plant phenolic acids have been found in plants and soils and some evidence suggests their involvement in biochemical interactions between plants (allelopathy) and organisms living in the soil. This study was conducted to compare the more common extraction procedures used in the recovery of water‐soluble phenolic acids from soil. Various extraction procedures were employed in the recovery of ferulic acid (4‐hydroxy‐3‐methoxycinnamic acid) from steam sterilized soil materials sampled from the A1 and B1 horizons of a Cecil soil (Typic Hapludults, clayey, kaolinitic, thermic) and a Portsmouth soil (Typic Umbraquualts, fine loamy, mixed, thermic). Ferulic acid was added (1000 mg/kg) to soil materials, allowed to equilibrate for 90 d, extracted and quantified by high‐performance liquid chromatography. Eight extracting solutions [H2O, methyl alcohol (CH3OH), 0.5 M sodium acetate (CH3CHOONa), 0.05 and 0.5 M diethylenetriaminepentaacetic acid (DTPA), 0.05 and 0.5 M ethylenediaminetetraacetic acid (EDTA), and 2 M NaOH] and various extraction times were used to recover ferulic acid from soil materials. Amounts of ferulic acid recovered from the A1 horizons were significantly lower than the amounts recovered from the B1 horizons for both Cecil and Portsmouth soil materials. Water and CH3OH recovered the least amounts of ferulic acid, whereas NaOH and 0.5 M DTPA recovered the most. In the B1 horizon materials, a major portion of the ferulic acid anions appeared to be absorbed by polyvalent cations (either exchangeable or nonexchangeable). This paper emphasizes the necessity of choosing the appropriate phenolic acid extractant for the soil phenolic acid fraction of interest.
An initial survey of the effects of aqueous solutions of ferulic acid and three of its microbial metabolic products at pH 4.5, 6.0, and 7.5 was determined on radicle growth of 11 crop species in Petri dishes. These bioassays indicated that cucumber, ladino clover, lettuce, mung bean, and wheat were inhibited by ferulic, caffeic, protocatechuic, and/or vanillic acids and that the magnitude of inhibition varied with concentration (0-2 mM), phenolic acid, and pH of the initial solution. The pH values of the initial solutions changed considerably when added to the Petri dishes containing filter paper and seeds. The final pH values after 48 hr were 6.6, 6.8, and 7.1, respectively, for the initial 4.5, 6.0, and 7.5 pH solutions. The amounts of the phenolic acids in the Petri dishes declined rapidly over the 48 hr of the bioassay, and the rate of phenolic acid decline was species specific. Cucumber was subsequently chosen as the bioassay species for further study. MES buffer was used to stabilize the pH of the phenolic acid solutions which ranged between 5.5 and 5.8 for all subsequent studies. Inhibition of radicle growth declined in a curvilinear manner over the 0-2 mM concentration range. At 0.125 and 0.25 mM concentrations of ferulic acid, radicle growth of cucumber was inhibited 7 and 14%, respectively. A variety of microbial metabolic products of ferulic acid was identified in the Petri dishes and tested for toxicity. Only vanillic acid was as inhibitory as ferulic acid. The remaining phenolic acids were less inhibitory to noninhibitory. When mixtures of phenolic acids were tested, individual components were antagonistic to each other in the inhibition of cucumber radicle growth. Depending on the initial total concentration of the mixture, effects ranged from 5 to 35% lower than the sum of the inhibition of each phenolic acid tested separately. Implications of these findings to germination bioassays are discussed.
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