Available phenolic acids

A range of extractants and extraction procedures has been used to extract phenolic acids from soil (Dalton 1999). Many of these extractants and extraction procedures, however, recover phenolic acids that are not directly involved in plant-plant allelopathic interactions (e.g., phenolic acids sorbed in the recalcitrant organic matter). Thus considerable efforts were made to identify extraction procedures that would provide reasonable estimates of available phenolic acids ( free phenolic acids in soil solutions and reversibly sorbed phenolic acids on soil particles) in soils (Dalton et al. 1983, 1987, 1989a, b Blum et al. 1994 Blum 1997 Dalton 1999). 

The actual effectiveness of neutral EDTA as an extractant of available phenolic acids was demonstrated as follows (Blum et al. 1994) ... 

Since the water content and solutions added to soil samples was determined and known, respectively, free , reversibly sorbed, and available phenolic acids of a soil sample could be expressed in either p,M or p,mol/g soil. In addition since under laboratory conditions the concentrations of phenohc acids added to sterile soils were known, the amount (% or p,mol/g soU) of irreversibly sorbed phenohc acids (e.g., bound into recalcitrant organic matter or onto clay minerals) could also be estimated. 

Dalton et al. (1989b) utilizing Mehlich 111, a mild chelating extractant, also observed that the recovery of available phenolic acids (femlic acid, vanillic acid, p-coumaric acid, p-hydroxybenzoic acid) from sterile soil (Cecil, Portsmouth and White Store) varied with soil type, horizon, time, and the type of phenolic acid added. When they allowed phenolic acids added to soil to equilibrate for 2 min before extraction, they noted a significant reduction in recovery of phenolic acids. Recovery declined with time up to 32 days. The decline was most rapid over the first 2 days. The presence of methoxy groups and acrylic side chains increased... 

When phenolic acids enter the soil environment they are reversibly and irreversibly sorbed to soil particles, polymerized, oxidized, reduced, leached, utilized by microbes, and taken up by roots. Rates for these various processes are highly variable and depend on soil type, biotic and physicochemical soil environmenL types and mixtures of phenohc acids in or added to soils, and time, among others. To eliminate the effects of soil microbes, soils may be autoclaved. Concentrations of individual available phenolic acids in soils at a given point in time may be estimated by extracting soils with appropriate extractants and HPLC analysis. Based on our soils, we recommend water for estimating soil solution concentrations and neutral EDTA for soil solution and reversibly sorbed phenolic acid concentrations. However, the effectiveness of neutral EDTA in recovering available phenolic acids in all other soils should not be assumed. Reversibly sorbed phenolic acids increased or decreased as soil solution concentrations and multivalent cations increased or decreased, respectively. 

Phenolic acids in soils occur either in a free state in the soil solution, reversibly sorbed to soil particles, fixed (irreversibly sorbed) very tightly to soil particles (e.g., recalcitrant organic matter, and clays), and/or on and in living and dead plant tissues/residues ( free , reversibly sorbed, and fixed). Of general interest to plant-plant allelopathic interactions are the free and reversibly sorbed states frequently referred to as the available fraction. Of particular interest is the active fraction of available phenolic acids, the fraction of available phenolic acids that actually interact with seeds, roots and microbes. Unfortunately we presently do not have a means of quantifying the active fraction, thus the focus on the available fraction. 

There is, however, a caveat for estimating available total phenolic acid concentrations. The estimates of the total available fraction of phenolic acids in soil extracts represent a crude estimate of what actually occurs in soil, not only because of the range of efficiencies of extraction procedures but also because different phenolic acids at the same concentration generate different absorbances with Folin Ciocalteu s phenol reagent (Fig. 3.7 Blum et al. 1991). In addition soil extracts also contain compounds, other than phenolic acids, that react with (i.e., reduce) the Folin Ciocalteu s phenol reagent (McAllister 1969 Box 1983). The assumption, therefore, was that available total phenolic acid values based on the Folin Ciocalteu s phenol reagent expressed as ferulic acid equivalence were relative values that were consistently related to the acmal total available phenolic acids (hereafter just called total phenolic acid) present in soil extracts. The extraction and quantification by HPLC analysis of available individual phenolic acids in soil do not have these particular problems. 

However, there is one more caveat for both total and individual phenolic acids. Soil extractions recover residual or net concentration, i.e., input - losses, for a point or various points in time. Since both input and losses are unknown between points of time the actual available total or individual phenolic acid concentrations in soil over time are also unknown. The concentrations of available phenolic acids interacting with roots could thus be greater, at times much greater, or lower, at times much lower, than the net concentrations determined from soil extracts. 

No additional changes occurred over the next week. These patterns of decline and/or increase over a 2-week period suggested not only that phenolic acids in the soil samples were primarily in a bound (sorbed) form but also that microbial utilization/synthesis of available phenolic acids was nutrient limited. 

If available phenolic acids in soil come from leachates of shoot tissues/residues above and on the surface of the soil then ... 

If available phenolic acids in soil come from root tissues/residues, then the distribution of available phenolic acids will be consistent with root tis-sue/residue distribution in the soil, movement of gravitational and capillary water, mass flow of soil solutions driven by transpirational pull , and the action of soil processes. Concentrations released will be highest shortly after glyphosate desiccation. Note This is also true for all other organic and inorganic compounds, and... 

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