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 mu mol mol(-1)) resulted in C-13-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 (CO2)-C-13 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 (CO2)-C-13 tracer induced a SOC delta C-13 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 (CO2)-C-13-tracer result, however, is less reliable because of lower significance, fewer replicates and heterogeneous isotopic distribution within plants.