Sources and composition of hydrolysable aliphatic lipids and phenols in soils from western Canada

The hydrolyzable lipid composition of grassland and forest soils, and their overlying vegetation (i.e., prairie grasses, aspen, and pine) were analyzed by alkaline hydrolysis and gas chromatography-mass spectrometry (GC-MS). Prior to hydrolysis, samples were pre-extracted with organic solvents to remove free lipids. The hydrolysis products from soils and plants consisted of series of aliphatic lipids (alkanoic acids, alkanols, alkanedioic acids, omega-hydroxyalkanoic acids, alpha-hydroxyalkanoic acids, mid-chain-substituted hydroxy and epoxy acids), benzyls and phenols. Cutin, suberin and plant waxes are believed to be the major sources for the detected lipids. The composition of cutin and suberin monomers in the soils is similar to the patterns observed in the corresponding vegetation reflecting the preservation of plant biomolecules such as cutin and suberin. The long-chain omega-hydroxyalkanoic and alpha,omega-alkanedioic acids (> C-16) as observed in the grass roots are typical biomarkers for suberin and indicate a major input of root biomass to the soils. Mid-chain-substituted hydroxy and epoxy acids (C-14-C-18) derived from the cutin of leaves were detected in the soils but in low abundance, indicating only a minor input of leaf material. A parameter to estimate the input of leaf vs. root organic matter is developed and applied based on the pattern and distribution of observed cutin and suberin biomarkers. The cutin/suberin ratios decrease from plant material to the mineral horizons likely due to the steady input of fresh root material in the soil horizons rather than the preferential degradation of cutin. The decrease of total hydrolysis products from the overlying vegetation to the soil is consistent with the degradation of hydrolysable biomarkers. Long-chain alpha-hydroxyalkanoic and alpha,omega-alkanedioic acids derived from suberin are observed to decompose without preference of specific constituents. In contrast, mid-chain-substituted hydroxy and epoxy acids appear to be preferentially degraded in soils compared to other cutin and suberin monomers such as omega-hydroxyalkanoic acids. However, the steady input of roots which are rich in omega-hydroxyalkanoic acids and low in mid-chain-substituted acids may in part influence the degradation parameters resulting in an apparent higher rate of degradation. (c) 2006 Elsevier Ltd. All rights reserved.