Soil Organic Carbon Isotope Tracing in Sorghum under Ambient CO2 and Free-Air CO2 Enrichment (FACE)

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 δ¹³C 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‰ δ¹³C 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 plants to 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.