Carbon isotopes and carbon turnover in cotton and wheat FACE experiments

The Maricopa cotton and wheat FACE (free-air CO₂ enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO₂ (δ ¹³C ≃ -37‰) used to elevate CO₂ concentration in field plots provided a strongly ¹³C-depleted tracer. Soil CO₂ and δ¹³C of soil organic carbon (SOC) in CO₂ -enriched and Control plots were measured between the final cotton FACE project (October 1991) and the end of the second wheat experiment (June 1994). The initial ¹³C-depletion in SOC of cotton FACE plots (measured by the difference in δ¹³C between FACE and Control plots) persisted at the same level (1.9‰) 1.5 years after the experiment ended. A similar depletion was observed in soil CO₂ evolved in the same plots, indicating ongoing decomposition of the new SOC. The SOC δ¹³C of wheat plots before and after two growing seasons showed increasing ¹³C-depletion in FACE relative to Control. Isotopic mass balance was consistent with 5-6% new carbon input from the two wheat crops. This is lower than the 12-13% calculated for FACE cotton and perhaps a consequence of the larger root system of cotton or the 3-year duration of the cotton experiments versus 2 years for the wheat.