SEPARATION OF SOIL ORGANIC AND INORGANIC CO FLUXES · 2006-11-02 · SEPARATION OF SOIL ORGANIC AND...
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SEPARATION OF SOIL ORGANIC AND INORGANIC CO 2 FLUXES William E. Emmerich and Dean A. Martens USDA-ARS Southwest Watershed Research Center Tucson, AZ INTRODUCTION Soil organic carbon is the second largest pool in the world and inorganic the third. Arid and semiarid rangeland soils contain most of the inorganic carbon in the form of carbonates. The determination of organic and inorganic carbon fluxes from soils is important in understanding the global carbon budget. Organic and inorganic carbon have different isotopic signatures. The purpose of this study was to develop a procedure to separate organic and inorganic soil carbon fluxes into the fraction from each source using the isotope signatures and present two years of results from two rangeland sites with different amounts of organic and inorganic soil carbon. METHODS A shrub (Lucky Hills) and grass (Kendall) site located on the Walnut Gulch Experimental Watershed in southeastern Arizona was selected for the study. The climate is semiarid with cool winters and warm summers. Mean annual precipitation is 356 mm and mean annual temperature is 17 o C. The Lucky Hills surface A horizon (0-6cm) had 8 gkg -1 organic carbon and 21 gkg -1 inorganic. The Kendall surface (0-3cm) horizon had 11 gkg -1 organic carbon and 7 gkg -1 inorganic. To collect the CO 2 flux from the soil a domed lid with a diameter of 0.54m was inserted 2 cm into the soil producing an enclosed volume of 0.02 m 3 . The CO 2 was collected for 30-60 min. depending on time of the year. Then a sample was pump out and bubbled through a solution of NaOH and SrCl 2 to precipitate the CO 2 out as SrCO 3 (Figure 1). During all sampling the CO 2 concentration was measured with a Qubit model S151 infra red analyzer calibrated at the sites. The in port was fitted with a soda lime filter to remove incoming CO 2 . Immediately after the lids were put down a CO 2 sample was collected at the same height as the lids to represent the CO 2 that was initially trapped under the lids. The strontium carbonate and soil samples collected with depth were analyzed at UC Davis Stable Isotope Facility on a Europa1 Hydra 20/20 continuous flow isotope ratio mass spectrometer. The carbon isotope signature was expressed relative to the Pee Dee Belemnite (PDB) standard as: 13 C(‰) = [[( 13 C/ 12 C sample) – ( 13 C/ 12 C standard PDB )]/ ( 13 C/ 12 C standard PDB )] 1000. FLUX SEPARATION PROCEDURES Upon inserting the collection lid into the soil a volume of air was trapped with a concentration of CO 2 (Ca) and a isotope signature ( 13 Ca) for the CO 2 . Both CO 2 concentration and 13 Ca were measured at the start of CO 2 collection. At sample collection time measured CO 2 concentration (Cmix) was a mixture of the Ca plus the flux of CO 2 (Cf ) from the soil. The isotope signature of this mixture ( 13 Cmix) was determined from the CO 2 sample at collection time. The number of moles (n) of air under the lid at the start of sample collection was calculated from: n = PV/RT Eq. 1. where P is the atmospheric pressure (determined as a function of elevation), V the volume under the lid, R the universal gas constant (0.0821 l atm/mole °K), and T temperature (°K) measured at time of sample collection. The number of moles of Figure 1. Equipment uses to collect CO 2 soil flux samples. air under the lid (Eq. 1) was multiplied by the Ca concentration (μmole/mole) at the start of sampling to give moles of CO 2 represented by (b). The number of moles of air under the lid at sample collection time was assumed to be the same as at the start of collection and multiplied by the Cmix concentration at sample collection to give the moles of CO 2 mixture represented by (a). The number of moles of CO 2 under the lid at sampling time is: a = b + c Eq. 2. where c is the number of moles of flux from the soil. A mass balance of the moles and the isotope signatures of the different sources of CO 2 can be represented by: a 13 Cmix = b 13 Ca + c 13 Cf Eq. 3. where 13 Cf is the isotope signature of the CO 2 flux from the soil. Combining Equations 2 and 3, 13 Cf was calculated. The diffusion coefficients for 12 CO 2 and 13 CO 2 differ by 4.4 ‰ due to the differences in mass (Craig, 1953). Therefore the 13 Cf and the isotope signature of the CO 2 in the soil atmosphere ( 13 Csa) would not be expected to be the same. Cerling et al. (1991) used a similar CO 2 soil surface collection procedure and collecting CO 2 soil profile samples found that the measured differences in ‰ and the theoretical due to diffusion coefficients were very close. The diffusion differences were used to calculate 13 Csa as: 13 Csa = 13 Cf + 4.4 ‰ Eq. 4. The 13 Csa is composed of the isotopic signature of the inorganic soil carbon ( 13 CIC) (primarily