Soil Lignin Phenolics: Comparison of the CuO Oxidation Vs. the Ester- and Ether-Linked Phenolics Methods.

Lignin phenolics, an important pool C in soils, can be used as indices for the soil organic matter formation and transformations. Assessment of soil lignin phenolics is needed to understand the C dynamics and ultimately plant nutrient cycling. Soils, and their respective silt and clay fractions, from forest, silvopasture, and pasture systems were assessed by HPLC for lignin phenolics using the Cu oxide oxidation method and ester- and ether-linked phenolics by NaOH. Distribution and recovery of lignin phenolics (p-hydroxyl, vanillyl, syringyl, and cinnamyl phenols) were dependent on the type od sample and the method used. For the soils, the CuO oxidation method extracted ~ 300% more lignin phenolics than the sum of the ester- and ether-linked phenolics, the syringyl and vanillyl phenols where the highest in the CuO. For the clay and silt fractions, the sum of the ester- and ether-linked phenolics were greater than the lignin phenolics extracted by the CuO oxidation method; p-hydroxyl and cinnamyl were greatest in the clay fraction, whereas vanillyl phenols were greatest in the silt fraction in the ester- and ether-linked phenolic approach. The differences on the lignin phenolics pools in the clay and silt fractions indicate that the soil organic matter associated with these fractions is chemically different. The differences on the recoveries within fractions are likely due to the different chemistries of the extractions and the samples. Comparison of the two approaches above for the determination of soil lignin phenolics must exerted with caution.