Isoprene emission from a subarctic peatland under enhanced UV-B radiation

Authors


Author for correspondence: Päivi Tiiva
Tel: +358 17 163204
Fax: +358 17 163191
Email: paivi.tiiva@uku.fi

Summary

  • • Isoprene is a reactive hydrocarbon with an important role in atmospheric chemistry, and emissions from vegetation contribute to atmospheric carbon fluxes. The magnitude of isoprene emissions from arctic peatlands is not known, and it may be altered by increasing UV-B radiation.
  • • Isoprene emission was measured with the dynamic chamber method from a subarctic peatland under long-term enhancement of UV-B radiation targeted to correspond to a 20% loss in the stratospheric ozone layer. The site type of the peatland was a flark fen dominated by the moss Warnstorfia exannulata and sedges Eriophorum russeolum and Carex limosa. The relationship between species densities and the emission was also assessed.
  • • Isoprene emissions were significantly increased by enhanced UV-B radiation during the second (2004) and the fourth (2006) growing seasons under the UV-B exposure. Emissions were related to the density of E. russeolum. The dominant moss, W. exannulata, proved to emit small amounts of isoprene in a laboratory trial.
  • • Subarctic fens, even without Sphagnum moss, are a significant source of isoprene to the atmosphere, especially under periods of warm weather. Warming of the Arctic together with enhanced UV-B radiation may substantially increase the emissions.

Introduction

Isoprene (2-methyl-1,3-butadiene) is the dominant volatile organic compound (VOC) released to the atmosphere by vegetation. The yearly global emission is estimated to be 440–660 Tg C (Guenther et al., 2006). Although isoprene is not emitted by all plant species (Kesselmeier & Staudt, 1999), the amount of assimilated C that is re-emitted back to the atmosphere as isoprene can form a substantial C loss at both species and ecosystem levels (Kesselmeier et al., 2002).

Northern peatlands are estimated to store as much as 270–370 Pg C (Turunen et al., 2002) and to emit substantial amounts of methane, an important greenhouse gas (Bartlett & Harris, 1993). Boreal Sphagnum peatlands have also been shown to emit significant amounts of isoprene (Janson & De Serves, 1998; Janson et al., 1999; Haapanala et al., 2006; Tiiva et al., 2007) and, to a lesser extent, other volatile organic compounds (Rinnan et al., 2005). However, no investigations have been made on isoprene emissions from subarctic peatlands dominated by mosses other than Sphagnum. Subarctic Siberian spruce (Picea abies ssp. obovata) and mountain birch (Betula pubescens ssp. czerepanovii) forests near the timber line have been shown to emit small amounts of isoprene (Rinne et al., 2000).

Isoprene has a significant role in atmospheric chemistry (Fuentes et al., 2000; Atkinson & Arey, 2003). It can increase the tropospheric ozone concentration or scavenge ozone; the balance is dependent on the concentration of NOx compounds and the level of UV radiation in the atmosphere (Finlayson-Pitts & Pitts, 1997). Isoprene can also extend the lifetime of methane by changing the oxidation capacity of the atmosphere (Bell et al., 2003). Furthermore, Claeys et al. (2004a, 2004b) have shown that oxidation of isoprene in the atmosphere leads to significant aerosol formation.

The function of isoprene emission from plants has received a lot of attention, but the significance of the emission remains unclear. The emission is dependent on temperature and light conditions (Kesselmeier & Staudt, 1999), and it can also be affected by environmental stress factors, such as increased tropospheric ozone concentration, as demonstrated in common reed (Phragmites australis) and downy oak (Quercus pubescens) (Velikova et al., 2005a, 2005b), or by UV-B enhancement as shown in gambel oak (Quercus gambelii) (Harley et al., 1996). The antioxidant role of isoprene has been shown in tobacco (Nicotiana tabacum), silver birch (Betula pendula) (Loreto et al., 2001) and common reed (Loreto & Velikova, 2001) under increased ozone concentration. The contribution of isoprene emission on thermotolerance has been demonstrated in oriental plane (Platanus orientalis) (Velikova et al., 2006), common reed (Velikova & Loreto, 2005) and the peat moss Sphagnum capillifolium (Hanson et al., 1999). Another hypothesis for the role of isoprene suggests that it is released from plants to dissipate extra energy and C under stressful conditions (Peñuelas & Llusià, 2004), for instance when photosynthesis is inhibited by limited CO2 supply (Magel et al., 2006).

