The fate of cutin and suberin of decaying leaves, needles and roots - Inferences from the initial decomposition of bound fatty acids

The lipid biopolymers cutin and suberin are frequently used as biomarkers to distinguish above - from below-ground plant tissue input in soil. Despite a growing number of studies, still little is known about their fate during decomposition. The aim of this study was to investigate the decomposition of bound fatty acids with a special emphasis on cutin and suberin and to evaluate the effect of inherent chemical properties on decomposition. We incubated fresh leaves, needles and roots of European beech and Norway spruce for 84 days in a laboratory experiment. Cutin and suberin derived monomers were obtained by a sequential extraction procedure with subsequent GC-MS measurement. We monitored the mass loss of the plant materials, changes in chemical composition using solid-state C-13 NMR spectroscopy and, from this, calculated relative amounts of biomolecule components (i.e., relative lignin content). Our results suggest that both cutin and suberin biopolymers are readily decomposed without any indication of suberin being more resistant than cutin. The concentrations of cutin and suberin derived monomers were exponentially correlated to the mass loss of the respective plant material and rapidly decreased (beech: cutin: 47.4 +/- 2.1%, suberin: 30.8 +/- 5.5%; spruce: cutin: 31.2 +/- 2.4%, suberin: 22.0 +/- 4.8% of the initial concentration) at the beginning of the incubation, but leveled off towards the end. This indicates that studies which assume a similar degradation of biomarker and source plant material might underestimate the turnover of root and shoot derived soil organic matter. Beside the tested inherent chemical properties of the lipids (number of C atoms in each monomer, type and location of chemical functional groups), the relative lignin content explained a considerable portion of the variation in lipid concentrations over time. We thus propose a two phase model for the initial decomposition of cutin and suberin: (1) in early phases, cutin or suberin that is not associated with lignin is readily consumed by microorganisms resulting in a rapid decrease of the respective polymer. (2) After the first phase, only cutin or suberin associated with lignin remains, resulting in a decomposition that proceeds with the initially low decay rate of lignin. However, a substantial part of the variation in lipid concentrations was not accounted for by the tested factors. This suggests that the decomposition of cutin and suberin is additionally modulated by a not yet quantified external factor. (C) 2016 Elsevier Ltd. All rights reserved.