Stable-carbon isotopes and soil organic carbon in wheat under CO₂ enrichment

• Stable-carbon isotopic tracers were enlisted in 1996 and 1997 wheat (Triticum aestivum) free-air CO₂ enrichment (FACE) experiments to detect entry of new C into soil organic carbon (SOC) pools. Any enhanced soil inputs might mitigate rising atmospheric CO₂. • The CO₂ used to enrich FACE plots (to ambient +190 μmol^-1 resulted in ¹³C-depleted wheat relative to ambient plants and the native SOC. To trace new C in control plots C₄-plant-derived exotic soils were placed into subplots in high-N FACE and control treatments, and a ¹³CO₂ gas tracer was pulsed to subplots in high-N control replicates. • Under high-N, isotopic mass balance showed 6% (P = 0.003) and 5% (P = 0.04) new C in 0-15-cm and 15-30-cm FACE SOC, respectively, after 2 yr. Results from the C₄-soil subplots were ambiguous, but the ¹³CO₂ tracer induced a sac δ¹³C increase (P = 0.08) at 15-30 cm in control-high N consistent with 6% new C. • We infer c, 3% year^-1 (30-40 g C m^-2 yr^-1) SOC turnover in surface soils at high-N under both ambient and elevated CO₂. The ¹³CO₂-tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants.