DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY Identification of Phenolic Acid Composition of Alkali-extracted Plants and Soils

and plant growth (Rice, 1984), and are structural and functional components of soil organic matter (Haider Phenolic acids (PAs) released from plant residues have been impliet al., 1975). Phenolic acids have also been shown to cated as important components in a variety of soil processes. To regulate the residue decomposition rate (Meentemeyer evaluate the role of plant PAs in soil processes, a quantitative alkaline extraction, solid-phase purification, and gas chromatographic protocol 1978; Berendse et al., 1987; Martens, 2000a), nutrient was developed for identification of the composition and concentration release from plant residues in soil (Tian et al., 1992) and of plant and soil PAs. Water-soluble or EDTA-exchangeable PAs increase long-term soil structure (Griffiths and Burns, were not detected in soil. Alkaline hydrolysis (1 M NaOH) at ambient 1972; Martens, 2000a,b). temperatures was required to extract ester-linked phenolics and alkaThe composition of lignin and other polyphenol polyline hydrolysis (4 M NaOH) with heat extracted ether-linked PAs mers in plant tissue, soil, or sediment is not amenable present in plant and soil material. Purification of NaOH-extracted to direct chemical analysis without prior isolation. TraPAs by polymeric solid-phase extraction with gas chromatographic ditional theories (Chen, 1992) have reported lignin polyflame ionization and mass spectral analysis of nonderivatized extracts mers to be composed of repeating units of coumaryl, resulted in a highly reproducible and accurate method for the saponificoniferyl, and sinapyl alcohols (monolignols or phenylable PAs. The method quantified plant and soil PAs as ethanone (acetylbenzene), benzaldehyde, and benzoicand cinnamic-acid derivpropane-like structures). Qualitative total lignin analyatives. The majority of soil PAs was identified as modified cinnamic sis for wood content or forage digestibility has always acids originating from vascular plant tissue. Comparison of the decentered on a proximate-gravimetric analysis of the scribed method with a standard acid digestion (12 M H2SO4 ) and strong-acid resistant fraction (12 M H2SO4 ) of plant gravimetric determination of lignin in plant residues found that interbiomass. Problems with this analysis include incomplete ferences formed by strong acid digestion of plant residues such as removal of carbohydrate and protein materials and consoybean [Glycine max (L.) Merr.] or clover (Trifolium pratense L.) densation of carbohydrate products with lignin, both containing higher carbohydrate and protein contents resulted in an resulting in erroneously high lignin contents (Dence, overestimation of plant lignin content when measured by the acid 1992). Quantitative analysis of lignin polymers has been digestion-gravimetric method. Since the majority of soil PAs originate commonly characterized on the basis of their nitrobenfrom vascular plants and are not microbial in origin, the composition of ester-linked PAs in soils may be an important indicator of the zene or cupric oxide oxidation products as 4-hydroxybequantity of plant residue C present in soils under different managenzaldehyde, vanillin, and syringaldehyde (Chen, 1992). ment systems. Interferences from reaction products and intensive sample preparation have limited the usefulness of the nitrobenzene and cupric oxide oxidations, respectively, for P plants are the ultimate source of esnonwoody plant or soil analysis (Chen, 1992). For a sentially all organic materials and are the direct condiscussion of the extensive sample preparation for cutributors of biological substances to soils (Hedges and pric oxide digestion and the limitation of the silylation Mann, 1979). Of the organic plant components added to derivatization procedure for gas chromatography of soil, lignin constitutes the second most abundant organic PAs, see Hedges and Ertel (1982). constituent in nature, next to cellulose (Harkin, 1973). Previous research provides substantial evidence supLignin is composed of polymerized PA monomers that porting the involvement of covalent bonding by substican be cross-linked to carbohydrates and proteins. Phetuted hydroxycinnamic acids (Hartley and Buchan, nolic acid monomers isolated from plant cell walls in1979; Fritz and Moore, 1987) bridging plant lignin (moclude derivatives of benzaldehyde, ethanone, and cinnolignol structures) to cell wall carbohydrates (Hartley, namic and benzoic acids (Hedges and Mann, 1979). 1973; Wallace et al., 1995) and proteins (Newby et al., Phenolic acids are among the most widespread classes 1980). The finding that the lignin of nonwoody plants of secondary metabolites and are known to be of great has a more dynamic structure incorporating hydroxycinsignificance in plant–soil systems (Siqueria et al., 1991). namates ester-linked to polysaccharides from the lignin They function as part of the structural plant matrix backbone was because of new extraction methodology (Siqueria, 1991), act as constitutive protection against for quantitative removal of ester-linked PAs using invading organisms (Vidhyasekaran, 1988), affect cell NaOH at ambient temperature or NaOH with heat and pressure to release the ether-linked lignin PAs (Provan et al., 1994). Sarakanen and Ludwig (1971) reported that Dean A. Martens, USDA-ARS Southwest Watershed Research Cenp-coumaric [3-(4-hydroxyphenyl)-2-propenoic acid] and ter, 2000 E. Allen Rd., Tucson, AZ 85718. Received 13 Sept. 2000. *Corresponding author ([email protected]). Abbreviations: DI, deionized; PA, phenolic acid. Published in Soil Sci. Soc. Am. J. 66:1240–1248 (2002).