Interannual and seasonal changes in the soil exchange rates of monoterpenes and other VOCs in a Mediterranean shrubland D. ASENSIO , J. P EN ˜ UELAS , P. P RIETO , M. E STIARTE , I. F ILELLA & J. L LUSIA ` Unitat Ecofisiologia CSIC-CEAB-CREAF, Centre for Ecological Research and Forestry Applications (CREAF), Edifici C, Universitat Auto `noma de Barcelona, 08193 Bellaterra (Barcelona), Catalonia, Spain Summary Information about soil VOC inventories and exchange rates in different soils is very scarce. Seasonality of soil VOC exchange rates is also largely unknown, despite the increasing interest in some soil volatile compounds, such as monoterpenes, because of their important role in soil ecology. We aimed to explore and quantify soil VOC exchange rates in a Mediterranean shrubland and their seasonality. Measurements of soil VOC exchange were taken using GC-MS and PTR-MS techniques, together with soil temperature, soil moisture and soil CO 2 efflux measurements, during two annual campaigns with contrasting pre- cipitation. Methanol, acetic acid, ethyl acetate, acetaldehyde, acetone, C 3 and C 4 carbonyls (such as methyl ethyl ketone), a-pinene and limonene, showed the highest emission rates. Maximum soil mono- terpene emission rates were very low (0.003 nmol m 2 s 1 ) compared with foliar monoterpene emission rates. The emission rates of the other VOCs were also low (maximum 0.8 nmol m 2 s 1 ) except for methanol (1.2 nmol m 2 s 1 ). Maximum soil uptake rates for some VOCs, such as methanol and ace- tonitrile (ranging from 0.1 to 0.5 nmol m 2 s 1 ) were, however, comparable with foliar uptake rates. Further studies are needed to corroborate these results and the possible importance of the soil VOC sink in regional chemistry-climate models. Long-term severe drought increased soil monoterpene emission rates in this Mediterranean shrubland. The increases seem to be linked to changes in the soil’s physical properties induced by low soil moisture. Unlike monoterpenes, other soil VOC emission rates decreased when soil moisture was low. The results suggest a seasonal control of soil temperature on the emission rates of monoterpenes and other VOCs. The emission rates increase with soil temperature. Positive correlations between the VOC exchange rates and the soil CO 2 fluxes suggest that phenology of roots and microorganisms also controls seasonal changes in soil VOCs in this Mediterranean shrubland. Introduction Volatile organic compounds (VOCs) are reactive trace substan- ces present in gaseous form in the troposphere, which interact with other atmospheric trace compounds, affecting distribu- tions of air pollutants such as NO x , PANs, and particles (Atkinson & Arey, 2003) and playing a central role in tropo- spheric ozone formation (Fuentes et al., 2000; Chen & Griffin, 2005). For this reason, considerable effort has been made to identify the sources and to quantify the amounts of VOCs (Lamb et al., 1987;Mu¨ ller, 1992). VOCs originate from three main sources: anthropogenic activities, biomass burning and the biosphere. The biosphere is the largest source of VOCs, its emissions surpassing several times those from anthropogenic and biomass burning sources (Guenther et al., 1995). Natural sources of VOC emissions to the atmosphere include marine and fresh water, soil and sedi- ments, microbial decomposition of organic material, geo- logical hydrocarbon reservoirs, plant foliage and woody material. Among terrestrial ecosystems, foliar emissions (mainly isoprene and terpenes) from woodlands are considered the largest source (Guenther et al., 1994; Fuentes et al., 1996). Isoprene and monoterpenes, and some partly oxygenated VOCs such as formaldehyde, acetaldehyde, acetone, methanol, ethanol, formic and acetic acids, are synthesized in plants and emitted in large amounts into the atmosphere (Guenther et al., 1994; Fall, 1999; Fall et al., 1999; Seco et al., 2007). Most stud- ies have focused on these compounds, because they are highly reactive and have a greater ozone formation potential. These volatile compounds are also involved in numerous Correspondence: Dolores Asensio. E-mail: loles@creaf.uab.cat Received 28 August 2007, revised version accepted 9 April 2008 878 # 2008 The Authors Journal compilation # 2008 British Society of Soil Science European Journal of Soil Science, October 2008, 59, 878–891 doi: 10.1111/j.1365-2389.2008.01057.x