Increasing levels of solar UV-B radiation (280–320 nm) resulting from the stratospheric ozone depletion have been reported, especially in the Arctic (ACIA, 2004). Although the increase in the concentration of ozone-depleting substances in the atmosphere has slowed down, the surface UV-B levels are expected to remain high for several decades (WMO, 2003). Research on UV-B effects on vegetation has been intensive, especially in the high latitudes where organisms are suggested to be weakly adapted to the increased UV-B radiation because of historically low levels of UV-B (Caldwell et al., 1980). However, studies on UV-B effects on isoprene emission from plants are negligible, and the potential effects of UV-B on ecosystem-level isoprene emission are so far unexplored. As Niemi et al. (2002) observed significant reduction in gross photosynthesis, net ecosystem exchange and methane emission from boreal peatland microcosms under enhanced UV-B radiation, it is possible that ecosystem-level isoprene emission is also affected.

The aim of the present study was to obtain the first estimate of the magnitude of isoprene emission from a subarctic peatland, and to assess the effects of long-term (four growing seasons) UV-B enhancement on these emissions in situ. We hypothesized that enhanced UV-B radiation would increase isoprene emission, because this response has previously been shown in a single species under realistic enhancement in UV-B radiation (Harley et al., 1996). To relate isoprene emission to photosynthesis, we measured the net C assimilation at the same site. To assess further the role of the vegetation cover, we measured isoprene emission of the dominant moss of the studied peatland in controlled laboratory conditions, and analysed the relationship between vascular plant densities, environmental factors and the emission. To our knowledge, this is the first report addressing the effects of enhanced UV-B radiation on isoprene emissions at the ecosystem scale.

Materials and Methods

Experimental design

Plots of a pristine subarctic mesotrophic flark fen were exposed to enhanced UV radiation for growing seasons 2003–06. The fen is a part of a larger peatland complex, Halssiaapa, in Sodankylä, Northern Finland (67°22′N, 26°38′E, 179 m asl). The vegetation of the plots consisted of a continuous cover of the moss Warnstorfia exannulata (Schimp.) Loeske. The vascular plants consisted mainly of sedges Eriophorum russeolum Fr. ex Hartm. and Carex limosa L. In addition there was some Carex magellanica Lam., Andromeda polifolia L., Vaccinium oxycoccos L., Menyanthes trifoliata L. and Scheuchzeria palustris L. in some of the plots (Table 1). The peat mosses Sphagnum flexuosum Dazy & Molk. and Sphagnum ripariumÅngstr. were present as minor patches in a few plots. The densities of vascular plant species in each plot were measured by counting the number of leaves or shoots in five subplots (10 × 10 cm). The water table, which was monitored during the experiment, fluctuated from above the soil surface to 19 cm below the surface during the measurement periods.

Table 1.  Mean leaf densities of Eriophorum russeolum, Carex limosa, Carex magellanica, Menyanthes trifoliata and Scheuchzeria palustris, and shoot densities of Vaccinium oxycoccos and Andromeda polifolia (leaves or shoots m−2 ± SE) in the ambient control, UV-A control and UV-B treatments in the middle of the 2003–06 growing seasons
Species2003200420052006
AmbientUV-AUV-BAmbientUV-AUV-BAmbientUV-AUV-BAmbientUV-AUV-B
  • *

    Species present in only one plot.

Eriophorum russeolum2370 ± 2802520 ± 2603540 ± 2001720 ± 2201950 ± 2002620 ± 1401940 ± 3001920 ± 2702390 ± 3401370 ± 2101290 ± 1702002 ± 160
Carex limosa 890 ± 230 940 ± 320 390 ± 140 660 ± 220 660 ± 190 190 ± 90 460 ± 160 550 ± 190 160 ± 40 970 ± 300 620 ± 240 150 ± 50
Carex magellanica  20 ± 17   0   1*  26 ± 18   0   5*  25 ± 17   0  5*  36 ± 25   1*   7*
Menyanthes trifoliata  60*  65 ± 45   0   7*  18 ± 12   0   8*  24 ± 16  0  33* 100 ± 64   0
Scheuchzeria palustris  70 ± 49 207 ± 83  93 ± 65   3*  43 ± 24  35 ± 25   0  53 ± 33  34 ± 23  65 ± 29 118 ± 49  51 ± 35
Vaccinium oxycoccos  36 ± 29  13 ± 13   6*  39 ± 32  12 ± 11   5*  41 ± 32  13 ± 12  5* 110 ± 83  47 ± 44  20 ± 20
Andromeda polifolia   4*  14 ± 10   0   4*  16 ± 11   0   3*  22 ± 15  0   2*  17 ± 14   0