CaCO 3 ) through the carbonate equilibrium equations (Rabenhorst et al., 1984) and the isotopic signature of the soil organic matter ( 13 COC) through the decomposition of the soil organic matter. The 13 Csa can be related to its components by: 13 Csa = d 13 CIC + e 13 COC Eq. 5. where d is the fraction of 13 CIC in 13 Csa and e is the fraction of 13 COC in 13 Csa with: 1 = d + e Eq. 6. The mass difference in the 13 C and 12 C causes an equilibrium- based isotopic fractionation to occur between the soil atmosphere CO 2 and the inorganic carbon (CaCO 3 ) in the soil (Emerich. et al., 1970). The inorganic carbon fractionation is represented by: 13 Csa = 13 CIC – 10.17‰ Eq. 7. The 13 CIC – 10.17‰ also represents the isotopic contribution to the 13 Csa from the inorganic carbon. The inorganic and organic isotopic contribution to the 13 Csa can be represented as: ( 13 CIC – 10.17‰) 13 Csa 13 COC Eq. 8. with a long term equilibrium established between the pedogenic carbonate and soil CO 2 plus the fractionation. Equation 5 can be rewritten including the inorganic carbon fractionation as: 13 Csa = d ( 13 CIC – 10.17‰) + e 13 COC Eq. 9. The fraction of CO 2 soil flux from the inorganic carbon was calculated by combining Equations 6 and 9: d =( 13 Csa - 13 COC)/( 13 CIC – 10.17‰ - 13 COC) Eq. 10. The fraction of CO 2 soil flux from organic carbon was calculated from Equation 6: e = 1 – d Eq.11. RESULTS The developed procedure was able to separate carbon dioxide flux from the soil into the fraction that originated from organic and inorganic sources in the soil. This involved determining the isotope signature of the carbon dioxide initially trapped in the collection lid, the mixture of trapped and soil carbon dioxide flux, and organic and inorganic soil carbon. Inorganic carbon isotope fractionation, and 12 C and 13 C diffusion coefficient values were used from other studies to complete the organic and inorganic carbon flux fraction calculations. The Lucky Hill site with the higher soil carbonates had soil CO 2 flux that was almost always dominated from inorganic sources (Figures 2 & 3). There seemed to be a fairly continuous flux of CO 2 from the carbonates in the soil with the organic fluxes influencing the fraction in response to precipitation events and the fall, winter, and spring decomposition of soil organic carbon. In 2003 there was an increase in organic CO 2 flux in response to the start of the summer rains. In 2004 this response did not occur as the summer precipitation events were all small and soil moisture would be limited for decomposition of organic carbon (Figure 3). In both years there was soil moisture at the end of the summer growing season to start the organic carbon decomposition in the fall, increasing the organic fraction of CO 2 flux. The Kendall site with higher soil organic and less inorganic carbon had a dominate soil CO 2 flux from the soil organic source (Figures 4 & 5). In 2003 during the very dry period at the end of the winter and start of the summer precipitation, the organic and inorganic fluxes became somewhat equal from a high organic and low inorganic fraction. The pattern repeated itself in 2004 to a lesser extent as it was not as dry. This pattern was thought to occur from the reduction in soil organic carbon decomposition and the precipitation of carbonates in the soil with the release of CO 2 from inorganic sources during the drying of the soil profile. The summer rains provided soil moisture for increased organic CO 2 flux and this continued into the fall with still more organic CO 2 flux fraction in both years. 0 0.2 0.4 0.6 0.8 1 Carbon Flux Fraction 0 5 10 15 20 25 Precipitation mm/d 0 50 100 150 200 250 300 350 Day of Year Figure 2. Lucky Hill 2003 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation. 0 0.2 0.4 0.6 0.8 1 Carbon Flux Fraction 0 5 10 15 20 25 Precipitation mm/d 0 50 100 150 200 250 300 350 Day of Year Figure 3. Lucky Hill 2004 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation. 0 0.2 0.4 0.6 0.8 1 Carbon Flux Fraction 0 5 10 15 20 25 Precipitation mm/d 0 50 100 150 200 250 300 350 Day of Year Figure 4. Kendall 2003 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation. 0 0.2 0.4 0.6 0.8 1 Carbon Flux Fraction 0 5 10 15 20 25 Precipitation mm/d 0 50 100 150 200 250 300 350 Day of Year Figure 5. Kendall 2004 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation. REFERENCES Craig, H. 1953. The geochemistry of the stable carbon isotopes. Geochimica et Cosmochimica Acta. 3:53-92. Emerich, K., D.H. Ehhalt, and J.C. Vogel. 1970. Carbon isotope fractionation during the precipitation of calcium carbonate. Earth and Planetary Science Letters. 8:363-371. Rabenhorst, M.C., L.P. Wilding, and L.T. West. 1984. Identification of pedogenic carbonates using stable carbon isotope and microfabric analysis. Soil Science Society of America Journal 48:125-132.
