Measurement of Carbon Dioxide Concentrations at … › crops › responses › 2006am ›...
1
Measurement of Carbon Dioxide Concentrations at Various Soil Depths Comparing Sensor and Gas Chromatography Data D.T. Lee 1 , Y. Zhang 1 , C.E. Clapp 2 , J.A.E Molina 1 , D. Chen 3 and A.J. Palazzo 4 (1)Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, (2)USDA-ARS & Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Cir, St. Paul, MN 55108, (3)California Department of Water Resources, P.O. Box 942836, Sacramento, CA 94236, (4)USACOE, CRREL, 72 Lyme Rd., Hanover, NH 03755 Gas Chromatography Analysis: Gas samples were analyzed using a GC coupled with a headspace autosampler (Teledyne Tekmar, Mason, OH) within a week of collection. Carbon dioxide was determined by the thermal conductivity detector (TCD) in the GC. The system was calibrated using analytical grade standards (Scott Specialty Gases, Plumsteadville, PA) (Venterea et al., 2002). In a parallel procedure to field sampling, 12 mL of standard CO 2 gas were injected into 9 mL glass vials. Soil CO 2 concentrations were calculated in parts per million (µmol L -1 ) of gas sample from the soil. Field Gas Sampling: Soil gas samples were collected by inserting the needle of a 12-mL polypropylene syringe through a septum cap and slowly withdrawing 12 mL. Three volumes of the gas tube were purged prior to each gas sample. Samples were immediately transferred to 9-mL glass vials sealed with butyl rubber septa (Alltech, Deerfield, IL). Un-evacuated vials, containing ambient air, were used for all gas sampling. Soil gas samples were injected into the vials and the concentrations adjusted for dilution with ambient air concentrations in the vials. Gas samples were generally taken between 900 and 1500 h local time when soil temperatures were expected to be close to their daily mean values. Gas Tube Preparation and Insertion: Soil gas samples were taken by Teflon tubes 0.16 cm I.D. (Fig 1b) adjacent to the CO 2 probes. Prior to field installation, each tube was cut to a specific length and punctured at the soil-contacting end (2 cm) to allow gas diffusion. A steel rod was inserted into the tube when positioning it to the targeted soil depth for structural support. The number and depth of tubing insertion matched adjacent buried CO 2 sensors. The gas samplings began after a rain event to assure proper soil to tube wall contact. CO 2 Probes: Carbon dioxide concentrations were measured in the RA and RE plots with CO 2 transmitters (model GMT220, Vaisala, Inc., Woburn, MA) (Fig. 1a). The voltage output sent from CO 2 transmitters, TDR waveguides, and thermistors were recorded by a datalogger (model CR10X, Campbell Scientific, Inc., Logan, UT), at 15 min intervals. Each sensor was sealed inside a PVC pipe with holes on the side covered by a gas permeable membrane. Six transmitters, TDR waveguides, thermistors, and Teflon tubes (Fig. 1b) were installed at depths of 10 to 60 cm with 10 cm spacing inside both RA and RE areas. Site: The experiment was carried out in a 6 x 9 m corn plot under a long-term field study started in 1980 at the University of Minnesota Research and Outreach Center, Rosemount, MN. Soil at the site was a Waukegan silt loam (Typic Hapludoll) with an underlying layer of gravel at 0.9 m. Two 0.75 by 1.25 m areas (Fig. 1b and 2) in a corn plot were used for RA (root active, in the row) or RE (root excluded, isolated by burying two layers of plastic film with a layer of aluminum foil in between to a depth of 0.9 m). Corn was planted on Julian day 139. Urea was broadcast at a rate of 150 kg N ha -1 one week after planting. Row spacing was 75 cm and planting density was 80,000 ha -1 . After harvesting corn grain, stover was removed and the soil was rototilled to a depth of 15 cm. Introduction: Measurement of field respiration is critical in studies of soil C and N dynamics and greenhouse gas emissions. CO 2 concentrations in soil can vary rapidly and widely with regard to factors such as tillage, fertilization, and rainfall, and are subject to both diurnal and seasonal variations. The objective of this study was to compare CO 2 concentrations measured by sensors with soil gas samples collected at the same depths in the rhizosphere and analyzed by gas chromatography (GC) under two treatments, with and without corn (Zea mays L.) roots, during the growing season. RE RA Fig. 2. Layout of RE and RA areas. Fig. 4. Soil temperature for RA and RE. RE RA o C o C Results and Discussion: Carbon dioxide concentrations were determined in the rhizosphere, and in root excluded soil at 6 depths by both sensor and GC methods. Both sensor and GC data (Fig 3.) indicated high soil CO 2 concentrations coincided with high soil temperatures (Fig 4.) during