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Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland a
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Alina Jasek , Miroslaw Zimnoch , Zbigniew Gorczyca , Ewa Smula & Kazimierz Rozanski
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AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland Published online: 20 Jan 2014.
Click for updates To cite this article: Alina Jasek, Miroslaw Zimnoch, Zbigniew Gorczyca, Ewa Smula & Kazimierz Rozanski (2014) Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland, Isotopes in Environmental and Health Studies, 50:2, 143-155, DOI: 10.1080/10256016.2014.868455 To link to this article: http://dx.doi.org/10.1080/10256016.2014.868455
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Isotopes in Environmental and Health Studies, 2014 Vol. 50, No. 2, 143–155, http://dx.doi.org/10.1080/10256016.2014.868455
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Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland Alina Jasek∗ , Miroslaw Zimnoch, Zbigniew Gorczyca, Ewa Smula and Kazimierz Rozanski AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland (Received 17 July 2013; accepted 8 October 2013) As urban atmosphere is depleted of 13 CO2 , its imprint should be detectable in the local vegetation and therefore in its CO2 respiratory emissions. This work was aimed at characterising strength and isotope signature of CO2 fluxes from soil in urban areas with varying distances from anthropogenic CO2 emissions. The soil CO2 flux and its δ 13 C isotope signature were measured using a chamber method on a monthly basis from July 2009 to May 2012 within the metropolitan area of Krakow, Southern Poland, at two locations representing different levels of anthropogenic influence: a lawn adjacent to a busy street (A) and an urban meadow (B). The small-scale spatial variability of the soil CO2 flux was also investigated at site B. Site B revealed significantly higher summer CO2 fluxes (by approximately 46 %) than site A, but no significant differences were found between their δ 13 CO2 signatures. Keywords: carbon-13; CO2 flux measurements; CO2 soil emissions; urban atmosphere
1.
Introduction
The biogenic carbon dioxide (CO2 ) fluxes into the atmosphere are one of the dominant components of the global carbon cycle. The total CO2 flux from continental biogenic sources is estimated to be approximately 120 PgC yr−1 and is almost entirely balanced by the terrestrial plants’ photosynthetic assimilation [1]. Although the variability of the carbon cycle on a decadal time scale is mostly dominated by gradually increasing anthropogenic CO2 fluxes to the atmosphere, the biogenic component is clearly visible in the seasonal changes of the atmospheric carbon budget, especially in the northern hemisphere. On the other hand, the role of human-dominated ecosystems in regional carbon budgets increases with progressive urbanisation through both enhanced anthropogenic CO2 emissions and the diminishing role of undisturbed natural ecosystems when compared with expanding urban agglomerations. This leads to an increasing interest in studying CO2 emissions in urban environments. Previous research [2] has shown that the increase in the CO2 concentration in the urban atmosphere is due to several factors, which can be divided into two main categories: (i) anthropogenic, e.g. combustion of fossil fuels associated with transportation and electricity/heat generation, both in power plants and individual households, and (ii) biogenic, including plant respiration, soil organic matter decomposition and CO2 emissions from water courses. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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Soil fluxes of CO2 were investigated in a wide range of ecosystems, from boreal tundra to tropical forest, including human-modified soils [3–7]. Anthropogenic and biogenic contributions of CO2 to an urban atmosphere were studied using several approaches, mostly involving carbon and oxygen isotopes in CO2 [7–12]. Levin et al. [8] used analyses of the 14 CO2 content in atmospheric CO2 samples collected weekly in a densely populated area of the Rhine valley in Germany to determine the contribution of fossil fuel-derived CO2 to the local urban atmosphere. Using isotope mass balance approaches, they were able to demonstrate that, apart from anthropogenic CO2 , also a significant contribution originating from local biogenic sources is present in the investigated urban atmosphere. Kuc et al. [9] used a similar approach using biweekly cumulative sampling of atmospheric CO2 in the urban atmosphere of Krakow. They found that both concentration and isotopic composition (14 C/12 C and 13 C/12 C) of atmospheric CO2 in the urban environment differ distinctly from atmospheric baseline conditions. These differences can be used to constrain the CO2 budget and to quantify anthropogenic and biogenic CO2 contributions to the local atmosphere [2]. Koerner and Klopatek [7] quantified CO2 sources across the metropolitan region of Phoenix, Arizona, by measuring biogenic CO2 soil fluxes and using fossil fuel combustion inventories from governmental and non-governmental databases to assess the anthropogenic component of the regional CO2 emissions. In the urban environment of Phoenix, Arizona, the anthropogenic sources were responsible for over 80 % of the total CO2 fluxes into the atmosphere. Clark-Thorne and Yapp [10] measured the stable isotope composition of atmospheric CO2 in about 150 samples collected in the urban atmosphere of Dallas, Texas, and evaluated sources and sinks of CO2 in the city’s atmosphere using the box-model approach. Zimnoch [11] performed systematic measurements of the stable isotope composition of CO2 emitted from combustion of various types of fossil fuel within the metropolitan area of Krakow, Poland, and found that the CO2 associated with the combustion of the three major categories of fossil fuel is characterised by distinct δ 13 C signatures: approximately −23.8, −51.8 and −30.6 ‰ for coal, methane and the fuel burned in car engines (diesel, gasoline and LPG), respectively. The concentrations of CO2 in soils vary with soil type, depth and season, ranging from approximately 5000 up to 40,000 ppm. These concentration levels are significantly higher than those encountered in the contemporary urban atmosphere (≈ 400–500 ppm). The main source of soil CO2 is the autotrophic (plant roots) and heterotrophic (soil microorganisms) respiration. Respiration activity, usually defined via CO2 emissions, is an important indicator of biological processes in soils, reflecting rates of organic matter decomposition and plant respiration. The existing gradient of CO2 concentration across the soil–atmosphere interface is a primary driving force for the diffusion-controlled transport of this gas and the biogenic emissions of CO2 into the atmosphere. The CO2 transport in the soil depends on the permeability of the given soil which in turn is controlled mainly by soil porosity and soil water content [12,13]. Here, we present the results of a systematic study aimed at characterising biogenic CO2 emissions in an urban environment. Temporal variations of the soil CO2 flux and its δ 13 C isotope composition were investigated at two contrasting sites in the city of Krakow, Southern Poland, between July 2009 and May 2012. Also, the spatial variability of the biogenic CO2 flux was quantified on the plot scale. An additional research question addressed in the framework of this study was whether the progressive modification of the δ 13 C isotope signature in atmospheric urban CO2 can also be traced in the isotopic composition of the biogenic CO2 flux from soils. Direct, systematic observations of biogenic CO2 emissions and their carbon isotope composition in urban areas are needed for a better characterisation of the regional carbon cycle under heavy anthropogenic stress. In particular, it is expected that such systematic observations of CO2 emissions from urban soils will help to better constrain regional carbon budgets aimed at the quantification of anthropogenic emissions of this gas related to the burning of fossil fuels. It is estimated that the European continent with its dense transportation network, extensive industrial
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infrastructure and high population density is currently responsible for more than 25 % of the global fossil fuel-derived CO2 emissions [14].
2.
