Phenolic composition of soil solutions: comparative study of lysimeter and centrifuge waters

The study of soil porewater provides information on the processes occurring in the soil. For example, phenolic compounds, which are strong complexing agents, are believed to be involved in the podzolisation process. In order to assess the importance of phenolics in elemental transfer, the phenolic (total and monomeric fraction) and elemental composition of lysimetric (tension-free lysimeter water, LW) and capillary (centrifugation water, CW) waters throughout the profile of a podzol (0–90 cm) were studied. In LW the concentrations of both organic and inorganic compounds increased down the soil profile between the A and the E horizons and then decreased below the BPs horizon. Concentrations were higher in CW than in LW (ratio CW-to-LW between 1 and 9 for Ca, Cd, Cu, DOC and total phenolics) except for Fe and Al. Phenolic carbon represented only a minor part of the total dissolved organic carbon (<3% LW, <10% CW), but DOC and total phenols content were highly correlated in LW. Total phenols were correlated with Cu and Cd concentrations in CW. Phenolic compounds, including p-hydroxybenzoic, vanillic, protocatechuic, p-coumaric acids, catechol and p-hydroxy-acetophenone (a spruce-specific metabolite), were identified in both LW and CW, but significantly larger concentrations were found in CW with CW-to-LW ratios of the sum of the 10 phenolic acids varying between 47 in the OA layer and 7 in the BP layer. However, the monomeric phenols in CW collected under the OA and E layers were only a small part of the total phenolic compounds. A better knowledge of the highly polymerized soluble fulvic acids is necessary in order to understand the significance of this phenolic fraction in relation to inorganic elements via complexation processes, or plant metabolism after root absorption.