The experiment consisted of 30 120 × 120-cm plots, 10 of which were exposed to enhanced UV-B radiation, another 10 to enhanced UV-A radiation (UV-A control), and the rest to ambient solar UV radiation (ambient control) in a completely randomized design. The UV-A control was included in the experiment in order to differentiate between the actual UV-B enhancement effects and the effect of the additional UV-A (< 5% of the ambient UV-A fluxes), which was an unwanted side-effect of the UV-B enhancement. An aluminium frame carrying four fluorescent tubes (Philips TL 40 W/12 RS, Philips Lighting, Eindhoven, the Netherlands) arranged as a square was positioned 120 cm above the peat surface on each plot. In the UV-B treatment, the tubes were encased in a cellulose acetate film (thickness 100 µm; Expopak, Jäminkipohja, Finland) which cuts off the radiation below 290 nm. In the UV-A control, the tubes were encased in a polyester film (thickness 125 µm; Melinex, KTA-yhtiöt, Helsinki, Finland) to cut off the UV-B and UV-C wavelengths. Each aluminium frame had two faders (Quicktronic HF 2 × 36/230–240 V DIM, Osram, Vantaa, Finland) controlling the current passing to the fluorescent tubes. The ambient control plots were equipped with wooden frames, which caused equivalent shading as the aluminium frames carrying the fluorescent tubes.

In the UV-B treatment, UV-B radiation was enhanced by a modulated exposure system. The target level of the enhancement was 46% above the ambient level, corresponding to a 20% decrease in the stratospheric ozone layer. However, for technical reasons the enhancement achieved was approx. 20% (Fig. 1). The flux of UV-B radiation was monitored continuously with an erythemally weighted sensor (PMA1102, Solar Light Co Inc., Glenside, PA, USA) in all 10 UV-B treatment plots, one UV-A control plot and one ambient control plot. In addition, the flux of UV-A radiation was monitored (PMA1111) in one plot of each treatment.

Figure 1.

Mean daily doses of erythemally weighted UV-B radiation under ambient solar radiation (dotted line) and the UV-B (solid line) treatment in growing seasons (a) 2004; (b) 2005; (c) 2006. Gaps in lines indicate absence of data due to technical failure.

Isoprene sampling and analysis

Samples for isoprene analysis were taken first on 20 July 2004, then in five campaigns between 2 and 20 July in 2005, and through the growing season of 2006 (7 June–15 August). During the measurements, a transparent polycarbonate chamber (thickness 1.5 mm, 60 × 60 cm, height 25 cm, Vink Finland, Kerava, Finland) was placed on an aluminium collar installed in each plot. The collar was filled with water to ensure air enclosure. The sample was pumped with a small battery-operated sampling pump (SKC 222-3, SKC Inc., Eighty Four, PA, USA in 2004 and 2005; 12 V. Rietschle Thomas, Puchheim, Germany in 2006) through an ATD steel sample tube (Perkin Elmer, Boston, MA, USA) filled with adsorbents. Tenax TA (150 mg, mesh 60/80, Supelco, Bellefonte, PA, USA) was used in 2004 and 2005, and a combination of Tenax TA and Carbopack B (100 mg of each, mesh 60/80, Supelco, Bellefonte, PA, USA) in 2006. The adsorbent was changed because of possible breakthrough during the unexpectedly high isoprene emissions in 2005, and because Carbopack B retains C5 compounds more efficiently than Tenax TA (Hakola et al., 2001). The absence of breakthrough was verified by two adsorbent tubes in a series in 2006.