Materials and methods
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2.1. Site description The presented study was conducted in the urban area of Krakow, Southern Poland. With about one million inhabitants, Krakow is one of the largest cities in Poland. The rapidly growing car traffic and industrial development characterise typical urban environments of central and eastern Europe. The climate is continental with a mean annual air temperature of approximately 8◦ C and annual precipitation rates fluctuating around 600 mm, the majority of which occur during the growing season. Predominant native soils in the study area are brown, alluvial and loamy sands, boulder loam and loess-like sediments. However, expanding urbanisation caused their partial degradation. Two sites were chosen for systematic measurements of the soil CO2 flux and its carbon isotope composition: (i) site A (50◦ 03 51 N, 19◦ 55 26 E), located in the immediate vicinity of one of the major streets in the city, and (ii) site B (50◦ 03 41 N, 19◦ 54 08 E), located within the large urban meadow ‘Blonia’, a 48-ha grassland recreation area situated close to the city centre, with limited car traffic in the direct neighbourhood. The aerial distance between the sites is approximately 2 km. Mickiewicz Avenue, where site A is located, was constructed in the 1930s. It consists of two lanes with a 20-m-wide green zone in between. This zone is a park-type area, covered by lawn, with sparse trees and shrubs. The lawn is cut twice a year. The street is one of the main routes between the northern and southern parts of the city. Hence, heavy traffic (approximately 5000 cars per h) is present there almost the whole day. The local vegetation is exposed to an atmosphere enriched in CO2 , originating mostly from burning of fossil fuel in cars. The CO2 flux measurements were performed in the centre of the green zone. The main soil types at this site are Technosols, before anthropogenic degradation being mostly Phaeozems and Fluvisols. The historical land-use record for the Blonia urban meadow, where site B is located, shows that this area was never urbanised. The present green area is only a small part of the original grassland dominating the region. Until the nineteenth century, the meadow was a neglected wasteland, subject to regular flooding by the near-by river. Until the twentieth century, the area was used as a natural pasture, mostly for cattle. Then, at the beginning of the twentieth century, the terrain was drained and transformed into a recreational area. The dominant soil types are Fluvisols. For two months in 2011 (October and November), a part of the Blonia meadow including measurement site B was designated for sheep grazing. 2.2. Sampling and measurement The soil CO2 flux and its δ 13 C isotope signature were measured using a closed, dynamic chamber system coupled with a VAISALA CarboCAP CO2 sensor and combined with a flask sampling system for isotopic analysis (Figure 1). The air was circulated through the entire system during a measurement run. The CO2 concentration measurements inside the system during a single flux measurement run were performed with a temporal resolution of one second. Each measurement run lasted approximately 20 min. In most cases single flux determination at a time was made at each site. The chamber method is a widely used technique for measuring gas exchange between soil and atmosphere. There are several possible modes of using chambers for measurements of mass fluxes. Respective methodologies were reviewed by several authors [15–17].
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Figure 1. Closed dynamic chamber system used for measurements of soil CO2 flux and its δ 13 C values. P, pump; FS, flow regulator; DA, drying agent (Mg(ClO4 )2 ); Flask, glass flask to collect air sample for isotope analysis; CO2 analyser, Vaisala CarboCAP sensor; and hPa, pressure sensor.
The chamber system used for this work was designed and built at the Faculty of Physics and Applied Computer Science at the AGH University of Science and Technology, Krakow. It consists of a rectangular container made of stainless steel, coupled with a collar hammered into the soil to the depth of ∼ 5 cm and fitted with a water seal to avoid gas leakages. A vent with a long capillary tube was attached to the chamber to compensate for the effect of increasing pressure in the chamber during the course of a measurement run [18]. The surface of the soil covered by the chamber was 0.1365 m2 , and the air volume enclosed in the chamber and other parts of the circulation system (glass flask, CO2 sensor, drying trap, connecting tubes, etc.) was 0.0285 m3 . The system was equipped with a regulator stabilising the air flow through the CO2 analyser (flow rate of approximately 0.3 l min−1 ), an KNF Neuberger pump (maximum pressure 1500 h Pa) integrated with a CarboCAP sensor, a pressure sensor to monitor pressure fluctuations inside the circulation system and a trap containing a drying agent (magnesium perchlorate, Mg(ClO4 )2 ) used to dry the air sampled for isotope analyses. In addition, outside atmospheric pressure was monitored during each measurement run. The system can be also operated without collecting air samples for stable isotope analysis. The CO2 flux from the soil measured using the chamber method can be calculated using the following general formula: f =
pV |dC(t) /dt|t=0 , RTS
(1)
where f is the molar flux density of CO2 , p is the atmospheric air pressure, C(t) is the function describing temporal evolution of CO2 concentration inside the chamber, |dC/dt|t=0 is the first derivative of C(t) function calculated for t = 0, R is the universal gas constant, T is the ambient air temperature, and V and S are the chamber volume and soil surface area covered by the chamber. The C(t) function can be derived through fitting the measurements of the CO2 concentration build-up inside the chamber over the course of a measurement run. Typically, the fitted function has a form C(t) = a(1 − e−bt ) + C0 , where a and b are fitting constants. In such a formulation of the C(t) function, its first derivative at t = 0 is a product of the fitting constants: |dC/dt|t=0 = ab. In practical applications of the chamber method, CO2 concentration measurements are usually limited to the initial part of the C(t) function and the data points are approximated by a linear function the slope of which, M, is used in Equation (1) instead of |dC/dt|t=0 . For typical parameters of chamber systems in use (volume, surface area) and typical soil CO2 fluxes, the measurement times required to cover the substantial portion of C(t) function before it reaches saturation would involve very long waiting times (tens of hours). Therefore, this simplified approach was used in the present study. The following operational form of Equation (1) was used: f =
BpVM , RTS
(2)
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where B stands for a numerical factor. For f (expressed in mmol m−2 h−1 ), M (in ppm h−1 ) and other variables expressed in SI units, the value of B equals 10−3 . It should be noted, however, that the simplified method of measuring CO2 fluxes based on Equation (2) underestimates the fluxes by a few percent. Since our work was focused mainly on temporal and spatial variability of biogenic CO2 emissions in urban environments and not so much on absolute values of the soil fluxes of this gas, for logistical reasons we decided to employ this simplified method, being aware of its drawbacks and limitations. During each measurement run, two air samples were collected in 1 l glass flasks. The first flask was filled with ambient atmospheric air before the beginning of the measurement run. The second sample representing air inside the circulation system was collected at the end of the measurement run. The flasks were pressurised to approximately 1500 hPa. The δ 13 C value of the soil-respired CO2 was calculated using the formula derived from the isotope and mass balance equations set up for the volume of air contained inside the circulation system: δ 13 Csource =
δ1 C1 − δ0 C0 , C1 − C 0
(3)
where C0 , δ0 and C1 , δ1 stand for concentration and δ 13 C value of CO2 inside the chamber at the beginning and the end of the measurement run, respectively. This approach is equivalent to the use of a Keeling plot, which is usually employed when at least several measurements of concentration and δ 13 C of CO2 are available [19,20]. The isotopic composition of CO2 is expressed in the delta (δ) notation as a per mille deviation from an internationally accepted standard. The δ 13 C values of CO2 are expressed relative to the Vienna Pee Dee Belemnite standard, on a scale such that NBS-19 calcite equals +1.95 ‰ exactly, and L-SVAC lithium carbonate is −46.6 ‰ [21]. The concentrations of CO2 in the flasks were measured using a Hewlett Packard 6890 gas chromatograph equipped with a Ni catalyst and flame ionization detector. After concentration measurement, CO2 was extracted cryogenically from the air sample, and its δ 13 C value was determined using a DELTA-S mass spectrometer (Finnigan MAT, now Thermo Fisher, Bremen, Germany). The typical uncertainty of CO2 concentration measurements was in the order of 2.0 ppm for the VAISALA CarboCAP CO2 sensor and 0.1 ppm for GC-based analyses. The overall uncertainty of the isotope measurements was in the order of 0.2 ‰. The soil temperatures at 5 cm depth and the mean soil water content for 0–5 cm depth intervals were measured using an Elmetron PT-101 thermometer and a Delta T HH2 moisture probe, respectively. To assess the spatial variability of the measured soil CO2 fluxes, a dedicated campaign was performed in October 2011. The CO2 fluxes were measured at site B over three days with similar atmospheric and weather conditions. In order to uniformly cover 48 ha of the urban meadow at site B, three sampling pathways were chosen: two close to the borders and one through the centre of the meadow (Figure 2). The CO2 fluxes were measured at every 100 m distance along these pathways. One measurement of the CO2 flux at the given point was performed. In total, the CO2 flux was measured at 42 points. At each point, the spatial coordinates were recorded using a GPS tracker (Garmin GPS72).
