Soil-phenolic acid interaction

The nature of soil-phenolic acid interaction adsorption-desorption. Adsorption of a solute from solution onto a solid matrix results in a higher solute concentration at the fluid-solid interface than in the solution. Huang and coworkers (27) observed a high sorption capacity of the mineral fraction of four latosols for phenolic acids. On the basis of their results, distribution coefficients,... 

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. 

WANG T.S.C., YEH K.L., CHENG S.Y. and YANG T.Z. 1971. Behaviour of soil phenolic acids. In Biochemical Interactions Among Plants, pp. 113-119. U.S. National Committee for the International Biological Program,... 

Since lignins are polymers of phenolics and are major plant constituents with resistance to microbial decomposition, they are the primary source of phenolic units for humic acid synthesis (178, 179). Once transformed, these humic acids become further resistant to microbial attack and can become bound to soils (180) form interactions with other high molecular weight phenolic compounds (ex. lignins, fulvic acids) and with clays (181) and influence the biodegradation of other organic substrates in soils (182, 183). 

It is also important to understand that most allelopathic effects apparently result from the combined actions of several allelochemicals, often with each below a threshold concentration for impact. In allelopathic situations which implicate phenolic acids, soil concentrations have ranged from below 10 to above 1000 ppm for each compound. The lower end of the spectrum is below a concentration required for an effect in current bioassays. Additive and synergistic effects have been demonstrated, however, for combinations of cinnamic acids (102), benzoic acids (103), benzoic and cinnamic acids (10 ). and -hydroxybenzaldehyde with coumarin (105). It appears that such combined interactions may be very important under field conditions. 

Approximately 40 to 50% of the total amount of phenolics sorbed was retained by the organic matter fraction (27). In surface soil layers, organic matter is frequently intimately associated with the mineral components present, providing a large surface area and reactive sites for surface interaction. Soil acidity has a major influence on phenolic adsorption by the organic carbon fraction, since the degree of dissociation of the phenolic acids is pH-dependent. Whitehead and coworkers (28) observed that the extractability of several phenolic acids was highly dependent upon the extractant pH between pH 6 and 14. The amount extractable continually increased with extractant pH thus the extracted acids could not be readily classified into distinct fractions. 

In plants, biosynthesis and exudation of allelochemicals follows developmental, diurnal, and abiotic/biotic stress-dependent dynamics. Compounds from 14 different chemical classes have been linked to allelopathic interactions, including several simple phenolic acids (e.g., benzoic and hydroxycinnamic acids) and flavonoids [Rice, 1984 Macias et al., 2007]. The existence of several soil biophysical processes that can reduce the effective concentration and bioactivity of these compounds casts doubts on their actual relevance in allelopathic interactions [Olofsdotter et al., 2002]. However, there are well-documented examples of phenylpropanoid-mediated incompatible interactions among plants. Several Gramineae mediate allelopathic interactions by means of... 

Figure 5.3. A humic acid macromolecule interacting with a surface of a clay mineral. The proposed macromolecular structure of the soil humic acid (HA) is based on the following common average characteristics of humic acids MW 6386 Da elemental analysis (%) C, 53.9 N, 5.0 H, 5.8 0,35.1 S, 0.5 C/N, 10.7 NMR information (%) aliphatic C, 18.1 aromatic C, 20.9 carbohydrate C, 23.7 metoxy C, 4.9 carboxylic C, 8.4 ketone C, 4.5 phenolic C, 4.2 functional groups (cmol/g) carboxyl, 376 phenol, 188 total acidity, 564. The structure was created using the ACD/ChemSketch program. [HA-clay complex Chen s group, unpublished (2008). Individual HA molecule Grinhut et al., 2007.]...

There is some confusion in the literature as to when it is appropriate to apply the term allelochemical to phenolic acids. Since phenolic acids and their derivatives are found essentially in all terrestrial soils, it should be understood that the presence of phenolic acids in soil does not automatically imply that these phenolic acids are functionally allelochemicals. In theory, phenolic acids in soils, depending on their chemical state, concentrations, and the organisms involved, can have no effect, a stimulatory effect, or an inhibitory effect on any given plant or microbial process. For phenolic acids in the soil to be classified as allelochemicals requires that a) the phenolic acids are in an active form (e.g., free and protonated), b) they are involved in chemically mediated plant, microbe, or plant/microbial interactions and c) the concentrations of the active forms in the soil solution are sufficient to modify plant or microbial behavior, either in a positive or negative manner.8,49 However, changes in microbial behaviour associated with the utilization of phenolic acids as a carbon or energy source would not qualify as an allelopathic response. 

