Elevated CO 2 stimulates N 2 O emissions in permanent grassland Claudia Kammann a, b, * , Christoph Mu ¨ ller b , Ludger Gru ¨ nhage a , Hans-Ju ¨ rgen Ja ¨ ger a a Department of Plant Ecology, University of Gießen, Heinrich-Buff-Ring 26-32, 35392 Gießen, Germany b School of Biology and Environmental Science, University College Dublin, Dublin 4, Ireland article info Article history: Received 11 December 2007 Received in revised form 5 April 2008 Accepted 11 April 2008 Available online 27 May 2008 Keywords: Elevated CO 2 FACE Moderate CO 2 enrichment N 2 O emissions Denitrification Soil N cycling Temperate grassland abstract To evaluate climate forcing under increasing atmospheric CO 2 concentrations, feedback effects on greenhouse gases such as nitrous oxide (N 2 O) with a high global warming potential should be taken into account. This requires long-term N 2 O flux measurements because responses to elevated CO 2 may vary throughout annual courses. Here, we present an almost 9 year long continuous N 2 O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N 2 O emissions were not different between plots that were later under ambient (A) and elevated (E) CO 2 treatments, respectively. However, over the entire experimental period (May 1998– December 2006), N 2 O emissions more than doubled under elevated CO 2 (0.90 vs. 2.07 kg N 2 O-N ha 1 y 1 under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N 2 O emissions due to elevated CO 2 when mineral N concentrations were above background values (e.g. shortly after N application in spring). N 2 O emissions under elevated CO 2 were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N 2 O emissions caused by elevated CO 2 equaled 4738 kg CO 2 -equivalents ha 1 , corresponding to more than half a ton (546 kg) of CO 2 ha 1 which has to be sequestered annually to balance the CO 2 -induced N 2 O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO 2 concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N 2 O emissions. Our results underline the need to include continuous N 2 O flux measurements in ecosystem-scale CO 2 enrichment experiments. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Not only CO 2 but also greenhouse gases (GHGs) such as N 2 O with a much higher global warming potential (GWP) than CO 2 have dramatically increased during the last century, caused by the supply of available N to soils and oceans via organic and inorganic amendments (IPCC, 2007). Nitrous oxide is predominantly produced in oxic and anoxic soil microsites through microbial ni- trification and denitrification, respectively (Granli and Bøckmann, 1994). With a GWP 298 times that of CO 2 over 100 years (IPCC, 2007), N 2 O has the potential to convert a positive ecosystem GHG balance (uptake) into a negative one (release) (Crutzen et al., 2007). It is well known that the production of N 2 O is tightly coupled to the soil organic carbon (SOC) status (Burford and Bremner, 1975; Sahrawat et al., 1986; Granli and Bøckmann, 1994; Li et al., 2005). Such an increase in the SOC status has been observed with elevated atmospheric CO 2 concentrations via increased plant litter inputs (meta-analysis: Jastrow et al., 2005; natural CO 2 spring: Kool et al., 2007). Thus, N 2 O emissions could theoretically rise with increasing atmospheric CO 2 concentrations on decadal time scales. Elevated CO 2 concentrations may alter N 2 O-producing soil processes (indirectly via plants) in the following ways. Denitrification-derived N 2 O emissions (N 2 O denit ) may be stimu- lated by increasing plant biomass and hence C-substrate availability (Smith and Tiedje, 1979a). The plant biomass, in particular belowground, usually increases under CO 2 enrichment (e.g. plant litter, root residues, root exudates; Zak et al., 2000; Jastrow et al., 2005). Such a carbon-supply increase is frequently discussed as the dominant mechanism causing higher N 2 O denit fluxes under elevated CO 2 , in particular when the soil mineral N availability for denitrifiers was high (Ineson et al., 1998; Baggs et al., 2003a, 2003b; Kettunen et al., 2006, 2007). N 2 O denit production may also be stimulated by increasing anaerobicity via rising soil respiration, or soil moisture (Smith and Tiedje, 1979b). Soil moisture increases often occurred under elevated CO 2 in grassland ecosystems, due to * Corresponding author. Department of Plant Ecology, University of Gießen, Heinrich-Buff-Ring 26-32, 35392 Gießen, Germany. Tel.: þ49 641 9935320; fax: þ49 641 9935309. E-mail address: Claudia.Kammann@bot2.bio.uni-giessen.de (C. Kammann). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2008.04.012 Soil Biology & Biochemistry 40 (2008) 2194–2205