3.
Results and discussion
3.1. Small-scale spatial variability of soil CO2 flux The results obtained during the sampling campaign carried out at site B over three days in October 2012 (17–19) were used to assess the spatial variability of the soil CO2 flux for the type of
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Figure 2. Spatial variability of soil CO2 flux at site B (urban meadow). The area of the meadow is approximately 48 ha. The measurements were performed during three days (17–19 October 2012) between 11:00 and 16:00 local time. Black points represent measured values. The surface was fitted to the experimental data using a Surface Fitting Tool with cubic interpolation algorithm (Matlab 2010B).
ecosystem represented by this site (urban meadow). Forty-two flux measurements performed on the site yield a mean CO2 flux equal to 13.3 ± 0.4 mmol m−2 h−1 . The quoted standard deviation of individual measurement (0.4 mmol m−2 h−1 ) is comparable to the calculated uncertainty using error propagation for single determinations of the CO2 flux. The constructed map (Figure 2) reveals a rather homogeneous distribution of CO2 fluxes, except for one region located in the eastern part of the meadow, close to the main entrance, where significantly higher fluxes were observed. The mean value of the CO2 flux in the eastern part is 17.3 ± 0.2 mmol m−2 h−1 . If measurements performed in this region are excluded from the evaluation of the mean CO2 flux, its mean value drops to 12.8 ± 0.4 mmol m−2 h−1 . Although the measurements presented in Figure 2 reveal that small-scale spatial variability of soil CO2 flux measured with a closed-chamber technique is of the same order as the standard uncertainty of single flux determination, it is advisable to perform several parallel measurements of the flux at adjacent points whenever possible. Thus, larger measurement errors may be avoided.
3.2. Temporal variability of soil CO2 flux The soil CO2 fluxes at both investigated sites reveal a strong seasonality (Figure 3) induced by the vegetation cycle and seasonal variation of soil parameters (temperature, water content). Minimum values of the CO2 fluxes were recorded during the winter months (December, January and February). Low soil temperatures limit respiration activity, while higher water contents reduce permeability of the soil during this period. A snow cover may additionally reduce the transport of CO2 between the soil and the atmosphere. Typical CO2 fluxes measured during winter varied between 1 and 5 mmol m−2 h−1 for both sites. During the vegetation period (April–October), the CO2 fluxes increase significantly, reaching maximum values of 25–30 mmol m−2 h−1 at site A and 40–50 mmol m−2 h−1 at site B. The periods when the flux maxima occur fluctuate from year to year. In 2010, the maximum CO2 flux was observed in June at both sites while in 2009 and 2011, the maximum flux at site B was delayed with respect to site A. The mean value of soil CO2 flux throughout the whole measurement period at both sites is 16.2 ± 1.7 mmol m−2 h−1 and is
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Figure 3. Seasonal variability of the CO2 flux at site A (closed diamonds, solid line) and site B (open squares, dotted line) during the period July 2009–May 2012. Shown are also standard uncertainties of the measured fluxes (one-sigma level). Each point represents a single determination of the flux.
comparable to the mean fossil fuel CO2 flux reported in the emissions database for global atmospheric research database for Krakow agglomeration in 2008 (approximately 17.8 mmol m−2 h−1 [22]). One of the important parameters controlling the respiration activity of soils is temperature. Mielnic and Dugas [23] showed that CO2 flux and soil temperature exhibit an exponential relationship. A clear temperature dependence was observed for the CO2 flux at both sites (Figure 4). The highest soil temperature (+21.5◦ C) was measured at site B in August 2011, while the lowest was recorded at site A in March 2010 (−4.5◦ C). The Q10 temperature sensitivity coefficient [24] was calculated for both sites using the firstorder exponential function, F = a exp(bt), where F stands for soil CO2 flux in μ mol m−2 s−1 and t for the soil temperature in ◦ C. The Q10 coefficients calculated for site A and B are 2.25 and 2.98, respectively. The Q10 coefficient determined for site A is in agreement with a value of 2.29 reported for cultivated Phaeozem soil located at a similar latitude [25]. The values of Q10 coefficients reported for Fluvisols are higher than the value obtained for the urban meadow (site B–2.98). In Switzerland, Samaritani et al. [26] obtained Q10 values of 3.6 for grass, 3.4 for pasture and 3.2 for mixed forest. Awasthi et al. [27] report Q10 values in a range from 2.5 to 5.0 for mixed Fluvisols, Cambisols, Rigosols and Luvisols in Nepal. A lower Q10 value on site A versus site B can also be a consequence of differences in soil characteristics at both sites. At site B, where the CO2 flux grows faster with temperature, the soil is less disturbed than at site A, where the soil was classified as typical Technosol. The historical land-use record for the Blonia meadow shows that this region of the city was never urbanised. The stronger flux-temperature dependence observed at site B can be related to that difference. Another important factor controlling the CO2 flux is soil water content which regulates the availability of water for plants during the growing season and changes the free air porosity of the soil. The application of the two-parameter fitting procedure to the available CO2 flux data for both sites, with soil temperature t and soil water content w as fitting parameters, revealed
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Figure 4. The relationship between soil CO2 flux and soil temperature measured at 5 cm depth, as observed at site A and site B during the period July 2009–May 2012. The data points represent individual determinations of the CO2 flux and soil temperature.
that soil water contents below 30% reduce the strength of the CO2 flux to the same extent as the drop of soil temperature below approximately 10◦ C (Figure 5). Such a dependence of the CO2 flux upon soil moisture is in agreement with qualitative predictions [28] which state that for low as well as high water contents, the CO2 emission from soils should be noticeably lower than for intermediate (approximately 30–50%) soil water contents. Both cases are related to different processes: for dry soils the respiration activity decreases due to limited water availability, while at high water contents the increasing anaerobic microbial activity combined with reduced soil permeability leads to reduction of the soil CO2 flux [29]. 3.3. δ13 C isotope signature of soil CO2 flux As anticipated, the measured δ 13 C values of atmospheric CO2 were slightly lower at site A (Figure 6(a)), where higher CO2 concentrations were also observed (Figure 6(b)). Heavy car traffic is permanently present in close vicinity to the plot where the flux measurements were performed at this site. The mean CO2 concentration difference between both sites (9.7 ± 3.8 ppm) is consistent with measured δ 13 C values; higher CO2 concentrations in a neighbourhood of car traffic are associated with lower δ 13 C signatures. The mean difference between individual δ 13 CO2 values recorded at both sites is 0.83 ± 0.15 ‰, with δ 13 C of atmospheric CO2 at site A being significantly lower than that at site B. A clear seasonal cycle of atmospheric δ 13 CO2 at both sites is visible in Figure 6(a), with minimum values occurring during winter, when anthropogenic sources of CO2 are stronger (house heating) and meteorological conditions favour longer periods of temperature inversions leading to accumulation of isotopically lighter CO2 within the boundary layer.
