Decreased rates of terpene emissions in Ornithopus compressus L. and Trifolium striatum L. by ozone exposure and nitrogen fertilization Joan Llusia a, b, * , Victoria Bermejo-Bermejo c ,H  ector Calvete-Sogo c , Josep Pe ~ nuelas a, b a CREAF, Cerdanyola del Vall es, 08193 Catalonia, Spain b CSIC, Global Ecology Unit CREAF-CEAB-UAB, Cerdanyola del Vall es, 08193 Catalonia, Spain c CIEMAT, Ecotoxicology of Air Pollution, Av. Complutense 40, 28040 Madrid, Spain article info Article history: Received 24 March 2014 Received in revised form 26 June 2014 Accepted 27 June 2014 Available online Keywords: BVOCS Terpenes Emissions Ozone Nitrogen deposition Annual pastures Gas exchange Legumes abstract Increasing tropospheric ozone (O 3 ) and nitrogen soil availability (N) are two of the main drivers of global change. They both may affect gas exchange, including plant emission of volatiles such as terpenes. We conducted an experiment using open-top chambers to analyze these possible effects on two leguminous species of Mediterranean pastures that are known to have different O 3 sensitivity, Ornithopus compressus and Trifolium striatum.O 3 exposure and N fertilization did not affect the photosynthetic rates of O. compressus and T. striatum, although O 3 tended to induce an increase in the stomatal conductance of both species, especially T. striatum, the most sensitive species. O 3 and N soil availability reduced the emission of terpenes in O. compressus and T. striatum. If these responses are confirmed as a general pattern, O 3 could affect the competitiveness of these species. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The coincidence of high solar radiation and the presence of hydroxyl radicals, NO x , and volatile organic compounds (VOCs) in the atmosphere from anthropogenic or natural origins shifts the equilibrium toward higher concentrations of ozone (O 3 )(Jenkin and Hayman, 1999) and thus favors its formation. Concentrations of O 3 at ground level have been increasing during the last century, from a mean level of 10 ppb (Anfossi and Sandroni, 1994) at the end of the 19th century to a present annual average concentration of 35e40 ppb in the mid-latitudes of the Northern Hemisphere (Fowler et al., 2008). Furthermore, the atmospheric deposition of reactive nitrogen (N) compounds has also increased globally from 32 Mt N y 1 in 1860 to ~112e116 Mt N y 1 (Pe~ nuelas et al., 2012, 2013). Current projections expect a further increase that will raise the total global annual N deposition to approximately twofold the current levels by 2050 (Galloway et al., 2004). NO, NO 2 and O 3 are in photochemical equilibrium in the steady state. The production of O 3 , and thus the ambient concentrations, depends on the input concentrations and ratios of NO, NO 2 and VOCs (Clapp and Jenkin, 2001). NO x emissions are mainly from vehicle exhausts and industrial processes, but both natural vege- tation and industry are responsible for VOC emissions. The release of VOCs, such as isoprene and mono- and sesquiterpenes, and of oxygenated compounds by plants (Fehsenfeld et al., 1992; Boland et al., 1995; Pe~ nuelas and Llusi a, 2001, 2003) contributes to the formation of aerosols (Zhang et al., 1992; Odum et al., 1996) and constitutes a significant input of reactive carbon (Fehsenfeld et al., 1992; Guenther et al., 1995) and the precursors of photochemical oxidants (Trainer et al., 1987, 2001; Roselle,1994; Simpson et al., 1995) into the atmosphere and hence might contribute to air pollution at regional scales. These compounds are important because of their atmospheric reactivity and influence on the con- centrations of tropospheric O 3 (Calogirou et al., 1996; Emeis et al., 1997; Georgopoulos et al., 1997; Kleinman et al., 1997; Pe~ nuelas and Staudt, 2010; Im et al., 2011). Tropospheric O 3 concentrations at mid-latitudes in the North- ern Hemisphere already exceed the threshold levels for the health of vegetation (The Royal Society, 2008; EEA, 2011) and are also * Corresponding author. E-mail address: j.llusia@creaf.uab.cat (J. Llusia). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2014.06.038 0269-7491/© 2014 Elsevier Ltd. All rights reserved. Environmental Pollution 194 (2014) 69e77