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Title: STABLE-CARBON ISOTOPES AND SOIL ORGANIC CARBON IN THE 1996 AND 1997 FACE WHEAT EXPERIMENTS

Author
item LEAVITT, S - UNIV OF ARIZONA
item PENDALL, E - UNIV OF ARIZONA
item PAUL, E - MICHIGAN STATE UNIV
item BROOKS, T - UNIV OF ARIZONA
item Kimball, Bruce
item Pinter Jr, Paul

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2001
Publication Date: 8/15/2001
Citation: Leavitt, S.W., Pendall, E., Paul, E.A., Brooks, T.J., Kimball, B.A., Pinter Jr, P.J. 2001. Stable-carbon isotopes and soil organic carbon in the 1996 and 1997 face wheat experiments. New Phytologist. 150(2):305-314.

Interpretive Summary: The carbon dioxide concentration (CO2) in the Earth's atmosphere is increasing, and consequent global warming and changing precipitation patterns have been predicted. However, higher levels of CO2 are also known to directly stimulate the growth of plants, and because such larger plants would have more roots and residue, there is a potential for storage of more carbon in the soil. Therefore, as part of free-air CO2 enrichment (FACE) experiments, measurements of the effects of elevated CO2 on the growth of wheat were made, and in addition, estimates of soil carbon storage were made both directly and by determining the ratios of 13C to 12C isotopes in the plants and in the soil. The altered isotopic ratios formed a tracer for new carbon added to the soil. The results indicated that after two wheat growing seasons, about 5% of the carbon in the soil was new from the wheat crops. However, the amounts were similar in both FACE and Control plots, suggesting the elevated CO2 did not significantly stimulate more soil C storage. The precision of the measurements was less than desired, however, because conditions associated with the control tracer were somewhat different from FACE and the soil variability was high. This research will aid in predicting the future levels of atmospheric CO2 and the extent to which the storage of carbon in the soil will mitigate the rate of rise, thereby slowing global change and benefitting many of earth's ecosystems and most of mankind.

Technical Abstract: The 1996 and 1997 wheat Free-Air CO2 Enrichment (FACE) CO2xN experiments offered opportunities to enlist isotopic tracers to detect entry of fresh carbon into the soil organic carbon (SOC) pools. The commercial CO2 used to enrich FACE plots to ambient+200 umol mol-1 had d13C of ca. -40 o/oo, resulting in d13C of ca. -18 to -19 o/oo in FACE plot air CO2. This was incorporated into labeled wheat plants ca. 12 o/oo 13C-depleted relative to ambient plants and ca. 17 o/oo depleted relative to the SOC of native soils. SOC content and d13C of SOC were analyzed for 3 depths (0-15, 15-30, 30-60 cm) for all treatments before and after each growing season. Under high-N, isotopic mass balance indicates ca. 5% new C in the 0-15 cm depth SOC. Less definitive calculations suggest a similar new C fraction in 15-30 cm high-N and 30-60 cm low-N FACE treatments. To quantify activity in the control (Blower) plots, exotic soils enriched in 13C (-17 to 20 o/oo from development under C4 plants) were placed into subplots in FACE and Blower high-N reps. 1 and 2. Substantial SOC losses (perhaps promoted by first-time cultivation of soils not in equilibrium with Maricopa farm conditions) and SOC d13C increases during the experiment, however, made it difficult to interpret isotopic changes. A 13C02 gas tracer pulsed to subplots in Blower reps. 1 and 2 imparted a labeling of the wheat 8-9 o/oo 13C-enriched relative to the native soil SOC. Consequent SOC d13C increases were consistent with ca. 5% new C fraction in Blower-high N. This suggests similar SOC changes under FACE and ambient conditions, but the pulsed signal in small subplots of 2 reps. may limit extrapolation to full plot scale.