Microbial Respiration in Arctic Upland and Peat Soils As a Source of Atmospheric Carbon
ONLINLESUPPLEMENTING MATERIAL
Microbial Respiration in Arctic Upland and Peat Soils as a Source of Atmospheric Carbon Dioxide
Christina Biasia,*, Simo Jokinena, Maija E. Marushchaka, Kai Hämäläinenb,1,Tatiana Trubnikovaa, Markku Oinonenb and Pertti J. Martikainena
aUniversity of Eastern Finland, Department of Environmental Science, P.O. Box 1627, 70211 Kuopio, Finland
bFinnish Museum of Natural History, University of Helsinki, Dating Laboratory, P.O.Box 64, 00014 Helsinki, Finland
1STUK-Radiation and Nuclear Safety Authority, P.O. Box 14, 00881 Helsinki, Finland
*Correspondance: Christina Biasi, Tel.: +358 40 3553810, Fax: +358 17 163750. Email:
Climatic characteristics of study site Seida
Mean long-term annual temperature is -5.6 °C and the mean annual precipitation amounts to 501 mm measured between 1977 and 2006 at the closest climate station in Vorkuta, 75 km from the research site (Komi Republican Center for Hydrometeorological and Environmental Monitoring). Mean annual temperature was 2-3 °C higher than this long-term mean in 2008 and 2007, respectively. The length of the thermic growing season, which is defined as a period when the daily mean air temperature is permanently above +5°C, was 80 days in 2007 and 79 days in 2008 comparable between both years.
Vegetation characteristics of the subsites and plant biomass during the study years
The vegetated areas of peat plateau are dominated by mosses, such as Dicranum sp.andHepaticae sp.in dry peat plateau(DPP) and Sphagnum sp. in moist peat plateau(MPP). The dominating vascular plants are Ledum decumbensin DPPand Rubus chamaemorusin MPP. In both subsites, also Betula nana and Vaccinium sp. are found and in the drier areas, lichens are abundant (for example,Cladina rangifera and C. maxima).The dominant vascular plants of tundra heath (STH) areBetula nanaand Carex globularis, and alsolichens (for example,Cladina rangifera and C. maxima) and mosses (for example,Sphagnum sp., Polytrichum strictum) are abundant.
We measured plant biomass and plant charactersitics by monitoring height of sedges and length of new growth and number of leaves of dominant vasuclar plants in 2007 and 2008. This was done periodically over the growing seasons 2007 and 2008 on specifically labelled plants next to collars where respiration measurements were carried out. The results for period DOY203-218, when this study was carried out, are shown in Figure S5 below.
Soil analysis
Soil moisture content was determined on a 10 g subsample by oven drying at 60 °C for 24 h. Soil pH was measured in a 1:2.5 soil to H2O solution (v/v). Carbon and nitrogen (N) contents were determined froma finely ground subsample by elemental analyzer (Thermo Finnigan Flash EA 1112 Series, San Jose, CA, USA), and soil organic matter (SOM) content was measured as loss on ignition. Dry bulk density of soils was determined by a core method.
A subsample of surface soil from each replicate was pooled andanalyzed for radiocarbon content by the Finnish Museum of Natural History, University of Helsinki. Additionally, radiocarbon analyses were performed for the soil profile of peat circles(PC). The sample pretreatment procedure for radiocarbon analyses followed the typical acid-alkali-acid method aiming to separate alkali insoluble fraction of the sample. This procedure removes possible carbonate contaminants and alkali-soluble humic acids. Pretreated samples were mixed with a stoichiometric excess of CuO and packed into glass ampoules, which were pumped into vacuum and torch-sealed. The packed samples were combusted at 520°C overnight. The released CO2 was collected and purified with liquid-N2 and ethanol traps at -196 and -85°C, respectively. After purifying and measuring the sample 13C value with IRMS, the CO2 samples were converted to graphite targets in presence of zinc powder and iron catalyst. AMS measurements on the targets were eventually performed at the Uppsala Tandem Laboratory.