the corn maturation period. Our previous study using GC analysis (Fig 5.) confirmed that CO 2 concentrations in the bulk soil, surrounding the sensors, are well represented by those of soil gas inside the PVC pipe where the sensor is housed. As observed in the field experiment, sensor CO 2 measurements are highly correlated to data collected through GC analysis for majority of soil depths (correlation R 2 values >0.988) (Fig 6a.). However, discrepancy between CO 2 measurements by sensor and GC were observed at certain soil depths. Data from RE 20 cm (Fig 6b) has shown high correlation between the two examined methods, but the relationship has a slope of 0.658. Even though the CO 2 concentration ratio of sensor over GC was lower (Fig. 6c), the ratio consistency enables linear calibration for correction. In general, a concern should be placed on disagreements between the two methods when computing CO 2 production based on sensor data. The Sensor method offers high temporal resolution but it requires periodic monitoring and regular calibration against GC data. Summary: The sensor method provides valuable high temporal resolution that has to be used in conjunction with the GC method. Carbon dioxide measurements by sensors represent those of adjacent soils but require linear calibration using CO 2 measurements by GC. RA RE µmol L-1 µmol L-1 Fig. 3. Sensor CO2 measurements (a) vs. GC CO2 (b) for RA and RE. (a) (b) RA RE µmol L-1 µmol L-1 Fig. 6. Comparison of CO2 concentration measured in field by sensor and GC methods for RA and RE. References: Chen, D., J.A.E. Molina, C.E. Clapp, R.T. Venterea, and A.J. Palazzo. 2005. Corn root influence on automated measurement of soil carbon dioxide concentrations. Soil Sci. 170:779-787. Venterea, R.T., M. Burger, and K.A. Spokas. 2005. Nitrogen oxide and methane emissions under varying tillage and fertilizer management. J. Environ. Qual. 34:1467-1477. Fig. 5. CO2 concentrations by GC inside sensor and its surrounding soil (Chen et al., 2005). Gas Tube (b) (a) Fig. 1. (a) CO2 transmitter; (b) Layout of buried sensors and gas tubes. RE RA Acknowledgment: This study was funded partially by the United States-Israel Binational Agricultural Research and Development Fund (IS-3517-04R). (b) R2=0.83 Slope=0.658 (c ) R2=0.956 Slope=0.597 (a) R2=0.988 Slope=0.1.0414 Julian Day (2006) 414 RA 60cm RE 20cm RA 40cm
Transcript of Measurement of Carbon Dioxide Concentrations at … › crops › responses › 2006am ›...
Measurement of Carbon Dioxide Concentrations at Various Soil Depths Comparing Sensor and Gas Chromatography DataD.T. Lee1, Y. Zhang1, C.E. Clapp2, J.A.E Molina1, D. Chen3 and A.J. Palazzo4
(1)Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, (2)USDA-ARS & Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Cir, St. Paul, MN 55108,(3)California Department of Water Resources, P.O. Box 942836, Sacramento, CA 94236, (4)USACOE, CRREL, 72 Lyme Rd., Hanover, NH 03755
Gas Chromatography Analysis:
Gas samples were analyzed using a GC coupled with a headspace autosampler (Teledyne Tekmar, Mason, OH)within a week of collection. Carbon dioxide was determined by the thermal conductivity detector (TCD) in the GC.The system was calibrated using analytical grade standards (Scott Specialty Gases, Plumsteadville, PA) (Venterea etal., 2002). In a parallel procedure to field sampling, 12 mL of standard CO2 gas were injected into 9 mL glass vials.Soil CO2 concentrations were calculated in parts per million (µmol L-1) of gas sample from the soil.
Field Gas Sampling:
Soil gas samples were collected by inserting the needle of a 12-mL polypropylene syringe through a septum cap andslowly withdrawing 12 mL. Three volumes of the gas tube were purged prior to each gas sample. Samples wereimmediately transferred to 9-mL glass vials sealed with butyl rubber septa (Alltech, Deerfield, IL). Un-evacuatedvials, containing ambient air, were used for all gas sampling. Soil gas samples were injected into the vials and theconcentrations adjusted for dilution with ambient air concentrations in the vials. Gas samples were generally takenbetween 900 and 1500 h local time when soil temperatures were expected to be close to their daily mean values.
Gas Tube Preparation and Insertion:
Soil gas samples were taken by Teflon tubes 0.16 cmI.D. (Fig 1b) adjacent to the CO2 probes. Prior to fieldinstallation, each tube was cut to a specific length andpunctured at the soil-contacting end (2 cm) to allowgas diffusion. A steel rod was inserted into the tubewhen positioning it to the targeted soil depth forstructural support. The number and depth of tubinginsertion matched adjacent buried CO2 sensors. Thegas samplings began after a rain event to assureproper soil to tube wall contact.
