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

Abstract

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, ω-hydroxyalkanoic acids, α-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 ω-hydroxyalkanoic and α,ω-alkanedioic acids (>C₁₆) 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 (C₁₄–C₁₈) 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 ω-hydroxyalkanoic and α,ω-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 ω-hydroxyalkanoic acids. However, the steady input of roots which are rich in ω-hydroxyalkanoic acids and low in mid-chain-substituted acids may in part influence the degradation parameters resulting in an apparent higher rate of degradation.

Introduction

Aliphatic lipids are common constituents of soil organic matter (SOM) but only limited information is available on their molecular composition (Kögel-Knabner et al., 1989, Dinel et al., 1990, Kögel-Knabner, 2000). Long-chain lipids from plant waxes and aliphatic biomacromolecules from leaves, bark and roots have been proposed as the major sources of the aliphatic constituents found in SOM (Kögel-Knabner, 2002). Analytical studies of SOM using nuclear magnetic resonance spectroscopy (NMR) and chemolytic methods have suggested that the biomacromolecules suberin and cutin are major sources of hydrolysable aliphatic lipids in SOM (Nierop, 1998, Kögel-Knabner, 2000, Naafs and van Bergen, 2002a, Nierop and Verstraten, 2003, Nierop et al., 2003). Cutin and suberin are biomacromolecules common in vascular plants and primarily function as barriers to prevent water loss (Kolattukudy and Espelie, 1989). Cutin is a major component of leaf cuticle and is mainly composed of short-chain (C₁₄–C₁₈) hydroxy- and epoxy acids (Holloway, 1982). Suberin occurs in the periderm of roots and barks and is composed of predominantly long-chain (C₂₀–C₃₂) aliphatic acids, diacids and ω-hydroxy acids and minor amounts of phenolic constituents (Kolattukudy and Espelie, 1989, Bernards, 2002). Since cutin is only produced by terrestrial plants, cutin-derived aliphatic lipids are used as markers for terrestrial inputs in to marine and riverine sediments (Goñi and Hedges, 1990a, Goñi and Hedges, 1990b, Gough et al., 1993, Prahl et al., 1994). Although the leaf cutins and suberin found in barks and roots of numerous plants have been analyzed (e.g., Eglinton and Hunnemann, 1968, Hunnemann and Eglinton, 1972, Holloway, 1982, Kolattukudy and Espelie, 1989, Goñi and Hedges, 1990a, Goñi and Hedges, 1990b), detailed studies of cutin and suberin in soils are limited and not much is known about the composition of these biomacromolecules in soils (Kögel-Knabner et al., 1989, Riederer et al., 1993, Nierop et al., 2001, Nierop et al., 2003, Naafs and van Bergen, 2002a, Naafs and van Bergen, 2002b, Nierop et al., 2003).

Lipids bound in cutin and suberin are not extractable with organic solvents, but they can be cleaved from SOM using chemolytic methods such as alkaline hydrolysis or boron trifluoride facilitated hydrolysis (Kögel-Knabner et al., 1989, Riederer et al., 1993, Kögel-Knabner, 2000). Depolymerization of cutin and suberin can also be achieved by thermochemolysis with tetramethylammonium hydroxide (del Rio and Hatcher, 1998, Nierop et al., 2001). Previous investigations of the hydrolysable lipids in soils demonstrate that the characteristic monomers of cutin and suberin are still observed indicating the preservation of cutin and suberin patterns (Almendros and Sanz, 1989, Almendros and Sanz, 1991, Riederer et al., 1993, Nierop et al., 2001, Nierop et al., 2003, Naafs and van Bergen, 2002a, Naafs and van Bergen, 2002b). Since the constituents of cutin and suberin are very similar, a ratio based on monomer characteristics for both biomacromolecules was proposed to estimate the input of aliphatic lipids derived from leaves (cutin) vs. roots (suberin) into soils (Kögel-Knabner et al., 1989). Incubation experiments with leaf material in soils, water, and sediments have been performed to study the degradation of cutin in natural environments (Goñi and Hedges, 1990c, Riederer et al., 1993, Opsahl and Benner, 1995). Although the general biodegradation of cutin and suberin in soils is supported by field and laboratory studies, it is unclear if the degradation progresses uniformly for all suberin and cutin monomers or if some of the constituents are preferentially degraded (Riederer et al., 1993, Nierop et al., 2001, Nierop et al., 2003, Naafs and van Bergen, 2002a, Naafs and van Bergen, 2002b).

In the present study, grassland and forest soils and their overlying source vegetation were subjected to alkaline hydrolysis to investigate the composition and degradation of cutin and suberin. Samples were pre-extracted to remove organic solvent soluble components such as free lipids, free carbohydrates, steroids, diterpenoids, triterpenoids, and phenols (Otto et al., 2005, Otto and Simpson, 2005). The direct comparison of the hydrolysis products from the biomacromolecules in plants and soils is advantageous to study the preservation and degradation of characteristic molecular markers (“biomarkers”) in soils. The comparative biomarker patterns for selected soils from Alberta, Canada, and their corresponding source vegetation (western wheatgrass, aspen, lodgepole pine) are presented.