The air flow in the sampling pumps was set to 205 ml min−1 with an M-5 bubble flowmeter (A.P. Buck, Orlando, FL, USA). This corresponds to a flow of 200 ml min−1 through the sample tube. To avoid air leakage from outside into the chamber, the chamber was maintained slightly overpressurized by pumping in charcoal-filtered air through a MnO2 scrubber at a rate of 215 ml min−1. During the 30-min sampling, the chamber air was circulated with a small fan, and the temperature and air humidity inside the chamber were monitored (Tinyview Plus, Gemini Data Loggers Ltd, Chichester, UK). In addition, photosynthetically active radiation (PAR) was monitored with a quantum sensor (Li-Cor, Lincoln, NE, USA). The sampling time of day was noted as minutes from midnight. The sample tubes were sealed immediately with Teflon-coated brass caps after the sampling, stored refrigerated and analysed at the University of Kuopio within 2 wk.

The samples were analysed by gas chromatography–mass spectrometry (Hewlett Packard 6890, MSD 5973, Palo Alto, CA, USA) after thermodesorption at 250°C and cryofocusing at –30°C with an automatic thermal desorber (ATD400, Perkin Elmer, Wellesley, MA, USA). Isoprene was separated using an HP-5 capillary column (50 m × 0.2 mm, film thickness 0.5 µm). The carrier gas was helium. The oven temperature was held at 40°C for 1 min and then raised to 210°C at a rate of 5°C min−1, and finally further to 250°C at a rate of 20°C min−1. Isoprene was identified according to the mass spectra in the Wiley data library, and quantified with a pure standard (Fluka, Buchs, Switzerland).

Emission rates were calculated by multiplying the isoprene concentration of the sample by the chamber volume to obtain the absolute amount of isoprene in the chamber. The surface topography and the water table of each plot were taken into account when determining the chamber volume. The absolute amount of isoprene in the chamber was then proportioned to the surface area of the plot and to the sampling time.

As biogenic isoprene emissions depend strongly on light and temperature, all emissions were made comparable by standardizing to the temperature of 30°C and the PAR of 1000 µmol m−2 s−1 by the classic algorithm established by Guenther et al. (1993), and only the standardized emissions are presented. The suitability of the algorithm to isoprene emissions from peatlands has been recognized and discussed further by Janson and De Serves (1998); Haapanala et al. (2006); Hellén et al. (2006).

Carbon assimilation

The net ecosystem CO2 exchange (NEE) was measured using the static chamber method (Alm et al., 1997) six times during the 2006 growing season. The CO2 concentration in the headspace of a cooled transparent chamber was analysed at 30-s intervals (one measurement lasted 4 min) using a portable infrared gas analyser (LCA-2, ADC Ltd, Hoddesdon, UK). PAR and chamber temperature were monitored during the measurements. CO2 flux rates were calculated from the linear change in the CO2 concentration in the chamber.

Laboratory trial

Patches of the moss W. exannulata were taken carefully from the fen on 5 July 2006. The moss was grown in a growth chamber (day : night temperature 20 : 16°C, relative humidity 40–60%, maximum light intensity 280 µmol m−2 s−1) and watered with distilled water until the isoprene emission measurements on 13 September 2006.

One day prior to the trial, the moss carpet (thickness 6 cm) was cut into 10 approx. 5 × 5-cm samples. The samples were enclosed in 1.5-l glass containers, which were flushed with charcoal and MnO2-filtered air through Teflon tubing at a flow rate of 215 ml min−1. Air samples were collected from the glass containers at a flow rate of 200 ml min−1 through Teflon tubing and Tenax TA/Carbopack B adsorbent tubes as described earlier. The headspace collection technique has been described in detail by Vuorinen et al. (2004). The 60-min sampling took place at a stable temperature of 22°C, and PAR of 100 µmol m−2 s−1 supplied by two lamps (LIVAL Shuttle Plus 24 W, Lival, Sipoo, Finland). The air samples were analysed by a GC–MS as described earlier. The moss samples were oven-dried (24 h at 60°C), and the emissions were proportioned to the biomass dry weight.

Statistical analyses

Statistical analyses were performed using spss 14.0 (SPSS Inc., Chicago, IL, USA). The overall effects of UV-B radiation on the repeated isoprene emission measurements in 2005 and 2006 and on NEE in 2006 were tested with linear mixed models. The analysis was followed by Dunnett's test for a pairwise comparison of the UV-B treatment with each of the controls at each measurement campaign. For the single-day isoprene measurement in 2004, one-way anova followed by Dunnett's test was applied. The field plot was used as the unit of replication. The differences in plant species densities in the measurement plots in mid-growing seasons 2003–06 were analysed with linear mixed models as repeated measurements.

