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

Summary Stable‐carbon isotopic tracers were enlisted in 1996 and 1997 wheat ( Triticum aestivum ) free‐air CO 2 enrichment (FACE) experiments to detect entry of new C into soil organic carbon (SOC) pools. Any enhanced soil inputs might mitigate rising atmospheric CO 2 . The CO 2 used to enrich FACE plots (to ambient +190 µmol mol −1 ) resulted in 13 C‐depleted wheat relative to ambient plants and the native SOC. To trace new C in control plots C 4 ‐plant‐derived exotic soils were placed into subplots in high‐N FACE and control treatments, and a 13 CO 2 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 C 4 ‐soil subplots were ambiguous, but the 13 CO 2 tracer induced a SOC δ 13 C 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 CO 2 . The 13 CO 2 ‐tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants.