Soil organic carbon isotope tracing in sorghum under ambient CO2 and Free-Air CO2 Enrichment (FACE)

Authors: LEAVITT, S.W - University Of Arizona; CHENG, L - University Of Arizona; WILLIAMS, D.G - University Of Wyoming; BROOKS, T - Delta State University; Kimball, Bruce; PINTER JR, P.J - Retired ARS Employee; Wall, Gerard - Gary; OTTMAN, M.J - University Of Arizona; MATTHIAS, A - University Of Arizona; PAUL, E.A - Colorado State University; THOMPSON, T.L - Virginia Tech; ADAM, N.R - Grand Canyon University

Submitted to: Land
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2022
Publication Date: 2/18/2022
Citation: Leavitt, S., Cheng, L., Williams, D., Brooks, T., Kimball, B.A., Pinter Jr., P., Wall, G.W., Ottman, M., Matthias, A., Paul, E., Thompson, T., Adam, N. 2022. Soil organic carbon isotope tracing in sorghum under ambient CO2 and Free-Air CO2 Enrichment (FACE). Land. 11(2). Article 309. https://doi.org/10.3390/land11020309.
DOI: https://doi.org/10.3390/land11020309

Interpretive Summary: The carbon dioxide concentration (CO2) in the Earth’s atmosphere is increasing, and predictions of consequent global warming and changing precipitation patterns have been made. However, higher levels of CO2 are also known to directly stimulate plant growth leading to larger plants with more roots and residue. Therefore, elevated CO2 can potentially result in storage of more carbon in the soil. As a part of a free-air CO2 enrichment (FACE) experiments on sorghum, measurements of the carbon isotope ratios were used to elucidate short term net carbon accumulation in soil. The net new soil carbon enhancement resulting from FACE was 5.8% under ample water and 7.7% under limited water. However, at the same time there was a loss of old pre-experiment carbon of about 6%, so the net soil carbon gain was very small. Thus, soil carbon storage under sorghum likely will be little affected by the increasing atmospheric CO2 concentration.

Technical Abstract: As atmospheric carbon dioxide concentrations, [CO2Air], continue their uncontrolled rise, the capacity of soils to accumulate or retain carbon is uncertain. Free-air CO2 enrichment (FACE) experiments have been conducted to better understand the plant, soil and ecosystem response to elevated [CO2], frequently employing commercial CO2 that imparts a distinct isotopic signal to the system for tracing carbon. We conducted a FACE experiment in 1998 and 1999, whereby sorghum (C4 photosynthetic pathway) was grown in four replicates of four treatments using a split-strip plot design: (i) ambient CO2/ample water (365 µmol mol-1, “Control–Wet”), (ii) ambient CO2/water stress (“Control–Dry”), (iii) CO2-enriched (560 µmol mol-1, “FACE–Wet”), and (iv) CO2-enriched/water stressed (“FACE–Dry”). The stable-carbon isotope composition of the added CO2 (in FACE treatments) was close to that of free atmosphere background values, so the subsequent similar 13C-enriched carbon signal photosynthetically fixed by C4 sorghum plants could be used to trace the fate of carbon in both FACE and control treatments. Measurement of soil organic carbon content (SOC (%) = gC/gdry soil × 100%) and d13C 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‰ d13C 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 13C-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 CO2 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 [CO2Air] 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 [CO2Air], 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 plantst o soils in this sorghum FACE experiment showing differences between FACE and Control treatments, which suggest more dynamic cycling of SOC under elevated [CO2Air] than in the Control treatment.