Blum, U., Shafer, S. R. and Lehman, M. E., 1999. Evidence for inhibitory allelopathic interactions involving phenolic acids in field soils concepts vs. an experimental model. Crit. Rev. Plant Sci. 18, 673-693... 

Dalton, B. R., 1999. The occurrence and behavior of plant phenolic acids in soil environments and their potential involvement in allelochemical interference interactions Methodological limitations in establishing conclusive proof of allelopathy. In Inderjit, Dakshini, K. M. M., and Foy, C. L., (Eds.), Principles and Practices in Plant Ecology Allelochemical Interactions, CRC Press, Boca Raton, FL, 57-74... 

What follows this introduction to plant-plant interactions (Chapter 1) are three additional chapters. The first chapter (Chapter 2) describes the behavior of allelopathic agents in nutrient culture and soil-microbe-seedling systems under laboratory conditions. Simple phenolic acids were chosen as the allelopathic agents for study in these model systems (see justifications in Section 2.2.6). The next chapter (Chapter 3) describes the relationships or lack of relationships between weed seedling behavior and the physicochemical environment in cover crop no-till fields and in laboratory bioassays. Here as well the emphasis is on the potential role of phenolic acids. The final chapter (Chapter 4) restates the central objectives of Chapters 2 and 3 in the form of testable hypotheses, addresses several central questions raised in these chapters, outlines why a holistic approach is required when studying allelopathic plant-plant interactions, and suggests some ways by which this may be achieved. 

Historically, simple phenolic acids have been the most frequently identified allelopathic agents (see literature reviews by Rice 1974, 1979, 1983, 1984, 1986). One would assume this was partly because of the fact that the necessary technology to isolate, identify, and quantify phenolic acids, even though crude in the early days, was readily available to most researchers. Furthermore, simple phenolic acids, such as the benzoic acid and cinnamic acid derivatives serve a variety of plant and ecosystem functions and are widespread in higher plants (Fig. 2.4 Bates-Smith 1956 Harborne 1982,1990 Goodwin and Mercer 1983 Siqueira et al. 1991). The ubiquitous distribution in nature and their apparent rapid turnover rates in soils, however, have lead to some controversy as to the importance of phenolic acids in plant-plant allelopathic interactions (Schmidt 1988 Schmidt and Ley 1999 Blum 2004, 2006). Finally, the behavior of phenolic acids in soil systems are somewhat similar to the behavior of a whole host of other organic acids (e.g., acetic acid, butyric acid, citric acid, formic acid, fiimaric acid, lactic acid, malonic acid, tannic acids and tartaric... 

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). 

Interactions of Phenolic Acids with Sterile and Non-sterUe Soils... 

The higher concentrations of phenolic acids required for a given percent inhibition between the two systems stem from the fact that nutrient cultures have a much more consistent environment than soil culture systems in that water, nutrients, and phenolic acids are evenly distributed in the treatment container and thus are readily available to interact with root surfaces. Soil systems, on the other hand, are much more complex heterogeneous environments in which roots must compete with a variety of soil sinks (e.g., clays, organic matter, and microbes) for water, nutrients, and phenolic acids. There is also mechanical resistance to the movement of water, nutrients, and phenolic acids and the growth of roots in soils. The slower development of inhibition after treatment and the slower recovery after phenolic acid depletion in soil systems is very likely related to the slower growth of seedlings in soil culture. 

Blum U (1996) AUelopathic interactions involving phenolic acids. J Nematol 28 259-267 Blum U (1997) The benefits of citrate over EDTA for extracting phenolic acids from soils and plant debris. J Chem Ecol 23 347-362... 

Blum U (2007) Can data derived from field and laboratory bioassays establish the existence of aUelopathic interactions in nature In Fujii Y, Hiradate S (eds) Allelopathy new concepts and methodology. Science Pubhshers, Enfield, NH, pp 31-38 Blum U, Austin MF, Shafer SR (1999a) The fates and effects of phenolic acids in a plant-microbe-soil model system. In Macias FA, Galindo JCG, Mohnillo JMG, Cutler HG (eds) Recent advances in allelopathy I. A science for the future. Cadiz University Press, Puerto Real, pp 159-166... 

---