Transcript of SEPARATION OF SOIL ORGANIC AND INORGANIC CO FLUXES · 2006-11-02 · SEPARATION OF SOIL ORGANIC AND...
SEPARATION OF SOIL ORGANIC AND INORGANIC CO2 FLUXESWilliam E. Emmerich and Dean A. Martens
USDA-ARS Southwest Watershed Research Center Tucson, AZ
INTRODUCTIONSoil organic carbon is the second largest pool in the world and inorganic the third. Arid and semiarid rangeland soils contain most of the inorganic carbon in the form of carbonates. The determination of organic and inorganic carbon fluxes from soils is important in understanding the global carbon budget. Organic and inorganic carbon have different isotopic signatures. The purpose of this study was to develop a procedure to separate organic andinorganic soil carbon fluxes into the fraction from each source using the isotope signatures and present two years of results from two rangeland sites with different amounts of organic and inorganic soil carbon.
METHODSA shrub (Lucky Hills) and grass (Kendall) site located on the Walnut Gulch Experimental Watershed in southeastern Arizona was selected for the study. The climate is semiarid with cool winters and warm summers. Mean annual precipitation is 356 mm and mean annual temperature is 17oC. The Lucky Hills surface A horizon (0-6cm) had 8 gkg-1 organic carbon and 21 gkg-1
inorganic. The Kendall surface (0-3cm) horizon had 11 gkg-1
organic carbon and 7 gkg-1 inorganic.To collect the CO2 flux from the soil a domed lid with a diameter of 0.54m was inserted 2 cm into the soil producing an enclosed volume of 0.02 m3. The CO2 was collected for 30-60 min. depending on time of the year. Then a sample was pump out and bubbled through a solution of NaOH and SrCl2 to precipitate the CO2 out as SrCO3 (Figure 1). During all sampling the CO2
concentration was measured with a Qubit model S151 infra red analyzer calibrated at the sites. The in port was fitted with a soda lime filter to remove incoming CO2. Immediately after the lids were put down a CO2 sample was collected at the same height as the lids to represent the CO2 that was initially trapped under the lids.The strontium carbonate and soil samples collected with depth were analyzed at UC Davis Stable Isotope Facility on a Europa1 Hydra 20/20 continuous flow isotope ratio mass spectrometer. The carbon isotope signature was expressed relative to the Pee Dee Belemnite (PDB) standard as:
�
13C(‰) = [[(13C/12C sample) –(13C/12C standardPDB)]/ (13C/12C standardPDB)]
�
1000.
FLUX SEPARATION PROCEDURESUpon inserting the collection lid into the soil a volume of air was trapped with a concentration of CO2 (Ca) and a isotope signature (
�
13Ca) for the CO2. Both CO2 concentration and
�
13Ca were measured at the start of CO2 collection. At sample collection time measured CO2 concentration (Cmix) was a mixture of the Ca plus the flux of CO2 (Cf ) from the soil. The isotope signature of this mixture (
�
13Cmix) was determined from the CO2 sample at collection time. The number of moles (n) of air under the lid at the start of sample collection was calculated from:
n = PV/RT Eq. 1.where P is the atmospheric pressure (determined as a function ofelevation), V the volume under the lid, R the universal gas constant (0.0821 l atm/mole °K), and T temperature (°K) measured at time of sample collection. The number of moles of
Figure 1. Equipment uses to collect CO2 soil flux samples.
air under the lid (Eq. 1) was multiplied by the Ca concentration (µmole/mole) at the start of sampling to give moles of CO2
represented by (b). The number of moles of air under the lid atsample collection time was assumed to be the same as at the start of collection and multiplied by the Cmix concentration at sample collection to give the moles of CO2 mixture represented by (a). The number of moles of CO2 under the lid at sampling time is:
a = b + c Eq. 2.where c is the number of moles of flux from the soil. A mass balance of the moles and the isotope signatures of the differentsources of CO2 can be represented by:
a
�
13Cmix = b
�
13Ca + c
�
13Cf Eq. 3.where
�
13Cf is the isotope signature of the CO2 flux from the soil. Combining Equations 2 and 3,
�
13Cf was calculated. The diffusion coefficients for 12CO2 and 13CO2 differ by 4.4 ‰ due to the differences in mass (Craig, 1953). Therefore the
�
13Cf and the isotope signature of the CO2 in the soil atmosphere (
�
13Csa) would not be expected to be the same. Cerling et al. (1991) used a similar CO2 soil surface collection procedure and collecting CO2
soil profile samples found that the measured differences in ‰and the theoretical due to diffusion coefficients were very close. The diffusion differences were used to calculate
�
13Csa as:�
13Csa =
�
13Cf + 4.4 ‰ Eq. 4. The
�
13Csa is composed of the isotopic signature of the inorganic soil carbon (
�
13CIC) (primarily CaCO3) through the carbonate equilibrium equations (Rabenhorst et al., 1984) and the isotopic signature of the soil organic matter (
�13COC) through the
decomposition of the soil organic matter. The
�
13Csa can be related to its components by:�
13Csa = d
�
13CIC + e
�
13COC Eq. 5.where d is the fraction of
�
13CIC in
�
13Csa and e is the fraction of �
13COC in
�
13Csa with:1 = d + e Eq. 6.
