KARST SOILS: DEPENDENCE OF CO2 CONCENTRATIONS ON PORE DIMENSION ODVISNOST KONCENTRACIJE CO2 OD VELIKOSTI POR V KRAŠKIH PRSTEH Martin BLECHA1-2 & Jiri FAIMON1 Abstract UDC 631.41:551.435.8(437.32) Martin Blecha & Jift Faimon: Karst soils: Dependence of CO2 concentrations on pore dimension CO2 concentrations were studied in the selected soils of the Moravian Karst, Czech Republic. The direct measurement in the air of drill-holes has indicated that the concentrations depend inversely on a pore dimension. The simplified relation between the drill-hole diameter and CO2 concentration, .„ Cco2-bD i-naD -, was proposed, where c0O2 is the CO2 concentration extrapolated to the zero drill-hole diameter in ppmv, cCO., is directly measured CO2 concentration in ppmv, and D is drillhole diameter in cm. a and b are parameters in cm 1 and ppmv cm1, respectively. For the karst soils formed at grass field and deciduous forest, the values of a and b parameters were determined as -0.146±0.012 (standard error) cm1 and 262.0±56.3 ppmv cm-1, respectively. The dependence between cCO2 and D was less obvious for the heavy clay soils of coniferous forest. To understand the dependence better, a conceptual model was created taking into account the concentration gradients and mass fluxes. Keywords: CO2 concentration, drill-hole diameter, karst soil, model. Izvleček UDK 631.41:551.435.8(437.32) Martin Blecha & Jift Faimon: Odvisnost koncentracije CO., od velikosti por v kraških prsteh Raziskovali smo koncentracijo CO2 v izbranih prsteh na Moravskem krasu v Češki republiki. Neposredne meritve v vrtinah so pokazale, da odvisnost koncentracije CO2 od premera vrtin zadovoljivo opiše enačba '^j^aD^, kjer je D [cm] premer vrtine v cm, cCO., je vrednost meritve, cco2 [ppmv] je ek-strapolirana koncentracija CO2 za D = 0, a [cm-1] in b [ppmv/cm] pa sta regresijska parametra. Za prsti na kraških travnikih smo dobili vrednosti parametrov a = -0,146±0,012 cm-1 in b = 262,0±56,3 ppmv-cm.-1. Odvisnost med cCO2 in D je manj značilna v glinenih prsteh iglastih gozdov. Merjene odvisnosti smo pojasnili z modelom, ki upošteva gradiente koncentracij in masne tokove. Ključne besede: koncentracije CO2, premer vrtin, kraška prst, modeliranje. INTRODUCTION Carbon dioxide is the key component in carbonate karst that affects (i) limestone dissolution (e.g. Stumm & Morgan 1996), (ii) calcite/aragonite speleothem growth (e.g. Dreybrodt 1988), or speleothem corrosion (Sarbu & Lascu 1997). Researchers believe that karst/cave CO2 is derived from karst soils (e.g. Ford & Williams 2007). The soil CO2 is produced by the respiration of (1) au-totrophs and (2) heterotrophs (Kuzyakov & Larionova 1 Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic, e-mail: faimon@sci.muni.cz 2 Research Institute for Soil and Water Conservation, Žabovreska 250, 256 27 Praha 5 - Zbraslav, Czech Republic, e-mail: blecha.martin@vumop.cz Received/Prejeto: 26.09.13 2005; Kuzyakov 2006). CO2 production may depend on temperature/moisture, soil profile depth, organic matter content, total rainfall, photosynthesis/solar radiation, and various anthropogenic factors such as soil tillage, or artificial change in vegetation cover. The role of abiotic sources is also considered (e.g., Serrano-Ortiz et al. 2010). Soil CO2 is generally an important part of the global carbon cycle (e.g., Schlesinger & Andrews 2000). The CO2 concentrations in karst soil air are typically measured in a range from 0.1 to 1.0 vol. % (Yoshimura et al. 2001; Li et al. 2002; Spötl et al. 2005; Kawai et al. 2006; Faimon & Ličbinska 2010; Sanchez-Canete et al. 2011; Faimon et al. 2012a). Some indices, e.g., karst water chemistry, enhanced CO2 levels in certain caves, limited total soil pore volumes, CO2 fluxes into external atmosphere, etc., question the soil capability for filling cave volume up to given concentrations. This indicates some more productive CO2 sources participating on karst CO2. The idea of an "underground CO2" was already proposed by Atkinson (1977). For the karst environment, an epikarstic source is sometimes hypothesized (Fairchild et al. 2000; Spötl et al. 2005; Faimon et al. 2012a; Cuesva et al. 2011, Peyraube et al. 2012, 2013). The hypothesis is supported by evident discrepancy between (1) CO2 concentrations directly measured in karst soils and (2) CO2 concentrations reconstructed from dripwater hy-drogeochemistry (see, Faimon et al. 2012b). Recently, Benavente et al. (2010) confirmed the existence of the enhanced CO2 concentrations deeply in subsoil by an in-situ measurement. Even thought we agree with the idea of the epikarstic source, we have