Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 6, 2001
Publication Date: N/A
Interpretive Summary: Photosynthetic plants are the ultimate source of all organic materials and the direct contributors of biological substances to soils. Agriculture has a tremendous potential to reduce carbon present in our atmosphere by fixing carbon into plants during the growing season, but this carbon also recycles back to the atmosphere following decomposition in soil. Some crop residue decomposes very rapidly and results in little change in soil organic matter, while other crop residue cycles more slowly and results in the greatest contribution to soil organic matter. Of the organic plant components added to soil, lignin and related compounds constitutes the second most abundant organic constituent in nature, next to cellulose. Lignin is composed of long chain of phenolic acids and the composition and quality of the phenolic acids have also been shown to regulate the residue decomposition rate, nutrient release from plant residues in soil and increase long-term soil structure. One of the big drawbacks to better understanding the role of phenolic acids in the carbon cycle has been the lack of documented methodology for extraction and characterization of plant and soil phenolic acids. This work present new methodology for agricultural scientists to further our understanding of how plant phenolic acids interact in the carbon cycle.
Technical Abstract: Certain phenolic acids (PA) released from plant residue 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 chromatography methodology 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 (1M NaOH) at ambient temperatures was required to extracted ester-linked phenolics and alkaline hydrolysis (4M NaOH) with heat extracted ether-linked PAs. Purification of NaOH- extracted PAs by polymeric solid phase extraction with nonderivatized gas chromatographic flame ionization/mass spectral analysis resulted in a highly reproducible and accurate method for the saponifiable PAs. The method quantified plant and soil PAs as ethanone, benzaldehyde, benzoic and cinnamic acid derivatives with the source of the majority of soil PAs identified as modified cinnamic acid from plant biomass. Comparison of the described method with a standard acid digestion (12M H2SO4) and gravimetric determination of lignin in plant residues found that interferences formed by strong acid digestion of plant residues containing higher carbohydrate and protein contents such as soybean (Glycine may L.) or clover (Trifolium pratense) resulted in an overestimation of plant lignin content. Since the majority of soil PAs originate from vascular plants and are not microbial in origin, the composition of ester-linked PAs in plants and soils may be an important indicator of the quantity of plant residue carbon present in soils under different management systems.