Turnover of non-polymeric leaf lipids in a loamy grassland soil

Abstract

Background: Leaves constitute a large input of lipids to soil, yet comparatively little is known about the fate of leaf lipids in soil. Our aim was to explore the initial stages of degradation of leaf lipids, both the loss of intact lipid and subsequent mineralisation. We focussed on intracellular lipids – triacylglycerols implicated in storage, membrane lipids such as phospholipids and galactolipids, and pigments – because they collectively constitute more than 1% of leaf mass. Methods: A mixture of U-13C lipids was extracted from leaves of wheat grown with 13CO2. The lipid mixture included the range of plant lipids soluble in organic solvent (e.g. free fatty acids, acylglycerols, pigments) but not polymeric lipids such as cutin and suberin. Mineralisation was deduced from 13CO2 efflux, while LC–MS examined degradation of intact 13C lipids. Results: There was no delay before lipids were mineralised. Instead, mineralisation was significant within minutes and reached a maximum within three hours. There was rapid loss (i.e. degradation) of a broad range of intact lipids including phospholipids, galactolipids, pigments (chlorophylls), and triacylglycerols. Around two-thirds of added lipid-C was respired over the course of 15 days, with one-third of lipid-C persisting in soil. Conclusions: Our study indicates that non-polymeric leaf lipids degrade quickly in soil, yet a fraction of lipid-C likely persisted in degradation products and/or microbial biomass. Persistence of lipid-C probably also reflected the presence of lipids that are more resistant to degradation (e.g. phaeophytins), and a fraction of added lipid being protected (e.g. by interaction with clays). Graphical abstract: [Figure not available: see fulltext.].