The correlation between all vascular plant species densities, time of day, depth of water table, relative humidity, temperature, PAR and the nonstandardized isoprene emission in the data of 2006 was studied with principal components analysis (PCA) using simca-p 11.0 (Umetrics Inc., Umeå, Sweden). A model with two principal components was extracted after unit variance (UV) scaling of the variables. The PCA results were also used in selection of covariates for the mixed models described above; variables that correlated with isoprene emission in the PCA, and were significant covariates in the mixed models analysis, were included. Temperature and PAR were excluded from the mixed models as all the data were standardized with the Guenther et al. (1993) algorithm.

Results

Isoprene emissions from a subarctic peatland

The subarctic fen under investigation was shown to be a substantial source of isoprene to the atmosphere (Figs 2a,3a; Table 2). Emissions varied from below the detection limit to 330 µg m−2 h−1 (1.35 nmol m−2 s−1) in 2004 and 2006, whereas in 2005 they were an order of magnitude higher, from below the detection limit to 8960 µg m−2 h−1 (36.5 nmol m−2 s−1). The highest standardized isoprene emissions, 200–8960 µg m−2 h−1, were measured on 2 July 2005. As the largest concentrations probably exceeded the breakthrough concentration of isoprene for the Tenax TA adsorbent tubes, the real emissions were probably even higher.

Figure 2.

(a) Mean standardized isoprene emission (+SE, n = 6–9) under ambient control (open bars), UV-A control (grey bars) and UV-B (black bars) treatments in July 2005; (b) mean water table height (grey bars, 0 indicates moss surface), chamber temperature (closed circles) and photosynthetically active radiation (PAR, open circles) during measurements. The first two campaigns (striped bars) were not included in the statistical analysis (see Results). No statistically significant treatment effects were observed.

Figure 3.

(a) Mean standardized isoprene emission (+SE, n = 10) under ambient control (open bars), UV-A control (grey bars) and UV-B (black bars) treatments during the 2006 growing season; (b) mean water table height (grey bars, 0 indicates moss surface), chamber temperature (closed circles) and photosynthetically active radiation (PAR, open circles) during measurements. Significant P values for repeated measurements are shown (linear mixed models). Significant difference from the UV-B treatment within the measurement point: *, P < 0.05; ***, P < 0.001 (Dunnett's test).

Table 2.  Mean isoprene emission, chamber temperature, photosynthetically active radiation (PAR) and water table height in the single-day measurement on 20 July 2004
ParameterAmbientUV-AUV-B
  1. Significant difference from UV-B treatment: *, P < 0.05; **, P < 0.01 (Dunnett's test).

Isoprene emission (µg m−2 h−1) (±SE, n = 4)  71 ± 32**  85 ± 34* 155 ± 32
Chamber temperature (°C)26.0 ± 1.025.1 ± 0.826.8 ± 1.5
PAR (µmol m−2 s−1) 628 ± 47 540 ± 50 562 ± 62
Water table (cm) 3.0 ± 1.1 5.9 ± 1.1 4.5 ± 1.1

Relationship between environmental factors, vegetation and isoprene emission

Principal components analysis was used to study relationships between environmental factors, vegetation and isoprene emission. The first principal component (PC), which explained 29% of variance in the data, mostly accounted for the variance in isoprene emission (Fig. 4). Temperature, PAR and time of day – all located to the same direction as isoprene emission on PC1 – showed positive correlation with isoprene emission. In contrast, relative humidity of the measurement chamber was negatively correlated with the emission, based on both the PCA and the significance of this variable as a covariate in the linear mixed model (P < 0.001). The depth of the water table also had a negative correlation with isoprene emission in the PCA, but it appeared insignificant as a covariate in the mixed models, and was therefore excluded from these models.

Figure 4.

Loading plot of the principal components analysis on environmental factors, vascular plant densities and isoprene emission in 2006. The variance explained by principal components 1 and 2 (PC1 and PC2) is shown in parentheses. WT, water table depth; RH percentage, relative humidity in the chamber; PAR, photosynthetically active radiation.

The loading plot of the PCA (Fig. 4) confirmed that the nonstandardized isoprene emission correlated with temperature and light in the measurements of 2006. Therefore standardization of the emissions by the Guenther et al. (1993) algorithm is considered applicable. However, even the standardized isoprene emissions were higher under warmer weather conditions. The weather conditions in July 2005 (Fig. 2b) were exceptionally warm and sunny for northern Finland, whereas in 2006 (Fig. 3b) the temperature was close to the long-term average, but precipitation was very low (Finnish Meteorological Institute, 2005, 2006).

