Identification of phenolic acid composition of alkali-extracted plants and soils

Phenolic acids (PAs) released from plant residues have been implicated as important components in a variety of soil processes. To evaluate the role of plant PAs in soil processes, a quantitative alkaline extraction, solid-phase purification, and gas chromatographic protocol was developed for identification of the composition and concentration of plant and soil PAs. Water-soluble or EDTA-exchangeable PAs were not detected in soil. Alkaline hydrolysis (1 M NaOH) at ambient temperatures was required to extract ester-linked phenolics and alkaline hydrolysis (4 M NaOH) with heat extracted ether-linked PAs present in plant and soil material. Purification of NaOH-extracted PAs by polymeric solid-phase extraction with gas chromatographic flame ionization and mass spectral analysis of nonderivatized extracts resulted in a highly reproducible and accurate method for the saponifiable PAs. The method quantified plant and soil PAs as ethanone (acetylbenzene), benzaldehyde, and benzoic- and cinnamic-acid derivatives. The majority of soil PAs was identified as modified cinnamic acids originating from vascular plant tissue. Comparison of the described method with a standard acid digestion (12 M H2SO4) and gravimetric determination of lignin in plant residues found that interferences formed by strong acid digestion of plant residues such as soybean [Glycine max (L.) Merr.] or clover (Trifolium pratense L.) containing higher carbohydrate and protein contents resulted in an overestimation of plant lignin content when measured by the acid digestion-gravimetric method. Since the majority of soil PAs originate from vascular plants and are not microbial in origin, the composition of ester-linked PAs in soils may be an important indicator of the quantity of plant residue C present in soils under different management systems.