Field-scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO 2 ), warming, and extended summer drought in all combinations. Here, we report on the short-term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO 2 significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co-dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO 2 had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH 4 1 consumption, gross mineralization, potential nitrification, denitrification and N 2 O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO 2 , as well as drought-induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO 2 concentration.

Summary 1.Recent findings indicate that the interactions among CO 2 , temperature and water can be substantial, and that the combined effects on the biological systems of several factors may not be predicted from experiments with one or a few factors. Therefore realistic multifactorial experiments involving a larger set of main factors are needed. 2. We describe a new Danish climate change-related field scale experiment, CLIMAITE, in a heath/ grassland ecosystem. CLIMAITE is a full factorial combination of elevated CO 2 , elevated temperature and prolonged summer drought. The manipulations are intended to mimic anticipated major environmental changes at the site by year 2075 as closely as possible. The impacts on ecosystem processes and functioning (at ecophysiological levels, through responses by individuals and communities to ecosystem-level responses) are investigated simultaneously. 3. The increase of [CO 2 ] closely corresponds with the scenarios for year 2075, while the warming treatment is at the lower end of the predictions and seems to be the most difficult treatment to increase without unwanted side effects on the other variables. The drought treatment follows predictions of increased frequency of drought periods in summer. The combination of the treatments does not create new unwanted side effects on the treatments relative to the treatments alone.

Thus, the total loss of N 2 O from nitrification and denitrification (E) can be described as The study was conducted in a sward consisting of white clover (Trifolium repens Analysis of gas samples from the field 179The 3.5 ml gas samples were pressurized by adding 2 ml N 2 before they were

In this study, we compare annual fluxes of methane (CH₄), nitrous oxide (N₂O) and soil respiratory carbon dioxide (CO₂) measured at nine European peatlands (<i>n</i> = 4) and shrublands (<i>n</i> = 5). The sites range from northern Sweden to Spain, covering a span in mean annual air temperature from 0 to 16 °C, and in annual precipitation from 300 to 1300 mm yr⁻¹. The effects of climate change, including temperature increase and prolonged drought, were tested at five shrubland sites. At one peatland site, the long-term (> 30 yr) effect of drainage was assessed, while increased nitrogen deposition was investigated at three peatland sites. <br><br> The shrublands were generally sinks for atmospheric CH₄, whereas the peatlands were CH₄ sources, with fluxes ranging from −519 to +6890 mg CH₄-C m⁻² yr⁻¹ across the studied ecosystems. At the peatland sites, annual CH₄ emission increased with mean annual air temperature, while a negative relationship was found between net CH₄ uptake and the soil carbon stock at the shrubland sites. Annual N₂O fluxes were generally small ranging from −14 to 42 mg N₂O-N m⁻² yr⁻¹. Highest N₂O emission occurred at the sites that had highest nitrate (NO₃⁻) concentration in the soil water. Furthermore, experimentally increased NO₃⁻ deposition led to increased N₂O efflux, whereas prolonged drought and long-term drainage reduced the N₂O efflux. Soil CO₂ emissions in control plots ranged from 310 to 732 g CO₂-C m⁻² yr⁻¹. Drought and long-term drainage generally reduced the soil CO₂ efflux, except at a hydric shrubland where drought tended to increase soil respiration. <br><br> In terms of fractional importance of each greenhouse gas to the total numerical global warming response, the change in CO₂ efflux dominated the response in all treatments (ranging 71–96%), except for NO₃⁻ addition where 89% was due to change in CH₄ emissions. Thus, in European peatlands and shrublands the effect on global warming induced by the investigated anthropogenic disturbances will be dominated by variations in soil CO₂ fluxes

. (2011). Effects of elevated atmospheric CO2, prolonged summer drought and temperature increase on N2O and CH4 fluxes in a temperate heathland. Soil Biology & Biochemistry, 43(8), 1660 -1670 . DOI: 10.1016 /j.soilbio.2011 were investigated solely and in all combinations in a temperate heathland. Field measurements of 28 N 2 O and CH 4 fluxes took place 1-2 years after the climate change manipulations were initiated. 29The soil was generally a net sink for atmospheric CH 4 . Elevated temperature (T) increased the 30 CH 4 uptake by on average 10 μg C m -2 h -1 , corresponding to a rise in the uptake rate of about 20 31 %. However, during winter elevated CO 2 (CO 2 ) reduced the CH 4 uptake, which outweighed the 32 positive effect of warming when analyzed across the study period. Emissions of N 2 O were 33 generally low (<10 µg N m -2 h -1 ). As single experimental factors, elevated CO 2 , temperature and 34 summer drought (D) had no major effect on the N 2 O fluxes, but the combination of CO 2 and 35 warming (TCO 2 ) stimulated N 2 O emission, whereas the N 2 O emission ceased when CO 2 was 36 combined with drought (DCO 2 ). We suggest that these N 2 O responses are related to increased 37 rhizodeposition under elevated CO 2 combined with increased and reduced nitrogen turnover rates 38 caused by warming and drought, respectively. The N 2 O flux in the multifactor treatment TDCO 2 39 was not different from the ambient control treatment. Overall, our study suggests that in the 40 future, CH 4 uptake may increase slightly, while N 2 O emission will remain unchanged in 41 temperate ecosystems on well-aerated soils. However, we propose that continued exposure to 42 altered climate could potentially change the greenhouse gas flux pattern in the investigated 43 heathland. 44 45 3