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Figure 5. The soil CO2 flux data for site A and site B obtained during the period July 2009–May 2012, as a function of soil temperature and soil water content. The surface was fitted using a Surface Fitting Tool with the cubic interpolation algorithm (Matlab 2010B).
Figure 6. (a) Seasonal δ 13 C variability of atmospheric CO2 at site A (closed diamonds, solid line) and site B (open squares, dotted line) measured during the period July 2009–May 2012, and their mean values (horizontal lines) with the standard uncertainties of the mean values marked by vertical bars. The standard uncertainty of isotope analyses is in the order of 0.2 ‰. The mean values of δ 13 Catm are −10.30 ± 0.18 ‰ for Site A and −9.56 ± 0.16 ‰ for Site B. (b) Seasonal variability of atmospheric CO2 concentration at site A (closed diamonds, solid line) and at site B (open squares, dotted line) measured during the reported period. The mean values are 419.0 ± 3.3 ppm (site A) and 408.5 ± 4.4 ppm (site B).
The question arises whether the observed difference in the isotopic composition of atmospheric CO2 at both sites is somehow imprinted in the isotopic signature of the soil CO2 flux recorded at those sites. It is expected that the fossil fuel signature in plant-assimilated CO2 should be imprinted in the soil CO2 flux, as one of its main components is the below-ground root respiration. Thus, the soil CO2 released at site A should contain less 13 C than that released at site B. The carbon isotopic signature of soil CO2 (δ 13 Csource ) calculated using Equation (3) has fluctuated at both sites between approximately −31 and −24 ‰, without apparent seasonality (Figure 7). The mean values of δ 13 Csource were −27.8 ± 1.5 ‰ and −28.1 ± 1.1 ‰ at sites A and B, respectively.
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Figure 7. Seasonal variability of δ 13 C in soil-respired CO2 at site A (rectangles) and site B (squares), measured during the period July 2009–May 2012. The presented δ 13 C values were calculated using two-component mixing model (see text).
Within the reported standard uncertainty, the mean δ 13 C values are non-distinguishable, suggesting dominance of C3 plants at both sites and lack of a discernible fossil fuel signal in the soil-respired CO2 . This surprising result may originate from several effects. First, one may argue that the observed difference in the isotopic composition of atmospheric CO2 at both sites may have its origin in the episodic character of the measurements performed, while autotrophic and heterotrophic respiration processes are operating during the whole day. In addition, as δ 13 Csource at each site is represented by only one measurement per month, the representativeness of δ 13 Csource values for the soil CO2 flux can be questioned. On the one hand, Bahn et al. [30] reported significant diurnal variability of δ 13 C in soil-respired CO2 , with lower δ 13 C values during the morning hours and higher during the night. They did not offer any explanation of this behaviour of δ 13 C. On the other hand, the study of Betson et al. [31] has not revealed any significant diurnal variability of δ 13 C in the soil-respired CO2 . A dedicated measurement campaign performed at site A on 9–10 July 2009 did not reveal any significant diurnal variability of δ 13 Csource either. In the light of this conflicting evidence, we tentatively assume that single measurements of soil-respired CO2 are representative at least for a whole day. Second, it is known that the strength of the assimilation process depends on the intensity of photosynthetically active radiation (PAR) [32]. Similarly, the intensity of vertical mixing within the boundary layer of the lower atmosphere also depends on solar radiation. Day et al. [33] have shown that lower atmospheric CO2 concentrations observed at higher PAR levels are a consequence of a more intensive convective mixing of the lower atmosphere and an increased intensity of photosynthesis. This mixing will likely smooth out the local gradients in terms of concentration
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and isotopic composition of the human-induced CO2 load on the near-ground atmosphere within the city. Thus, similar δ 13 C signatures of atmospheric CO2 are expected at both investigated sites. Third, a long residence time of carbon in the soil reduces the isotope variability of the input signal. Thus, the expected δ 13 C difference of heterotrophic CO2 flux components at both sites can be significantly smaller than natural spatial and temporal variability of δ 13 Csource observed at those sites. The average age of carbon in soil organic matter is reported to range from 200 to 1200 years, depending on the soil type. This age is higher than the duration of the industrial era (approximately 150 years) during which a gradual modification of atmospheric δ 13 C values due to burning of fossil fuels was recorded. However, the age of carbon measured in the total soil respiration flux is much lower, in the order of several years for temperate soils [34]. Using 13 C-labelling experiments Bahn et al. [30] found that CO2 assimilated during photosynthesis is rapidly transported to plant roots and respired (within a few hours). Thus, depending on the relative contribution of heterotrophic and autotrophic respiration in the soil, the fossil fuel imprint in the δ 13 C isotope signature of the soil CO2 flux can be smaller or larger. The obtained results do not allow us to draw definitive conclusions with respect to a possible anthropogenic imprint in the carbon isotope composition of the soil CO2 flux. More measurements of δ 13 Csource at other sites of contrasting characteristics in terms of anthropogenic impact are needed to clarify this problem.
4.
Conclusions
The results of this study indicate that the CO2 flux from urban soils varies mostly with soil temperature, but is also dependent on the soil characteristics. The observed strong seasonal variability of the CO2 flux was mainly driven by temperature, which controls respiration activity.Although in the reported range of soil temperatures (from −4.5◦ C to +21.5◦ C), a strong temperature dependence of the CO2 flux was observed, multi-parameter analysis suggest that the soil water content is also important. A decrease in the soil water content below approximately 30% reduces the intensity of the CO2 flux by the same amount as the drop of soil temperature below approximately 10◦ C. Because measurements of soil CO2 flux are point measurements, the question of small-scale spatial variability of this parameter is critical. The assessment of this variability performed for site B suggests that for homogeneous ecosystems such as the investigated urban meadow, the measure of the spatial variability of the CO2 flux (standard deviation) determined during the vegetation period is comparable to the uncertainty of a single determination of the flux at the given point. This is good news for soil CO2 flux measurements using the chamber methodology. Nevertheless, it is recommended to make several flux measurements at a given site on near-by locations in order to better characterise the measured parameter and its spatial variability. The two observation sites A and B were intentionally selected in the framework of this study to represent different degrees of anthropogenic impact in terms of fossil fuel CO2 load of the local atmosphere. However, no discernible signal of this anthropogenic CO2 could be detected in the isotopic composition of the local soil CO2 flux. Several possible reasons for this lack of fossil fuel signal are proposed: (i) episodic character of δ 13 Csource measurements, (ii) a distinct link between the photosynthetic activity of plants and the intensity of vertical mixing of the local atmosphere, leading to preferential ‘sampling’ of the atmospheric CO2 by local vegetation during periods of intense mixing, and (iii) long residence time of carbon in the soil reducing isotope variability of the input signal. It is clear that more detailed studies would be required to follow the fate of anthropogenic CO2 in an urban biosphere. Especially, analyses of soil parameters (porosity, density, organic carbon content and its 13 C and 14 C isotope signatures), combined with measurements of soil CO2 flux
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and its carbon isotope composition (both 13 C/12 C and 14 C/12 C ratios) would be required to gain a deeper insight into this problem. Funding The support of this work through the funds from Polish Ministry of Science and Higher Education [project No. 817/NCOST/2010/0] and the statutory funds of the AGH University of Science and Technology [project No.11.11.220.01] is kindly acknowledged.
