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

Abstract

Summary 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 mol−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 SOC 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.