The second PC (explained variance 14%) was mainly related to the leaf density of A. polifolia, V. oxycoccos and M. trifoliata, which did not correlate with isoprene emission (Fig. 4). On the contrary, the leaf density of E. russeolum, and to a lesser extent that of C. magellanica, correlated positively with isoprene in the PCA. However, C. magellanica was present in only a few plots. The densities of E. russeolum and C. limosa were highly significant covariates associated with the standardized isoprene emissions in 2006 (P < 0.001, linear mixed models) but not in 2004 and 2005. However, the relationship of the species densities with isoprene emission was not consistent over the treatments (density × treatment interaction, P = 0.026 for E. russeolum and P < 0.001 for C. limosa), and thus the interaction terms were included in the mixed model analysis.

The leaf densities of E. russeolum and C. limosa were already significantly different in the randomly assigned treatment plots at the beginning of the experiment (Tables 1; P < 0.001, linear mixed models). Eriophorum russeolum was more abundant and C. limosa less abundant in the UV-B plots than in the control plots. The density of E. russeolum varied significantly from year to year, but enhanced UV-B had no effect on the temporal changes (year, P < 0.001; year × treatment interaction, P > 0.8). The density of C. limosa did not show significant variation from year to year. As the density of E. russeolum and C. limosa was associated with the isoprene emissions, and as the densities differed between the treatments, the use of the leaf densities of these species as covariates in the statistical analysis was essential in order to remove their contribution to isoprene emissions.

The moss W. exannulata, which dominated the ground layer at the experimental site, emitted isoprene at a standardized rate of 156 ± 18 ng g−1(DW) h−1 (62.4 ± 7.2 µg m−2 h−1, mean ± SE of 10 samples) in the laboratory trial.

Effects of enhanced UV-B radiation on isoprene emissions and carbon assimilation

In the single day measurement of 20 July 2004, isoprene emission from the subarctic fen was significantly higher under enhanced UV-B radiation than under the ambient (P = 0.008, Dunnett's test) and the UV-A control (P = 0.048; Table 2).

In July 2005, isoprene emissions were unexpectedly large, 1100 ± 160 µg m−2 h−1 averaged across the treatments (±SE). The largest emission peaks may have exceeded the breakthrough concentration of isoprene in the Tenax TA adsorbent tubes, therefore we decided to omit from the statistical analyses the two first measurement campaigns with the highest emissions (2–3 and 4–5 July, striped bars in Fig. 2a). In the remaining three campaigns (6–9, 11–14 and 19–20 July), the standardized emissions were on average 483 ± 66 µg m−2 h−1, and they were not affected by UV-B (P = 0.261, linear mixed models; Fig. 2a). The average emission rate, 483 µg m−2 h−1, is used in the following discussion.

During the fourth UV-B exposure season (2006) the standardized isoprene emissions were increased slightly by enhanced UV-B radiation (P = 0.056, linear mixed models; Fig. 3a). Averaged over the growing season, the standardized emissions were 41 ± 6, 51 ± 9 and 66 ± 10 µg m−2 h−1 under ambient control, UV-A control and enhanced UV-B radiation, respectively. The emissions followed changes in temperature over the growing season, but remained below 100 µg m−2 h−1 until the beginning of August. Enhanced UV-B did not affect the emissions in June or July, but increased them in August, when the emissions were higher overall. A highly significant increase in the standardized isoprene emission was detected on 8–9 August 2006, when the emissions were 360 and 177% higher under enhanced UV-B compared with the ambient (P < 0.001, Dunnett's test) and the UV-A control (P < 0.001; Fig. 3a), respectively. Furthermore, a week later the emissions were still at a significantly higher level under enhanced UV-B compared with the ambient control (P = 0.008).

The nonstandardized isoprene emission data were also analysed statistically, and similar results were obtained.

The NEE during the 2006 growing season was not significantly affected by UV-B (P = 0.122, linear mixed models), although the NEE was slightly higher under enhanced UV-B on 4–5 and 26–27 July compared with the ambient control (P = 0.08 and P = 0.078, Dunnett's test; Fig. 5). The mean NEE under ambient control, UV-A control and UV-B treatment was 127.4 ± 15.3, 154.1 ± 14.1 and 171.4 ± 14.0 mg CO2 m−2 h−1, respectively. The leaf density of E. russeolum was a significant covariate determining NEE (P = 0.012, linear mixed models).