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Isotopes in Environmental and Health Studies Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gieh20
Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland a
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Alina Jasek , Miroslaw Zimnoch , Zbigniew Gorczyca , Ewa Smula & Kazimierz Rozanski
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AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland Published online: 20 Jan 2014.
Click for updates To cite this article: Alina Jasek, Miroslaw Zimnoch, Zbigniew Gorczyca, Ewa Smula & Kazimierz Rozanski (2014) Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland, Isotopes in Environmental and Health Studies, 50:2, 143-155, DOI: 10.1080/10256016.2014.868455 To link to this article: http://dx.doi.org/10.1080/10256016.2014.868455
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Isotopes in Environmental and Health Studies, 2014 Vol. 50, No. 2, 143–155, http://dx.doi.org/10.1080/10256016.2014.868455
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Seasonal variability of soil CO2 flux and its carbon isotope composition in Krakow urban area, Southern Poland Alina Jasek∗ , Miroslaw Zimnoch, Zbigniew Gorczyca, Ewa Smula and Kazimierz Rozanski AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland (Received 17 July 2013; accepted 8 October 2013) As urban atmosphere is depleted of 13 CO2 , its imprint should be detectable in the local vegetation and therefore in its CO2 respiratory emissions. This work was aimed at characterising strength and isotope signature of CO2 fluxes from soil in urban areas with varying distances from anthropogenic CO2 emissions. The soil CO2 flux and its δ 13 C isotope signature were measured using a chamber method on a monthly basis from July 2009 to May 2012 within the metropolitan area of Krakow, Southern Poland, at two locations representing different levels of anthropogenic influence: a lawn adjacent to a busy street (A) and an urban meadow (B). The small-scale spatial variability of the soil CO2 flux was also investigated at site B. Site B revealed significantly higher summer CO2 fluxes (by approximately 46 %) than site A, but no significant differences were found between their δ 13 CO2 signatures. Keywords: carbon-13; CO2 flux measurements; CO2 soil emissions; urban atmosphere
1.
Introduction
The biogenic carbon dioxide (CO2 ) fluxes into the atmosphere are one of the dominant components of the global carbon cycle. The total CO2 flux from continental biogenic sources is estimated to be approximately 120 PgC yr−1 and is almost entirely balanced by the terrestrial plants’ photosynthetic assimilation [1]. Although the variability of the carbon cycle on a decadal time scale is mostly dominated by gradually increasing anthropogenic CO2 fluxes to the atmosphere, the biogenic component is clearly visible in the seasonal changes of the atmospheric carbon budget, especially in the northern hemisphere. On the other hand, the role of human-dominated ecosystems in regional carbon budgets increases with progressive urbanisation through both enhanced anthropogenic CO2 emissions and the diminishing role of undisturbed natural ecosystems when compared with expanding urban agglomerations. This leads to an increasing interest in studying CO2 emissions in urban environments. Previous research [2] has shown that the increase in the CO2 concentration in the urban atmosphere is due to several factors, which can be divided into two main categories: (i) anthropogenic, e.g. combustion of fossil fuels associated with transportation and electricity/heat generation, both in power plants and individual households, and (ii) biogenic, including plant respiration, soil organic matter decomposition and CO2 emissions from water courses. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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Soil fluxes of CO2 were investigated in a wide range of ecosystems, from boreal tundra to tropical forest, including human-modified soils [3–7]. Anthropogenic and biogenic contributions of CO2 to an urban atmosphere were studied using several approaches, mostly involving carbon and oxygen isotopes in CO2 [7–12]. Levin et al. [8] used analyses of the 14 CO2 content in atmospheric CO2 samples collected weekly in a densely populated area of the Rhine valley in Germany to determine the contribution of fossil fuel-derived CO2 to the local urban atmosphere. Using isotope mass balance approaches, they were able to demonstrate that, apart from anthropogenic CO2 , also a significant contribution originating from local biogenic sources is present in the investigated urban atmosphere. Kuc et al. [9] used a similar approach using biweekly cumulative sampling of atmospheric CO2 in the urban atmosphere of Krakow. They found that both concentration and isotopic composition (14 C/12 C and 13 C/12 C) of atmospheric CO2 in the urban environment differ distinctly from atmospheric baseline conditions. These differences can be used to constrain the CO2 budget and to quantify anthropogenic and biogenic CO2 contributions to the local atmosphere [2]. Koerner and Klopatek [7] quantified CO2 sources across the metropolitan region of Phoenix, Arizona, by measuring biogenic CO2 soil fluxes and using fossil fuel combustion inventories from governmental and non-governmental databases to assess the anthropogenic component of the regional CO2 emissions. In the urban environment of Phoenix, Arizona, the anthropogenic sources were responsible for over 80 % of the total CO2 fluxes into the atmosphere. Clark-Thorne and Yapp [10] measured the stable isotope composition of atmospheric CO2 in about 150 samples collected in the urban atmosphere of Dallas, Texas, and evaluated sources and sinks of CO2 in the city’s atmosphere using the box-model approach. Zimnoch [11] performed systematic measurements of the stable isotope composition of CO2 emitted from combustion of various types of fossil fuel within the metropolitan area of Krakow, Poland, and found that the CO2 associated with the combustion of the three major categories of fossil fuel is characterised by distinct δ 13 C signatures: approximately −23.8, −51.8 and −30.6 ‰ for coal, methane and the fuel burned in car engines (diesel, gasoline and LPG), respectively. The concentrations of CO2 in soils vary with soil type, depth and season, ranging from approximately 5000 up to 40,000 ppm. These concentration levels are significantly higher than those encountered in the contemporary urban atmosphere (≈ 400–500 ppm). The main source of soil CO2 is the autotrophic (plant roots) and heterotrophic (soil microorganisms) respiration. Respiration activity, usually defined via CO2 emissions, is an important indicator of biological processes in soils, reflecting rates of organic matter decomposition and plant respiration. The existing gradient of CO2 concentration across the soil–atmosphere interface is a primary driving force for the diffusion-controlled transport of this gas and the biogenic emissions of CO2 into the atmosphere. The CO2 transport in the soil depends on the permeability of the given soil which in turn is controlled mainly by soil porosity and soil water content [12,13]. Here, we present the results of a systematic study aimed at characterising biogenic CO2 emissions in an urban environment. Temporal variations of the soil CO2 flux and its δ 13 C isotope composition were investigated at two contrasting sites in the city of Krakow, Southern Poland, between July 2009 and May 2012. Also, the spatial variability of the biogenic CO2 flux was quantified on the plot scale. An additional research question addressed in the framework of this study was whether the progressive modification of the δ 13 C isotope signature in atmospheric urban CO2 can also be traced in the isotopic composition of the biogenic CO2 flux from soils. Direct, systematic observations of biogenic CO2 emissions and their carbon isotope composition in urban areas are needed for a better characterisation of the regional carbon cycle under heavy anthropogenic stress. In particular, it is expected that such systematic observations of CO2 emissions from urban soils will help to better constrain regional carbon budgets aimed at the quantification of anthropogenic emissions of this gas related to the burning of fossil fuels. It is estimated that the European continent with its dense transportation network, extensive industrial
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infrastructure and high population density is currently responsible for more than 25 % of the global fossil fuel-derived CO2 emissions [14].
2.