Incubation experiment
Field-moist soil (25 g) was weighed in 500 ml jars and placed in incubators in the laboratory. Incubation temperature was set to approximate field conditions (± 2°C) (10°C for organic soils of MPP and mineral soils of STH and at 15°C for the other samples). A pre-incubation period of one month, where jars were only loosely covered and water content was adjusted, was carried out to deplete C stemming from residual fine roots including mycorrhiza which contribute to CO2 flux. Soil respiration rates were then determined by closing the jars and sampling headspace air (20 ml) initially and 4 times over the following six hour incubation time. The CO2 concentration of headspace air was determined by gas chromatography (GC)(HP 5890 series II, Hewlett-Packard with a thermal conductivity (TC) detector for CO2). Soil respiration was calculated from the linear increase in CO2 concentration over the incubation period. For comparison of basal soil respiration rates between different soils, all rates were normalized to 15°C using Q10 values derived from Arrhenius equations (see main text).
For radiocarbon dating of CO2, the jars were flushed with CO2 free air (equivalent five-times chamber volume) to remove background ambient air from the headspace. Jars were thereafter closed until enough CO2 had accumulated to allow sampling for 14C. Collection of14CO2 and 13CO2was done following the procedure used for the field and described in the main body of the text. Carbon dioxide collected in the molecular sieves was analyzed for 14C within one week.
14CO2 collection
After finishing the SR measurements, CO2 was collected for 14CO2 analysis by the molecular sieve sampling techniqueusing a protocol modified after Schuur and Trumbore (2006). Prior to the field sampling, the molecular sieve tubes were filled and regenerated according to the procedure described by Hämäläinenand others (2010). Particularly, the tubes containing 20 g of molecular sieve grains were heated to 500°C and evacuated until the pressure within the tube reached 1×10-2 mbar. The sieve tubes were then closed by valves at both ends and taken to the field site for collections. In the field, chamber air was scrubbed first through an external soda lime cartridge by pump (flow rate: 4 l min-1) to remove background atmospheric CO2. To achieve that, the equivalent of five chamber-volumes was scrubbed. Then, the pump was switched off and CO2(consisting of SMR and RR) was allowed to accumulate for about 10 minutes while monitoring the concentration with IRGA. The chamber air was then passed through anhydroneto dry the air and the tubes filled with molecular sieves (type 13X, Merck 1.05703.0250) by pumping for 20 minutes or until the saturation of the sieve tube was observed. The CO2 samples in the sieve tubes were treated in the laboratory as follows. The sieve tube was first evacuated to remove the unwanted volatile material inside. CO2 was released by keeping the sieve 2hat500°C. The released CO2was then treated correspondingly to soil samples, grafitized and analyzed by AMS.A full molecular sieve yielded 1.3 ± 0.5 mg of C which is sufficient for radiocarbon dating. The advantage of CO2 collection with molecular sieves over the traditional NaOH trapping method is that risks of isotope fractionation and contamination are largely reduced (Bauer and others 1992; Hardie and others 2005).
To determine the 13C of respiration, which was needed for eventual correction of contamination of molecular sieves with air and incomplete scrubbing of atmospheric CO2 in the chamber system, some Keeling plots were taken at the end of the sampling following the procedure described in Biasi and others (2008a) and simultaneously an air sample was taken for 13C analysis.The 13CO2 values were determined by gas chromatography coupled to an isotope ratio mass spectrometer (GC-IRMS) in the laboratories of Kuopio (Biasi and others 2008b; Biasi and others 2005)
14C notation
The 14C content is expressed as fraction modern (F14C) (Reimer and others 2004), representing the proportion of 14C activity in the sample compared to the 14C standard activity. As an isotopic fractionation can occur due to the mass difference of the C isotopes, all samples are routinely corrected for mass-dependent fractionation using the 13C values. The typical accuracy of the F14C values values was ±0.004. F14Cis approximately equal to 1.0 in 1950, before the nuclear weapons test. Higher values reflect the time of nuclear weapon testing (bomb C peak) whereas lower values the pre-bomb times.To obtain the approximate age for the corresponding C fixation by photosynthetic activity, the radiocarbon data were converted to calendar years via atmospheric calibration data (Levin and others 2008; Levin and Kromer 2004) and IntCal09(Reimer and others 2009), and by using CaliBomb(Stuiver and Reimer 1986), calib.qub.ac.uk/CALIBomb) and Oxcal 4.1 software.Often two possible values are provided for post-bomb results (upslope or down-slope of the bomb curve). In some cases, we could rule out one value by assuming the following: mineral soil is older than overlying organic soil; CO2 respired is younger than bulk soil C; and SR is younger than SMR due to contribution of roots. If some uncertainty remained, both age results were given. In any case, post-bomb age values are considered as approximations, because the atmospheric 14C concentration after nuclear bomb testing was not uniform around the globe and the calibration data available for this study were from Central Europe.
