Macronutrients and metals released from soils by solutions of naturally occurring phenols

Abstract

Phenolic compounds produced by plants enter the soil by leaching and litter decomposition. The goal of this work is to determine the effect of phenolic compounds on solubility of plant macronutrients and metals in agroforestry systems. Soils from forest and pasture systems were repeatedly extracted with water (control) or phenolic solutions and then compared to a Mehlich 3 reference. The phenolics were aqueous solutions of tannic acid or β–1,2,3,4,6‐penta‐O‐galloyl‐D‐glucose (PGG) (hydrolyzable tannins), procyanidin (condensed tannin), or small phenolics catechin, gallic acid, or methyl gallate. The concentration of the macronutrients Ca, Mg, K, P, and S, and the metals Fe, Al, Mn, and Zn in the supernatants was determined by inductively‐coupled plasma spectroscopy. Cumulative extraction of macronutrients was generally similar to or less than the amount obtained by the Mehlich 3 extraction with the lowest recoveries obtained with the water control, PGG, and procyanidin. Metals tended to be somewhat more extractable from forest soil, especially with gallic acid, tannic acid or PGG treatments. Three mechanisms affected extraction of analytes by phenol‐containing solutions: (1) pH‐driven dissolution (Ca and Mg), (2) chelation of the metal (Al) by the polyphenol, or (3) reduction of the metal (Fe and Mn). Relatively low extraction of nutrients by some polyphenols is attributed to the tendency of some phenols to sorb to soil. This study demonstrates that tannins and related compounds change the solubility of macronutrients and metals in soils by a complex process that is not easily predictable from simple chemical properties of the phenolics.

Single and Combined Fe and S Deficiency Differentially Modulate Root Exudate Composition in Tomato: A Double Strategy for Fe Acquisition?

“…This pattern was independent from the growing condition, albeit showing different variations according to the starvations (Figure 5). In particular, phenols and coumaric acids showed a general increase in all the conditions; indeed, the exudation of phenols has already been shown to be involved in the response to Fe starvation, possibly for the mobilization of sparingly soluble Fe sources [52][53][54][55]. Moreover, phenolics can increase the solubility and the uptake of metals such as Fe by chelation or reduction [52,[54][55][56].…”

“…In particular, phenols and coumaric acids showed a general increase in all the conditions; indeed, the exudation of phenols has already been shown to be involved in the response to Fe starvation, possibly for the mobilization of sparingly soluble Fe sources [52][53][54][55]. Moreover, phenolics can increase the solubility and the uptake of metals such as Fe by chelation or reduction [52,[54][55][56]. Similar results were found by other authors who showed that the Arabidopsis phenylpropanoid pathway is stimulated in roots by Fe-deficiency [54,57].…”

Short-Term Soil Flushing with Tannic Acid and Its Effect on Metal Mobilization and Selected Properties of Calcareous Soil

“…Because the original pH in the TA solution was acidic (pH 4.0), it favored the release of alkaline cations from the calcareous soil, resulting in a sudden increase in the pH of the leachate. Due to the fact of its acidic nature, TA can efficiently extract Ca and Mg from soil [ 34 ]. Between the 10th and 30th hour of soil flushing, the pH in the leachate stabilized ( Figure 3 a).…”

“…After the soil was saturated with TA, its concentration in the leachate stabilized and was close to the concentration in the original solution. The tendency of TA to sorb to soil can be a critical determinant of metal mobilization, as this tendency limits its ability to form soluble extractable complexes with metals [ 34 ]. A decrease in flushing solution concentration is typical during soil flushing.…”

“…These phenols take part in redox reactions by forming quinones and donating electrons [ 47 ]. Phenolic compounds are known to rapidly mobilize or solubilize soil metals, such as Ca, Al, and Fe, probably through chelation and oxidation/reduction reactions [ 34 , 48 , 49 ]. The reducing capacity of TA is greater in acidic than in alkaline conditions [ 50 ].…”