Materials and methods
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2.1. Site description The presented study was conducted in the urban area of Krakow, Southern Poland. With about one million inhabitants, Krakow is one of the largest cities in Poland. The rapidly growing car traffic and industrial development characterise typical urban environments of central and eastern Europe. The climate is continental with a mean annual air temperature of approximately 8◦ C and annual precipitation rates fluctuating around 600 mm, the majority of which occur during the growing season. Predominant native soils in the study area are brown, alluvial and loamy sands, boulder loam and loess-like sediments. However, expanding urbanisation caused their partial degradation. Two sites were chosen for systematic measurements of the soil CO2 flux and its carbon isotope composition: (i) site A (50◦ 03 51 N, 19◦ 55 26 E), located in the immediate vicinity of one of the major streets in the city, and (ii) site B (50◦ 03 41 N, 19◦ 54 08 E), located within the large urban meadow ‘Blonia’, a 48-ha grassland recreation area situated close to the city centre, with limited car traffic in the direct neighbourhood. The aerial distance between the sites is approximately 2 km. Mickiewicz Avenue, where site A is located, was constructed in the 1930s. It consists of two lanes with a 20-m-wide green zone in between. This zone is a park-type area, covered by lawn, with sparse trees and shrubs. The lawn is cut twice a year. The street is one of the main routes between the northern and southern parts of the city. Hence, heavy traffic (approximately 5000 cars per h) is present there almost the whole day. The local vegetation is exposed to an atmosphere enriched in CO2 , originating mostly from burning of fossil fuel in cars. The CO2 flux measurements were performed in the centre of the green zone. The main soil types at this site are Technosols, before anthropogenic degradation being mostly Phaeozems and Fluvisols. The historical land-use record for the Blonia urban meadow, where site B is located, shows that this area was never urbanised. The present green area is only a small part of the original grassland dominating the region. Until the nineteenth century, the meadow was a neglected wasteland, subject to regular flooding by the near-by river. Until the twentieth century, the area was used as a natural pasture, mostly for cattle. Then, at the beginning of the twentieth century, the terrain was drained and transformed into a recreational area. The dominant soil types are Fluvisols. For two months in 2011 (October and November), a part of the Blonia meadow including measurement site B was designated for sheep grazing. 2.2. Sampling and measurement The soil CO2 flux and its δ 13 C isotope signature were measured using a closed, dynamic chamber system coupled with a VAISALA CarboCAP CO2 sensor and combined with a flask sampling system for isotopic analysis (Figure 1). The air was circulated through the entire system during a measurement run. The CO2 concentration measurements inside the system during a single flux measurement run were performed with a temporal resolution of one second. Each measurement run lasted approximately 20 min. In most cases single flux determination at a time was made at each site. The chamber method is a widely used technique for measuring gas exchange between soil and atmosphere. There are several possible modes of using chambers for measurements of mass fluxes. Respective methodologies were reviewed by several authors [15–17].
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Figure 1. Closed dynamic chamber system used for measurements of soil CO2 flux and its δ 13 C values. P, pump; FS, flow regulator; DA, drying agent (Mg(ClO4 )2 ); Flask, glass flask to collect air sample for isotope analysis; CO2 analyser, Vaisala CarboCAP sensor; and hPa, pressure sensor.
The chamber system used for this work was designed and built at the Faculty of Physics and Applied Computer Science at the AGH University of Science and Technology, Krakow. It consists of a rectangular container made of stainless steel, coupled with a collar hammered into the soil to the depth of ∼ 5 cm and fitted with a water seal to avoid gas leakages. A vent with a long capillary tube was attached to the chamber to compensate for the effect of increasing pressure in the chamber during the course of a measurement run [18]. The surface of the soil covered by the chamber was 0.1365 m2 , and the air volume enclosed in the chamber and other parts of the circulation system (glass flask, CO2 sensor, drying trap, connecting tubes, etc.) was 0.0285 m3 . The system was equipped with a regulator stabilising the air flow through the CO2 analyser (flow rate of approximately 0.3 l min−1 ), an KNF Neuberger pump (maximum pressure 1500 h Pa) integrated with a CarboCAP sensor, a pressure sensor to monitor pressure fluctuations inside the circulation system and a trap containing a drying agent (magnesium perchlorate, Mg(ClO4 )2 ) used to dry the air sampled for isotope analyses. In addition, outside atmospheric pressure was monitored during each measurement run. The system can be also operated without collecting air samples for stable isotope analysis. The CO2 flux from the soil measured using the chamber method can be calculated using the following general formula: f =
pV |dC(t) /dt|t=0 , RTS
(1)
where f is the molar flux density of CO2 , p is the atmospheric air pressure, C(t) is the function describing temporal evolution of CO2 concentration inside the chamber, |dC/dt|t=0 is the first derivative of C(t) function calculated for t = 0, R is the universal gas constant, T is the ambient air temperature, and V and S are the chamber volume and soil surface area covered by the chamber. The C(t) function can be derived through fitting the measurements of the CO2 concentration build-up inside the chamber over the course of a measurement run. Typically, the fitted function has a form C(t) = a(1 − e−bt ) + C0 , where a and b are fitting constants. In such a formulation of the C(t) function, its first derivative at t = 0 is a product of the fitting constants: |dC/dt|t=0 = ab. In practical applications of the chamber method, CO2 concentration measurements are usually limited to the initial part of the C(t) function and the data points are approximated by a linear function the slope of which, M, is used in Equation (1) instead of |dC/dt|t=0 . For typical parameters of chamber systems in use (volume, surface area) and typical soil CO2 fluxes, the measurement times required to cover the substantial portion of C(t) function before it reaches saturation would involve very long waiting times (tens of hours). Therefore, this simplified approach was used in the present study. The following operational form of Equation (1) was used: f =
BpVM , RTS
(2)
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where B stands for a numerical factor. For f (expressed in mmol m−2 h−1 ), M (in ppm h−1 ) and other variables expressed in SI units, the value of B equals 10−3 . It should be noted, however, that the simplified method of measuring CO2 fluxes based on Equation (2) underestimates the fluxes by a few percent. Since our work was focused mainly on temporal and spatial variability of biogenic CO2 emissions in urban environments and not so much on absolute values of the soil fluxes of this gas, for logistical reasons we decided to employ this simplified method, being aware of its drawbacks and limitations. During each measurement run, two air samples were collected in 1 l glass flasks. The first flask was filled with ambient atmospheric air before the beginning of the measurement run. The second sample representing air inside the circulation system was collected at the end of the measurement run. The flasks were pressurised to approximately 1500 hPa. The δ 13 C value of the soil-respired CO2 was calculated using the formula derived from the isotope and mass balance equations set up for the volume of air contained inside the circulation system: δ 13 Csource =
δ1 C1 − δ0 C0 , C1 − C 0
(3)
where C0 , δ0 and C1 , δ1 stand for concentration and δ 13 C value of CO2 inside the chamber at the beginning and the end of the measurement run, respectively. This approach is equivalent to the use of a Keeling plot, which is usually employed when at least several measurements of concentration and δ 13 C of CO2 are available [19,20]. The isotopic composition of CO2 is expressed in the delta (δ) notation as a per mille deviation from an internationally accepted standard. The δ 13 C values of CO2 are expressed relative to the Vienna Pee Dee Belemnite standard, on a scale such that NBS-19 calcite equals +1.95 ‰ exactly, and L-SVAC lithium carbonate is −46.6 ‰ [21]. The concentrations of CO2 in the flasks were measured using a Hewlett Packard 6890 gas chromatograph equipped with a Ni catalyst and flame ionization detector. After concentration measurement, CO2 was extracted cryogenically from the air sample, and its δ 13 C value was determined using a DELTA-S mass spectrometer (Finnigan MAT, now Thermo Fisher, Bremen, Germany). The typical uncertainty of CO2 concentration measurements was in the order of 2.0 ppm for the VAISALA CarboCAP CO2 sensor and 0.1 ppm for GC-based analyses. The overall uncertainty of the isotope measurements was in the order of 0.2 ‰. The soil temperatures at 5 cm depth and the mean soil water content for 0–5 cm depth intervals were measured using an Elmetron PT-101 thermometer and a Delta T HH2 moisture probe, respectively. To assess the spatial variability of the measured soil CO2 fluxes, a dedicated campaign was performed in October 2011. The CO2 fluxes were measured at site B over three days with similar atmospheric and weather conditions. In order to uniformly cover 48 ha of the urban meadow at site B, three sampling pathways were chosen: two close to the borders and one through the centre of the meadow (Figure 2). The CO2 fluxes were measured at every 100 m distance along these pathways. One measurement of the CO2 flux at the given point was performed. In total, the CO2 flux was measured at 42 points. At each point, the spatial coordinates were recorded using a GPS tracker (Garmin GPS72).