Post-processing of measured F14C values of CO2 from molecular sieves
The measured F14C values of CO2 collected in molecular sieveswere first corrected for blank values (contamination with atmosphere) using the 13C values measured and isotope mixing models as well as mass balance approaches as described in Schuur and Trumbore (2006).
The contribution of air was on average 11.8% for field samples, and 4.3% for laboratory samples. Samples with greater than 50% air contamination (one replicate of MPP and one replicate of DPP, field samples) were omitted from the study.
Figure S1
Fig.S1: Basal soil respiration rates from laboratory incubations with root free soil, expressed on a volume basis. DPP, MPP, PC and STH represent dry peat plateau, moist peat plateau, peat circle and shrub tundra heath, respectively. Patterned grey bars indicate mineral soil of STH, patterned white bars organic soil of STH. Different letter denote significant differences between subsites at p ≤ 0.05 (KruskalWallice test).
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Figure S2
Fig.S2: Basal soil respiration rates as determined from laboratory incubations (n=3, SE) are correlated with age of soil (SE of analysis). The upper figure includes values of peat circles (PC). The lower figure excludes them and shows then a significant correlation (p < 0.05). This analysis assumes that the age of MPP is 45 years, but near significant correlation (p < 0.1) is also found if the age of MPP is assumed to be 28 years. It shows that respiration rates of PC are very high for this ancient soil thus breaking the correlation between age and respiration rates.
Figure S3
Fig.S3: Soil microbial respiration rates as determined with 14C partitioning approach (white bars) and trenching approach (black bars) in Seida study site, Arctic tundra. Both techniques were employed in two consecutive years in peak season (between 22 July and 6 August). The star indicates a significant difference between the methods at p < 0.05 (Kruskal-Wallis test).
Figure S4
Fig.S4: Correlations between soil microbial respiration (SMR) and net ecosystem carbon exchange (NEE). Upper graph: soil microbial respiration (SMR) was determined with root trenching. Both fluxes were measured over the growing season 2008 (n=5 for SMR, n=3 for NEE; SE).Lower Graph: Both fluxes were determined in 2007; SMR rateswere calculated by 14C partitioning approach (n=2-3 for SMR, n=3 for NEE; SE). Sunny, clear days prevailed during this period.
Figure S5
Figure S5: Plant characteristics of dominate vascular plant species. Upper graph: height of Carex sp., and length of new growth of all other vascular plants. Lower graph: leaf number of Carex sp., and leaf number of all other vascular plantsof vegetation in 2007 and 2008 growing season (all data are means of 12 measurements for each plant taken between DOY 203-218). There was no significant difference in a single parameter between both years, indicating that plant biomass was comparable in 2007 and 2008.
Table S1: Pearson's Correlation Coefficient (R2) and p-values Between Basal Soil Respiration Rates (g CO2 g-1DW h-1) and Relevant Variables as Determined from Root-free Soil during LaboratoryIncubations (univariate analysis)
Correlations that were near significant (p< 0.10) are in italic and correlations that were significant (p< 0.05) are in bold.
Abbreviations are as followed: BD (bulk density), SOM (soil organic matter), WC (water content); age of soil is in years.
aassuming that age of soil from MPP is ~28 yrs
bassuming that age ofsoil from MPP is ~45 yrs
cassuming that age of CO2 respired from DPP is ~12 yrs
dassuming that age of CO2 respired from DPP is ~49 yrs
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Table S2: Pearson's Correlation Coefficient (R2) and p-values BetweenSoil Microbial Respiration (SMRTR; ing CO2 m-2 season-1)and Relevant Variables as Determined with Root Trenching Approach in 2008, Seida Study Site (univariate analysis)
Correlations that were near significant (p< 0.10) are in italic and correlations that were significant (p< 0.05) are in bold.
Abbreviations are as followed: BD (bulk density), VWC (volumetric water content); WT (water table), AL (active layer), NEE (net ecosystem carbon exchange), ER (ecosystem respiration).
aassuming that age of soil from MPP is ~28 yrs
bassuming that age ofsoil from MPP is ~45 yrs
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