Figure 5.

Mean net ecosystem CO2 exchange (NEE, +SE, n = 7–10) under ambient control (open bars), UV-A control (grey bars) and UV-B (black bars) treatments during the 2006 growing season. Statistically slightly significant difference from UV-B treatment within the measurement point: +, P < 0.1 (Dunnett's test).

Discussion

Subarctic fen as an isoprene source

The subarctic fen under investigation was a significant source of isoprene to the atmosphere. The emissions were especially high during warm weather periods as experienced during the measurements in July 2005. The isoprene emission averaged over this warm period was 483 µg m−2 h−1, while it was 53 µg m−2 h−1 averaged over the 2006 growing season.

Our results indicate that peatlands are more important isoprene sources than forests in the subarctic. Rinne et al. (2000) observed with the gradient method a low mean isoprene flux, 14.4 µg m−2 h−1, from a subarctic mountain birch- (Betula pubescens ssp. czerepanovii) and Siberian spruce- (Picea abies ssp. obovata) dominated forest in Finland. Although the different methods can partly account for the different emission rates, the emissions from the subarctic forest were clearly lower than those from our peatland site. Furthermore, our results showed that isoprene emissions from the subarctic fen under warm weather conditions were of the same magnitude as emissions previously reported for boreal peatlands (Janson & De Serves, 1998; Janson et al., 1999) and Norway spruce- (Picea abies) dominated boreal forests (Janson et al., 1999; Hakola et al., 2003).

In the present study we show that subarctic fens without Sphagnum dominance can be substantial sources of isoprene. The vegetation of our site consisted of a continuous cover of the moss W. exannulata and sedges E. russeolum and C. limosa, whereas earlier assessments on peatland isoprene emissions have been conducted exclusively on Sphagnum peatlands. Sphagnum and several other moss species have been reported to emit isoprene strongly in laboratory trials (Hanson et al., 1999). Hellén et al. (2006) detected the mean standard emission potentials of 50–200 µg m−2 h−1 from a southern boreal Sphagnum fen in Finland with the static chamber technique in the 2004 growing season. Haapanala et al. (2006) observed the standard emission potential of 680 µg m−2 h−1 on the same fen with the relaxed eddy accumulation technique in the 2004 and 2005 growing seasons. Janson and De Serves (1998) reported the mean standardized isoprene emission of 1290 µg m−2 h−1 from wet microsites of a Sphagnum fen in eastern Finland in chamber measurements in August 1997. We showed here, for the first time, that the moss W. exannulata is also an isoprene-emitting species. Our results showed higher peak emissions under very warm weather conditions, but lower average emissions in a cooler and drier summer than the earlier observations. It appears that the vast peatland areas of the boreal and subarctic regions are important sources of isoprene regardless of the dominant moss species.

The densities of E. russeolum and, to a lesser extent, C. limosa and C. magellanica (although the latter was present in only a few plots) correlated with isoprene emission in the measurements of 2006. Other vascular species were not clearly related to the emissions. Thus we suggest that, in addition to the moss W. exannulata, the sedge E. russeolum was an important isoprene emitter in the studied ecosystem.

The low precipitation in 2006 and the concomitant low water table may have caused drought stress to the mosses, which may have led to decreased isoprene emission during this growing season. Severe drought stress has been demonstrated to reduce isoprene emissions in live oak (Quercus virginiana) (Pegoraro et al., 2004). The low water table may also have increased the contribution of soil microbial activity to isoprene production and uptake, as microbes are known to both emit and degrade isoprene in oxic conditions (Fall & Copley, 2000; Schöller et al., 2002). However, the contribution of soil microbes to the isoprene emission could not be separated in this study.

As the net CO2 flux into the fen vegetation during the 2006 growing season was, on average, 150.7 mg CO2 m−2 h−1, the C content of the mean isoprene emission, 53 µg m−2 h−1, made up 0.1% of the assimilated C during that time. In July 2005, however, when the average NEE was 257.6 mg CO2 m−2 h−1 (J.K. Haapala and co-workers, unpublished data), the C released as isoprene made up 0.4% of the assimilated C. However, the highest isoprene emission peaks reached over 10% of the mean C assimilation rate. This is in accordance with earlier studies suggesting that < 2% of the recently fixed C is normally released as isoprene, but even 50% losses have been reported under stressful conditions (Sharkey & Loreto, 1993; Harley et al., 1999; Pegoraro et al., 2004).