3.
Results and discussion
3.1. Small-scale spatial variability of soil CO2 flux The results obtained during the sampling campaign carried out at site B over three days in October 2012 (17–19) were used to assess the spatial variability of the soil CO2 flux for the type of
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Figure 2. Spatial variability of soil CO2 flux at site B (urban meadow). The area of the meadow is approximately 48 ha. The measurements were performed during three days (17–19 October 2012) between 11:00 and 16:00 local time. Black points represent measured values. The surface was fitted to the experimental data using a Surface Fitting Tool with cubic interpolation algorithm (Matlab 2010B).
ecosystem represented by this site (urban meadow). Forty-two flux measurements performed on the site yield a mean CO2 flux equal to 13.3 ± 0.4 mmol m−2 h−1 . The quoted standard deviation of individual measurement (0.4 mmol m−2 h−1 ) is comparable to the calculated uncertainty using error propagation for single determinations of the CO2 flux. The constructed map (Figure 2) reveals a rather homogeneous distribution of CO2 fluxes, except for one region located in the eastern part of the meadow, close to the main entrance, where significantly higher fluxes were observed. The mean value of the CO2 flux in the eastern part is 17.3 ± 0.2 mmol m−2 h−1 . If measurements performed in this region are excluded from the evaluation of the mean CO2 flux, its mean value drops to 12.8 ± 0.4 mmol m−2 h−1 . Although the measurements presented in Figure 2 reveal that small-scale spatial variability of soil CO2 flux measured with a closed-chamber technique is of the same order as the standard uncertainty of single flux determination, it is advisable to perform several parallel measurements of the flux at adjacent points whenever possible. Thus, larger measurement errors may be avoided.
3.2. Temporal variability of soil CO2 flux The soil CO2 fluxes at both investigated sites reveal a strong seasonality (Figure 3) induced by the vegetation cycle and seasonal variation of soil parameters (temperature, water content). Minimum values of the CO2 fluxes were recorded during the winter months (December, January and February). Low soil temperatures limit respiration activity, while higher water contents reduce permeability of the soil during this period. A snow cover may additionally reduce the transport of CO2 between the soil and the atmosphere. Typical CO2 fluxes measured during winter varied between 1 and 5 mmol m−2 h−1 for both sites. During the vegetation period (April–October), the CO2 fluxes increase significantly, reaching maximum values of 25–30 mmol m−2 h−1 at site A and 40–50 mmol m−2 h−1 at site B. The periods when the flux maxima occur fluctuate from year to year. In 2010, the maximum CO2 flux was observed in June at both sites while in 2009 and 2011, the maximum flux at site B was delayed with respect to site A. The mean value of soil CO2 flux throughout the whole measurement period at both sites is 16.2 ± 1.7 mmol m−2 h−1 and is
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Figure 3. Seasonal variability of the CO2 flux at site A (closed diamonds, solid line) and site B (open squares, dotted line) during the period July 2009–May 2012. Shown are also standard uncertainties of the measured fluxes (one-sigma level). Each point represents a single determination of the flux.
comparable to the mean fossil fuel CO2 flux reported in the emissions database for global atmospheric research database for Krakow agglomeration in 2008 (approximately 17.8 mmol m−2 h−1 [22]). One of the important parameters controlling the respiration activity of soils is temperature. Mielnic and Dugas [23] showed that CO2 flux and soil temperature exhibit an exponential relationship. A clear temperature dependence was observed for the CO2 flux at both sites (Figure 4). The highest soil temperature (+21.5◦ C) was measured at site B in August 2011, while the lowest was recorded at site A in March 2010 (−4.5◦ C). The Q10 temperature sensitivity coefficient [24] was calculated for both sites using the firstorder exponential function, F = a exp(bt), where F stands for soil CO2 flux in μ mol m−2 s−1 and t for the soil temperature in ◦ C. The Q10 coefficients calculated for site A and B are 2.25 and 2.98, respectively. The Q10 coefficient determined for site A is in agreement with a value of 2.29 reported for cultivated Phaeozem soil located at a similar latitude [25]. The values of Q10 coefficients reported for Fluvisols are higher than the value obtained for the urban meadow (site B–2.98). In Switzerland, Samaritani et al. [26] obtained Q10 values of 3.6 for grass, 3.4 for pasture and 3.2 for mixed forest. Awasthi et al. [27] report Q10 values in a range from 2.5 to 5.0 for mixed Fluvisols, Cambisols, Rigosols and Luvisols in Nepal. A lower Q10 value on site A versus site B can also be a consequence of differences in soil characteristics at both sites. At site B, where the CO2 flux grows faster with temperature, the soil is less disturbed than at site A, where the soil was classified as typical Technosol. The historical land-use record for the Blonia meadow shows that this region of the city was never urbanised. The stronger flux-temperature dependence observed at site B can be related to that difference. Another important factor controlling the CO2 flux is soil water content which regulates the availability of water for plants during the growing season and changes the free air porosity of the soil. The application of the two-parameter fitting procedure to the available CO2 flux data for both sites, with soil temperature t and soil water content w as fitting parameters, revealed
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Figure 4. The relationship between soil CO2 flux and soil temperature measured at 5 cm depth, as observed at site A and site B during the period July 2009–May 2012. The data points represent individual determinations of the CO2 flux and soil temperature.
that soil water contents below 30% reduce the strength of the CO2 flux to the same extent as the drop of soil temperature below approximately 10◦ C (Figure 5). Such a dependence of the CO2 flux upon soil moisture is in agreement with qualitative predictions [28] which state that for low as well as high water contents, the CO2 emission from soils should be noticeably lower than for intermediate (approximately 30–50%) soil water contents. Both cases are related to different processes: for dry soils the respiration activity decreases due to limited water availability, while at high water contents the increasing anaerobic microbial activity combined with reduced soil permeability leads to reduction of the soil CO2 flux [29]. 3.3. δ13 C isotope signature of soil CO2 flux As anticipated, the measured δ 13 C values of atmospheric CO2 were slightly lower at site A (Figure 6(a)), where higher CO2 concentrations were also observed (Figure 6(b)). Heavy car traffic is permanently present in close vicinity to the plot where the flux measurements were performed at this site. The mean CO2 concentration difference between both sites (9.7 ± 3.8 ppm) is consistent with measured δ 13 C values; higher CO2 concentrations in a neighbourhood of car traffic are associated with lower δ 13 C signatures. The mean difference between individual δ 13 CO2 values recorded at both sites is 0.83 ± 0.15 ‰, with δ 13 C of atmospheric CO2 at site A being significantly lower than that at site B. A clear seasonal cycle of atmospheric δ 13 CO2 at both sites is visible in Figure 6(a), with minimum values occurring during winter, when anthropogenic sources of CO2 are stronger (house heating) and meteorological conditions favour longer periods of temperature inversions leading to accumulation of isotopically lighter CO2 within the boundary layer.