The mass difference in the 13C and 12C causes an equilibrium-based isotopic fractionation to occur between the soil atmosphere CO2 and the inorganic carbon (CaCO3) in the soil (Emerich. et al., 1970). The inorganic carbon fractionation is represented by:�
13Csa = �
13CIC – 10.17‰ Eq. 7.
The
�
13CIC – 10.17‰ also represents the isotopic contribution to the
�
13Csa from the inorganic carbon. The inorganic and organic isotopic contribution to the
�
13Csa can be represented as:(
�
13CIC – 10.17‰) � �
13Csa � �
13COC Eq. 8.with a long term equilibrium established between the pedogeniccarbonate and soil CO2 plus the fractionation. Equation 5 can be rewritten including the inorganic carbon fractionation as:�
13Csa = d (
�
13CIC – 10.17‰) + e
�
13COC Eq. 9.The fraction of CO2 soil flux from the inorganic carbon was calculated by combining Equations 6 and 9:
d =(
�
13Csa -
�
13COC)/(
�
13CIC – 10.17‰ -
�
13COC) Eq. 10. The fraction of CO2 soil flux from organic carbon was calculated from Equation 6:
e = 1 – d Eq.11.
RESULTSThe developed procedure was able to separate carbon dioxide flux from the soil into the fraction that originated from organic and inorganic sources in the soil. This involved determining the isotope signature of the carbon dioxide initially trapped in thecollection lid, the mixture of trapped and soil carbon dioxide flux, and organic and inorganic soil carbon. Inorganic carbon isotopefractionation, and 12C and 13C diffusion coefficient values were used from other studies to complete the organic and inorganic carbon flux fraction calculations.The Lucky Hill site with the higher soil carbonates had soil CO2
flux that was almost always dominated from inorganic sources (Figures 2 & 3). There seemed to be a fairly continuous flux ofCO2 from the carbonates in the soil with the organic fluxes influencing the fraction in response to precipitation events andthe fall, winter, and spring decomposition of soil organic carbon. In 2003 there was an increase in organic CO2 flux in response to the start of the summer rains. In 2004 this response did not occur as the summer precipitation events were all small and soilmoisture would be limited for decomposition of organic carbon (Figure 3). In both years there was soil moisture at the end of the summer growing season to start the organic carbon decomposition in the fall, increasing the organic fraction of CO2
flux.The Kendall site with higher soil organic and less inorganic carbon had a dominate soil CO2 flux from the soil organic source (Figures 4 & 5). In 2003 during the very dry period at the end of the winter and start of the summer precipitation, the organic and inorganic fluxes became somewhat equal from a high organic and low inorganic fraction. The pattern repeated itself in 2004 to a lesser extent as it was not as dry. This pattern was thought to occur from the reduction in soil organic carbon decomposition and the precipitation of carbonates in the soil with the release of CO2 from inorganic sources during the drying of the soil profile. The summer rains provided soil moisture for increased organic CO2 flux and this continued into the fall with still more organic CO2 flux fraction in both years.
0
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Figure 2. Lucky Hill 2003 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation.
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Figure 3. Lucky Hill 2004 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation.
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Figure 4. Kendall 2003 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation.
0
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0 50 100 150 200 250 300 350Day of Year
Figure 5. Kendall 2004 fraction of soil carbon flux from organic ( ) and inorganic ( ) sources and daily precipitation.
REFERENCESCraig, H. 1953. The geochemistry of the stable carbon isotopes.
Geochimica et Cosmochimica Acta. 3:53-92.Emerich, K., D.H. Ehhalt, and J.C. Vogel. 1970. Carbon isotope fractionation during the
precipitation of calcium carbonate. Earth and Planetary Science Letters. 8:363-371.Rabenhorst, M.C., L.P. Wilding, and L.T. West. 1984. Identification of pedogenic
carbonates using stable carbon isotope and microfabric analysis. Soil Science Society of America Journal 48:125-132.