CO2 Probes:
Carbon dioxide concentrations were measured in theRA and RE plots with CO2 transmitters (modelGMT220, Vaisala, Inc., Woburn, MA) (Fig. 1a). Thevoltage output sent from CO2 transmitters, TDRwaveguides, and thermistors were recorded by adatalogger (model CR10X, Campbell Scientific, Inc.,Logan, UT), at 15 min intervals. Each sensor wassealed inside a PVC pipe with holes on the sidecovered by a gas permeable membrane. Sixtransmitters, TDR waveguides, thermistors, andTeflon tubes (Fig. 1b) were installed at depths of 10 to60 cm with 10 cm spacing inside both RA and REareas.
Site:
The experiment was carried out in a 6 x 9 m corn plot under a long-term fieldstudy started in 1980 at the University of Minnesota Research and OutreachCenter, Rosemount, MN. Soil at the site was a Waukegan silt loam (TypicHapludoll) with an underlying layer of gravel at 0.9 m. Two 0.75 by 1.25 mareas (Fig. 1b and 2) in a corn plot were used for RA (root active, in the row) orRE (root excluded, isolated by burying two layers of plastic film with a layer ofaluminum foil in between to a depth of 0.9 m). Corn was planted on Julian day139. Urea was broadcast at a rate of 150 kg N ha-1 one week after planting.Row spacing was 75 cm and planting density was 80,000 ha-1. After harvestingcorn grain, stover was removed and the soil was rototilled to a depth of 15 cm.
Introduction:
Measurement of field respiration is critical in studies of soil C and N dynamicsand greenhouse gas emissions. CO2 concentrations in soil can vary rapidly andwidely with regard to factors such as tillage, fertilization, and rainfall, and aresubject to both diurnal and seasonal variations. The objective of this study was tocompare CO2 concentrations measured by sensors with soil gas samplescollected at the same depths in the rhizosphere and analyzed by gaschromatography (GC) under two treatments, with and without corn (Zea maysL.) roots, during the growing season.
RE RA
Fig. 2. Layout of RE and RA areas.
Fig. 4. Soil temperature for RA and RE.
RE
RA
oC
oC
Results and Discussion:
Carbon dioxide concentrations were determined in the rhizosphere, and in root excluded soil at 6 depths by both sensor and GCmethods. Both sensor and GC data (Fig 3.) indicated high soil CO2 concentrations coincided with high soil temperatures (Fig 4.)during the corn maturation period. Our previous study using GC analysis (Fig 5.) confirmed that CO2 concentrations in the bulk soil,surrounding the sensors, are well represented by those of soil gas inside the PVC pipe where the sensor is housed. As observed in thefield experiment, sensor CO2 measurements are highly correlated to data collected through GC analysis for majority of soil depths(correlation R2 values >0.988) (Fig 6a.). However, discrepancy between CO2 measurements by sensor and GC were observed atcertain soil depths. Data from RE 20 cm (Fig 6b) has shown high correlation between the two examined methods, but the relationshiphas a slope of 0.658. Even though the CO2 concentration ratio of sensor over GC was lower (Fig. 6c), the ratio consistency enableslinear calibration for correction. In general, a concern should be placed on disagreements between the two methods when computingCO2 production based on sensor data. The Sensor method offers high temporal resolution but it requires periodic monitoring andregular calibration against GC data.
Summary:
The sensor method provides valuable high temporal resolution that has to be used in conjunction with the GC method. Carbondioxide measurements by sensors represent those of adjacent soils but require linear calibration using CO2 measurements by GC.
RA
RE
µmol L-1
µmol L-1
Fig. 3. Sensor CO2 measurements (a) vs. GC CO2 (b) for RA and RE.
(a) (b)
RA
RE
µmol L-1
µmol L-1
Fig. 6. Comparison of CO2 concentration measured in field bysensor and GC methods for RA and RE.
References:Chen, D., J.A.E. Molina, C.E. Clapp, R.T. Venterea, and A.J. Palazzo. 2005. Corn root influence on automated measurement
of soil carbon dioxide concentrations. Soil Sci. 170:779-787.Venterea, R.T., M. Burger, and K.A. Spokas. 2005. Nitrogen oxide and methane emissions under varying tillage and fertilizer
management. J. Environ. Qual. 34:1467-1477.
Fig. 5. CO2 concentrations by GC inside sensor and its surrounding soil (Chen et al., 2005).
Gas Tube
(b)(a)
Fig. 1. (a) CO2 transmitter; (b) Layout of buried sensors and gas tubes.
RE RA
Acknowledgment: This study was funded partially by the United States-Israel Binational Agricultural Research and Development Fund (IS-3517-04R).
(b)
R2=0.83Slope=0.658
(c ) R2=0.956Slope=0.597
(a)
R2=0.988Slope=0.1.0414
Julian Day (2006)
414
RA 60cm
RE 20cm
RA 40cm
