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Soil Organic Carbon Isotope Tracing in Sorghum under Ambient CO 2 and Free-Air CO 2 Enrichment (FACE)

Author

Listed:
  • Steven W. Leavitt

    (Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA)

  • Li Cheng

    (Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA)

  • David G. Williams

    (Department of Botany, University of Wyoming, Laramie, WY 82071, USA)

  • Talbot Brooks

    (The Geospatial Center, Delta State University, Cleveland, MS 38732, USA)

  • Bruce A. Kimball

    (U.S. Department of Agriculture-Agricultural Research Service, U.S. Arid-Land Agricultural Research Center, Maricopa, AZ 85138, USA)

  • Paul J. Pinter

    (U.S. Department of Agriculture-Agricultural Research Service, Phoenix, AZ 85044, USA)

  • Gerard W. Wall

    (U.S. Department of Agriculture-Agricultural Research Service, U.S. Arid-Land Agricultural Research Center, Maricopa, AZ 85138, USA)

  • Michael J. Ottman

    (Department of Plant Sciences, University of Arizona, Tucson, AZ 85721, USA)

  • Allan D. Matthias

    (Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721, USA
    Deceased.)

  • Eldor A. Paul

    (Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA)

  • Thomas L. Thompson

    (School of Plant and Environmental Sciences, Virginia Tech, Blacksburg, VA 24061, USA)

  • Neal R. Adam

    (College of Science, Engineering and Technology, Grand Canyon University, Phoenix, AZ 85019, USA)

Abstract

As atmospheric carbon dioxide concentrations, [CO 2Air ], continue their uncontrolled rise, the capacity of soils to accumulate or retain carbon is uncertain. Free-air CO 2 enrichment (FACE) experiments have been conducted to better understand the plant, soil and ecosystem response to elevated [CO 2 ], frequently employing commercial CO 2 that imparts a distinct isotopic signal to the system for tracing carbon. We conducted a FACE experiment in 1998 and 1999, whereby sorghum (C 4 photosynthetic pathway) was grown in four replicates of four treatments using a split-strip plot design: (i) ambient CO 2 /ample water (365 μmol mol −1 , “Control–Wet”), (ii) ambient CO 2 /water stress (“Control–Dry”), (iii) CO 2 -enriched (560 μmol mol −1 , “FACE–Wet”), and (iv) CO 2 -enriched/water stressed (“FACE–Dry”). The stable-carbon isotope composition of the added CO 2 (in FACE treatments) was close to that of free atmosphere background values, so the subsequent similar 13 C-enriched carbon signal photosynthetically fixed by C 4 sorghum plants could be used to trace the fate of carbon in both FACE and control treatments. Measurement of soil organic carbon content (SOC (%) = g C/ g dry soil × 100%) and δ 13 C at three depths (0–15, 15–30, and 30–60 cm) were made on soils from the beginning and end of the two experimental growing seasons. A progressive ca. 0.5‰–1.0‰ δ 13 C increase in the upper soil SOC in all treatments over the course of the experiment indicated common entry of new sorghum carbon into the SOC pools. The 0–15 cm SOC in FACE treatments was 13 C-enriched relative to the Control by ca. 1‰, and according to isotopic mass balance, the fraction of the new sorghum-derived SOC in the Control–Wet treatment at the end of the second season was 8.4%, 14.2% in FACE–Wet, 6.5% in Control–Dry, and 14.2% in FACE–Dry. The net SOC enhancement resulting from CO 2 enrichment was therefore 5.8% (or 2.9% y −1 of experiment) under ample water and 7.7% (3.8% y −1 of experiment) under limited water, which matches the pattern of greater aboveground biomass increase with elevated [CO 2Air ] under the Dry treatment, but no parallel isotopic shifts were found in deeper soils. However, these increased fractions of new carbon in SOC at the end of the experiment do not necessarily mean an increase in total SOC content, because gravimetric measurements of SOC did not reveal a significant increase under elevated [CO 2Air ], at least within the limits of SOC-content error bars. Thus, new carbon gains might be offset by pre-experiment carbon losses. The results demonstrate successful isotopic tracing of carbon from plants to soils in this sorghum FACE experiment showing differences between FACE and Control treatments, which suggest more dynamic cycling of SOC under elevated [CO 2Air ] than in the Control treatment.

Suggested Citation

  • Steven W. Leavitt & Li Cheng & David G. Williams & Talbot Brooks & Bruce A. Kimball & Paul J. Pinter & Gerard W. Wall & Michael J. Ottman & Allan D. Matthias & Eldor A. Paul & Thomas L. Thompson & Nea, 2022. "Soil Organic Carbon Isotope Tracing in Sorghum under Ambient CO 2 and Free-Air CO 2 Enrichment (FACE)," Land, MDPI, vol. 11(2), pages 1-15, February.
  • Handle: RePEc:gam:jlands:v:11:y:2022:i:2:p:309-:d:753166
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    References listed on IDEAS

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    1. Bruce A. Hungate & Elisabeth A. Holland & Robert B. Jackson & F. Stuart Chapin & Harold A. Mooney & Christopher B. Field, 1997. "The fate of carbon in grasslands under carbon dioxide enrichment," Nature, Nature, vol. 388(6642), pages 576-579, August.
    2. William H. Schlesinger & John Lichter, 2001. "Limited carbon storage in soil and litter of experimental forest plots under increased atmospheric CO2," Nature, Nature, vol. 411(6836), pages 466-469, May.
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