Decline of soil volatile organic compounds from a Mediterranean deciduous forest under a future drier climate

Biogenic volatile organic compounds (BVOCs) are crucial for ecosystem functioning, atmospheric chemistry and climate. While modulation of BVOC emissions from living vegetation with biotic and abiotic factors is well documented, how these factors drive soil BVOC emissions remain less understood, particularly in Mediterranean forests. To fill this gap, this pioneer study investigates whether BVOC fluxes from natural soil covered by litter (referred to as forest soil) vary under natural and amplified long-term water stress (35% annual rain exclusion over the past 10 years) in a deciduous oak Mediterranean forest (Quercus pubescens Willd.) compared to natural climate conditions. This aim has only been tackled in a single evergreen Mediterranean forest so far. Using proton transfer reaction time of flight mass spectrometer (PTR-ToF-MS) we also provide, for the first time, a detailed diurnal cycle of soil BVOCs in relation to air temperature, air humidity, and biotic factors including soil respiration and litter content in lignin, cellulose and hemicellulose. The main results revealed that forest soil represents a source of most BVOCs (e.g., acetaldehyde, acetone, acrolein, hexanol, monoterpenes) with maximum values at mid-day (42 μgC.m⁻². h⁻¹) in response to higher temperatures while it acts as a clear sink of isoprene. Total soil BVOC emission rates, together with soil respiration, decreased by 43% after a decade of partial rain restriction. These results will contribute to enhance further modeling of soil BVOC emissions under various climate scenarios both at regional and global scales.