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Figure 5. The soil CO2 flux data for site A and site B obtained during the period July 2009–May 2012, as a function of soil temperature and soil water content. The surface was fitted using a Surface Fitting Tool with the cubic interpolation algorithm (Matlab 2010B).
Figure 6. (a) Seasonal δ 13 C variability of atmospheric CO2 at site A (closed diamonds, solid line) and site B (open squares, dotted line) measured during the period July 2009–May 2012, and their mean values (horizontal lines) with the standard uncertainties of the mean values marked by vertical bars. The standard uncertainty of isotope analyses is in the order of 0.2 ‰. The mean values of δ 13 Catm are −10.30 ± 0.18 ‰ for Site A and −9.56 ± 0.16 ‰ for Site B. (b) Seasonal variability of atmospheric CO2 concentration at site A (closed diamonds, solid line) and at site B (open squares, dotted line) measured during the reported period. The mean values are 419.0 ± 3.3 ppm (site A) and 408.5 ± 4.4 ppm (site B).
The question arises whether the observed difference in the isotopic composition of atmospheric CO2 at both sites is somehow imprinted in the isotopic signature of the soil CO2 flux recorded at those sites. It is expected that the fossil fuel signature in plant-assimilated CO2 should be imprinted in the soil CO2 flux, as one of its main components is the below-ground root respiration. Thus, the soil CO2 released at site A should contain less 13 C than that released at site B. The carbon isotopic signature of soil CO2 (δ 13 Csource ) calculated using Equation (3) has fluctuated at both sites between approximately −31 and −24 ‰, without apparent seasonality (Figure 7). The mean values of δ 13 Csource were −27.8 ± 1.5 ‰ and −28.1 ± 1.1 ‰ at sites A and B, respectively.
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Figure 7. Seasonal variability of δ 13 C in soil-respired CO2 at site A (rectangles) and site B (squares), measured during the period July 2009–May 2012. The presented δ 13 C values were calculated using two-component mixing model (see text).
Within the reported standard uncertainty, the mean δ 13 C values are non-distinguishable, suggesting dominance of C3 plants at both sites and lack of a discernible fossil fuel signal in the soil-respired CO2 . This surprising result may originate from several effects. First, one may argue that the observed difference in the isotopic composition of atmospheric CO2 at both sites may have its origin in the episodic character of the measurements performed, while autotrophic and heterotrophic respiration processes are operating during the whole day. In addition, as δ 13 Csource at each site is represented by only one measurement per month, the representativeness of δ 13 Csource values for the soil CO2 flux can be questioned. On the one hand, Bahn et al. [30] reported significant diurnal variability of δ 13 C in soil-respired CO2 , with lower δ 13 C values during the morning hours and higher during the night. They did not offer any explanation of this behaviour of δ 13 C. On the other hand, the study of Betson et al. [31] has not revealed any significant diurnal variability of δ 13 C in the soil-respired CO2 . A dedicated measurement campaign performed at site A on 9–10 July 2009 did not reveal any significant diurnal variability of δ 13 Csource either. In the light of this conflicting evidence, we tentatively assume that single measurements of soil-respired CO2 are representative at least for a whole day. Second, it is known that the strength of the assimilation process depends on the intensity of photosynthetically active radiation (PAR) [32]. Similarly, the intensity of vertical mixing within the boundary layer of the lower atmosphere also depends on solar radiation. Day et al. [33] have shown that lower atmospheric CO2 concentrations observed at higher PAR levels are a consequence of a more intensive convective mixing of the lower atmosphere and an increased intensity of photosynthesis. This mixing will likely smooth out the local gradients in terms of concentration
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and isotopic composition of the human-induced CO2 load on the near-ground atmosphere within the city. Thus, similar δ 13 C signatures of atmospheric CO2 are expected at both investigated sites. Third, a long residence time of carbon in the soil reduces the isotope variability of the input signal. Thus, the expected δ 13 C difference of heterotrophic CO2 flux components at both sites can be significantly smaller than natural spatial and temporal variability of δ 13 Csource observed at those sites. The average age of carbon in soil organic matter is reported to range from 200 to 1200 years, depending on the soil type. This age is higher than the duration of the industrial era (approximately 150 years) during which a gradual modification of atmospheric δ 13 C values due to burning of fossil fuels was recorded. However, the age of carbon measured in the total soil respiration flux is much lower, in the order of several years for temperate soils [34]. Using 13 C-labelling experiments Bahn et al. [30] found that CO2 assimilated during photosynthesis is rapidly transported to plant roots and respired (within a few hours). Thus, depending on the relative contribution of heterotrophic and autotrophic respiration in the soil, the fossil fuel imprint in the δ 13 C isotope signature of the soil CO2 flux can be smaller or larger. The obtained results do not allow us to draw definitive conclusions with respect to a possible anthropogenic imprint in the carbon isotope composition of the soil CO2 flux. More measurements of δ 13 Csource at other sites of contrasting characteristics in terms of anthropogenic impact are needed to clarify this problem.
4.
Conclusions
The results of this study indicate that the CO2 flux from urban soils varies mostly with soil temperature, but is also dependent on the soil characteristics. The observed strong seasonal variability of the CO2 flux was mainly driven by temperature, which controls respiration activity.Although in the reported range of soil temperatures (from −4.5◦ C to +21.5◦ C), a strong temperature dependence of the CO2 flux was observed, multi-parameter analysis suggest that the soil water content is also important. A decrease in the soil water content below approximately 30% reduces the intensity of the CO2 flux by the same amount as the drop of soil temperature below approximately 10◦ C. Because measurements of soil CO2 flux are point measurements, the question of small-scale spatial variability of this parameter is critical. The assessment of this variability performed for site B suggests that for homogeneous ecosystems such as the investigated urban meadow, the measure of the spatial variability of the CO2 flux (standard deviation) determined during the vegetation period is comparable to the uncertainty of a single determination of the flux at the given point. This is good news for soil CO2 flux measurements using the chamber methodology. Nevertheless, it is recommended to make several flux measurements at a given site on near-by locations in order to better characterise the measured parameter and its spatial variability. The two observation sites A and B were intentionally selected in the framework of this study to represent different degrees of anthropogenic impact in terms of fossil fuel CO2 load of the local atmosphere. However, no discernible signal of this anthropogenic CO2 could be detected in the isotopic composition of the local soil CO2 flux. Several possible reasons for this lack of fossil fuel signal are proposed: (i) episodic character of δ 13 Csource measurements, (ii) a distinct link between the photosynthetic activity of plants and the intensity of vertical mixing of the local atmosphere, leading to preferential ‘sampling’ of the atmospheric CO2 by local vegetation during periods of intense mixing, and (iii) long residence time of carbon in the soil reducing isotope variability of the input signal. It is clear that more detailed studies would be required to follow the fate of anthropogenic CO2 in an urban biosphere. Especially, analyses of soil parameters (porosity, density, organic carbon content and its 13 C and 14 C isotope signatures), combined with measurements of soil CO2 flux
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and its carbon isotope composition (both 13 C/12 C and 14 C/12 C ratios) would be required to gain a deeper insight into this problem. Funding The support of this work through the funds from Polish Ministry of Science and Higher Education [project No. 817/NCOST/2010/0] and the statutory funds of the AGH University of Science and Technology [project No.11.11.220.01] is kindly acknowledged.
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