The observed C loss as isoprene in July 2005 was 6% of the mean methane efflux, 7.0 mg CH4-C m−2 h−1, from the fen during that time (S.K. Mörsky and co-workers, unpublished data). However, the highest isoprene emission peaks during the warm July 2005 were as high as the mean methane emission. This underlines the importance of isoprene emissions in the C balance of the fen ecosystem.

Effects of enhanced UV-B radiation

The UV-B enhancement predominantly increased isoprene emission, as hypothesized, although the achieved enhancement was lower than the targeted level. Significantly higher emission under enhanced UV-B was observed in the single-day measurement during the second growing season, and at the end of the fourth growing season under the exposure. In contrast, we did not detect significant UV-B effects on the emissions during the warm July in the third growing season. The density of E. russeolum, which was by chance higher in the UV-B treatment, was taken into account in the statistical analyses, and therefore cannot have affected the differences between treatments.

The increased emissions can be partly explained by the changes in C assimilation at the same experimental site. The C assimilation was slightly higher under UV-B enhancement at the end of the 2006 growing season when isoprene emissions were significantly increased by UV-B. J.K. Haapala and co-workers (unpublished data) observed decreased ecosystem respiration coupled with unaffected gross photosynthesis under enhanced UV-B in the plots of the present study in 2003–05. As the growth of the most abundant plant species, E. russeolum, was not affected (Scharf, 2006), the part of the assimilated C that was neither used to build biomass nor respired may have been released as isoprene. These results do not indicate UV-B-induced damage in photosynthesis. Isoprene emission is related to recent C assimilation, but the coupling is incomplete, suggesting the use of stored C in isoprene synthesis when photosynthesis is limited (Affek & Yakir, 2003; Schnitzler et al., 2004).

To our knowledge, there are no earlier data on UV-B effects on isoprene emissions from mixed-species ecosystem plots. Current knowledge even at a single-species scale is scarce. Harley et al. (1996) observed increased isoprene emission from gambel oak (Q. gambelii) under UV-B enhancement, corresponding to a 30% loss in the ozone layer. However, the increased emission was a result of morphological changes and increased leaf biomass rather than of physiological stimulation of isoprene synthesis (Harley et al., 1996). In general, increased isoprene synthesis has been suggested to be related to acute thermal and oxidative stress, for example caused by ozone, and the increased synthesis is usually rapidly induced in response to exposure (Loreto & Velikova, 2001; Loreto et al., 2001; Velikova et al., 2005a, 2005b). It has been shown that oxidative stress and the subsequently stimulated antioxidant defence are also caused by exposure to enhanced UV-B (Turunen & Latola, 2005), which suggests that the UV-B-induced increase in isoprene emissions may also result from physiological stimulation of isoprene synthesis.

Why isoprene emission was not consistently increased by enhanced UV-B cannot be explained with our data. The limited number of successful measurements in 2005 may not have caught the actual treatment effects, which appeared later in the season the year after. However, the significantly higher emission under UV-B in the single measurement of 2004 supports our observation that the emissions are indeed increased by UV-B. In general, it is important that measurements are conducted over extended periods and under a variety of environmental conditions, in order to obtain a realistic estimate of the response pattern when working with mixed vegetation compositions in the field.

To conclude, subarctic peatlands form a more important source of isoprene to the atmosphere than forests of the same area. They are a significant source of isoprene, especially in warm weather periods. The emissions appear to be higher under enhanced UV-B radiation, which suggests that if stratospheric ozone depletion leads to a further increase in the flux of solar UV-B to the biosphere, isoprene emissions from the subarctic peatlands may increase.

Acknowledgements

We thank Timo Oksanen for constructing the equipment, Juhani Tarhanen for technical assistance in the GC–MS analyses, Hanne Suokanerva for maintenance of the exposure system, and Nina Kontinaho and Mia Åberg for field assistance. We are grateful to Jaana Haapala for the vegetation data 2003–05. The study was financially supported by the Academy of Finland (decisions 202300 and 200884), the Emil Aaltonen Foundation, the Finnish Konkordia Fund, and the European Science Foundation (VOCBAS programme).

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