primarily concentrated on karst soils and its efficiency to fill enlarged pores by CO2. The purpose of this study is to demonstrate how the diameter of drill-hole in soil profile can influence CO concentrations. METHODS RESEARCH LOCATION lic). It represents a belt of Middle and Upper Devonian The study was performed in the Moravian Karst, the limestones, 3-6 km wide and 25 km long (correspond-largest karst area in the Bohemian Massif (Czech Repub- ing to 94 km2 area). Typical soils consist of Rendzic Lep- Fig. 1: Research location with monitoring sites. tab. 1: The soils and sampling sites. Site coordinates envir. vegetation cover pedogenic substrate soil type b. dens. g/cm3 por. org. mat. wt. % abbrevn. Harbechy Plateau 49°21'34"N 16°43'49"E agricult. field after harvest (wheat) loam loesses Haplic Luvisol 1.049 0.60 5.40 AFH Lažanky I 49°21'24"N 16°42'55"E meadow grassy devonian limestone Rendzic Leptosols 0.702 0.72 13.22 GML Lažanky II 49°20'47"N 16°43'50"E forest deciduous loam loesses Haplic Luvisol 0.880 0.65 8.88 DFL Rudice 49°19'53"N 16°42'34"E forest coniferous loam loesses Stagnosols 1.086 0.57 8.51 CFR envir. - environment; b. dens. - bulk density; por. - porosity; org. mat. - organic matter tosols, Haplic Luvisols, and Albeluvisols. The research sites were located at the meadow and deciduous forests at Lažanky (Blansko), the agricultural field near the sinkhole Společnak at Vilemovice (Harbechy Plateau), and the coniferous forest at Rudice, see Fig. 1. The details on these sites/soils are illustrated in Tab. 1. MONITORING At every research location, shallow holes, 25 cm deep, and 7.0, 5.0, 2.7, and 2.0 cm in diameter were manually drilled into soils by using hand augers. These drill-holes were arranged into a line as follows: The 7-cm-hole was in the middle and further holes with decreasing diameters were on both sides. The drill-hole spacing was 20 cm each from other. The walls of drill-holes were reinforced by a plastic net. The top of the drill-hole was sealed by a plastic cap. The CO2 levels, temperature, and relative humidity in drill-hole air were repeatedly measured throughout two periods. The 1st period lasted from August 27 until September 13, 2012. The second began on May 5 and ended on May 17, 2013. The results were recorded between 3-6 P.M. The hand-held sensor FYA600-CO2H (Ahlborn, Germany) (±50 ppmv +2% of the values in the range < 5000 ppmv; ±100 ppmv +3% of the values in the range of 5000-10000 ppmv) working on principle of two-channel infrared absorption spectrometer (NDIR technology) was used to measure the CO2 concentration. Since the sensor is cylindrical, 18 mm in diameter, it was placed directly into the drill-hole air at a depth of about of 11-12 cm. The sensor FHA646E1 (Ahlborn, Germany) was used to measure the temperature and relatively humidity (±0.4 °C in the range from -20 to 0 °C and ±0.1 °C in the range from 0 to +70 °C, and ±2% RH in the range from 0 to 100% RH at 25 °C). The sensors were plugged into the drill-hole by a rubber selvage to prevent CO2 from escaping. The data were recorded after the stabilization of measured value. All the data were gathered by the data logger ALMEMO 2590 4S (Ahlborn, Germany). RESULTS The temperature of the external atmosphere varied between 15 and 25 °C except for September 13, 2012, being dropped to 11°C. In all the drill-holes, the temperature ranged from 9 to 19 °C and developed in conformity with the external atmosphere. The relative humidity of the air in the holes ranged from 92 to 100%. The CO2 concentrations varied based on both time and drill-hole diameter. The CFR site was the only location where the CO2 concentrations did not show any trend (Fig. 2). The enhanced concentrations of CO2 (between 2382 and 7716 ppmv) were systematically measured in the drill-holes with the smallest diameter. In contrast, the lowest concentrations were found in the drill-holes with the biggest diameter (between 568 and 3192 ppmv). Absolute minimum in concentrations (568 ppmv) was observed in the 7-cm drill-hole at the AFH site on September 13, 2012. The highest maximum of carbon dioxide concentration, 7716 ppmv, was measured in the 2-cm drill-holes at the DFL site on May 9, 2013. Fig. 2: CO2 concentrations measured in the soil drill-holes of various diameters at the sites AFH (A), DFL(B), GML(C), and CRF (D). The drill-holes were 25 cm deep. The distance between the individual holes was 20 cm. DATA ANALYSIS CO2 CONCENTRATIONS VS. DRILL-HOLE DIAMETER The results of the correlation analysis of the variables, drill-hole diameter and measured CO2 concentrations, are shown in Tab. 