Photodegradation of typical Mediterranean plant litters: variations with Quercus species and the leaf-surface exposed

Photodegradation can play a key role in litter decomposition under the context of climate change which hinders organic matter turnover because of available water scarcity. Here, we aimed at testing the effect of a two-month sunlight exposure with different radiation intensities (either in winter or summer) on the chemical signature of two typical Mediterranean species, i.e., Quercus ilex and Q. pubescens . We also tested whether the side of the leaf exposed mattered in the chemical modification observed. To do so, two holistic chemical approaches, FTIR-ATR (Fourier transform infrared–attenuated total reflectance) and solid-state NMR of 13 C, were used. ANOVA from solid-state NMR of 13 C revealed that after exposure in summer, certain markers of lignin (phenol and aromatics) decreased in leaves whatever the Quercus spp. and it depended on the side of the leaf exposed. Lignin transformation thus occurred via dearomatization and/or dephenolization. Moreover, for both species, when leaves were exposed at their topside, their NMR chemical signatures were negatively correlated with alkyl assigned to cutin and waxes, showing that the thin layer of cuticle was attacked by photodegradation. FTIR-ATR spectra and NMR data also highlighted that a weaker irradiation (winter vs summer exposure) strongly limited lignin content decrease. Finally, lignin, cutin and wax oxidation, by enhancing the availability of polysaccharides from lignocellulose, may favor further litter transformation when suitable climate conditions (mainly humidity) are recovered.