Identification of Phenolic Acid Composition of Alkali‐extracted Plants and Soils

Abstract

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.