2. The strong negative correlations predominate for the AFH site (the correlations that are significant at a = 0.05 appear in nine cases; the correlations significant at a = 0.10 appear in additional four cases). The negative correlation for the DFL and GML sites are only slightly less convincing (at each site, the correlations significant at a = 0.05 are visible in seven cases; the correlations significant at a = 0.10 appear in additional three cases). In contrast, the correlations for the CRF site seemed to be inconclusive. They are paradoxically positive: the correlation significant at a = 0.05 appear in two cases; the correlations significant at a = 0.10 appear in one case). TEMPERATURE EFFECT The correlations between the logarithm of CO2 concentration and reciprocal temperature in Kelvins were tested, based on the assumption that CO2 concentrations correspond with CO2 production and that the production obeys the Arrhenius equation. However, both the variables, ln(cCO2) and 1/T, correlate only sporadically, which is demonstrated in Tab. 3. Two negative correlations significant at a = 0.05 were found for the AFH and GML sites. Only one significant negative correlation was found for the DFL soil. Paradoxically, just positive correlations predominate in case of the CFR site. REGRESSION ANALYSIS The data on CO2 concentrations and diameters were regressed by the equation ccO2 = s D + c0CO2), (1) where cCO2 is the measured CO2 concentration, s is the slope of dependence, D is the diameter [cm] and c(0CO2) is the CO2 concentration extrapolated to a zero D. The discovered linear dependence parameters (eq. 1) are shown in Tab. 4. For all the parameters, standard error and p-values are given. The dependence slope s ranged between -910.7 and -49.7 ppmv cm-1 for all the AFH, GML, and DFL sites; the higher the s value, the stronger the dependence of CO2 concentration on the diameter D. The significance of the s-parameter is consistent with the results of the correlation analysis. The y-intercept, cJCO2), ranged from 2466 to 8395 ppmv for all of the AFH, GML, and DFL sites and changed with the slope s. All these c"CO2) parameters are significant at a = 0.05. For the CRF sites, the s-parameters are paradoxically positive with high uncertainty in most cases. The significant values for CRF-12 and CRF-15 are the tab. 2: pearsons correlations between soil cCO2 and drill-hole diameter. ----------- AFH-11 AFH-12 AFH-13 AFH-14 AFH-15 AFH-16 AFH-17 -0.94 -0.98 -0.98 -0.95 -0.94 -0.90 -0.99 AFH-21 AFH-22 AFH-23 AFH-24 AFH-25 AFH-26 AFH-27 -0.94 -1.00 -0.98 -0.98 -0.97 -0.96 -1.00 DFL-11 DFL-12 DFL-13 DFL-14 DFL-15 DFL-16 DFL-17 -0.98 -0.98 -0.95 -0.98 -0.90 -0.83 -0.76 DFL-21 DFL-22 DFL-23 DFL-24 DFL-25 DFL-26 DFL-27 -0.90 -0.92 -0.90 -0.90 -0.96 -0.99 -1.00 GML-11 GML-12 GML-13 GML-14 GML-15 GML-16 GML-17 -0.99 -0.87 -0.93 -0.75 -0.97 -0.96 -0.99 GML-21 GML-22 GML-23 GML-24 GML-25 GML-26 GML-27 -0.98 -0.85 -0.98 -0.95 -0.92 -0.95 -0.55 CFR-11 CFR-12 CFR-13 CFR-14 CFR-15 CFR-16 CFR-17 0.64 0.97 0.55 0.91 0.96 0.82 0.85 CFR-21 CFR-22 CFR-23 CFR-24 CFR-25 CFR-26 CFR-27 -0.19 -0.26 -0.48 -0.12 0.14 -0.56 -0.01 The correlations highligted are significant at a = 0.05 The correlation by italic are significant at a = 0.10 only exception, cant. The c0Cq2) parameters are more signifi- The slopes s = dcCO2/dD follow the equation dD (2) 400 200 - 0 - -200 - -400 - E u "5 E Q. a a "D 8 -600 o •o -800 - -1000 - -1200 + V ^ A AFH X ^ □ GML • DFL + CFR n \ A \ • A \ A A \ A \ • il \ • \ y = -0.168x +385.2 \ \ • R^ = 0.85 \ \ s V o o O o o o o o o o O o o o o o o o o o O o o o o o o o o o CN CO 'd- m CD r- oo O) o Co [ppmv] where a, b are the parameters and the other symbols have their standard meaning. The parameters were found through regression analysis. They are listed in Tab. 5 by the monitoring sites. For the individual sites, aparameter varied between -0.13 and -0.16 cm-1, and b-parameter ranged from 88 to 422 ppmv cm-1. For the total combined data of all the sites, a = -0.178 cm-1 and b = 421.2 ppmv cm-1 (see Fig. 3). For the meadow and deciduous forest soils without the CFR soil, a = -0.158 cm-1 and b = 310.6 ppmv cm-1. Fig. 3: Relation between the slopes and zero diameter concentrations. tab. 4: The regression parameters of the dependence cCO., = s D + c s-parameter parameter whole model site date s std. err. (a> P C0(CO2) std. err. P R2 p AFH-11 P b std. err. P R2 P AFH -0.134 0.011 0.000 66.7 55.1 0.250 0.93 0.000 GML -0.133 0.017 0.000 262.9 55.2 0.000 0.84 0.000 DFL -0.140 0.014 0.000 317.8 80.0 0.002 0.89 0.000 CRF -0.114 0.028 0.001 341.4 69.3 0.000 0.58 0.001 as w CRF