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 (delta(13)C approximate to-37 parts per thousand) used to elevate CO2 concentration in field plots provided a strongly C-12-depleted tracer. Soil CO2 and delta(13)C 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 C-13-depletion in SOC of cotton FACE plots (measured by the difference in delta(13)C between FACE and Control plots) persisted at the same level (1.9 parts per thousand) 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 delta(13)C of wheat plots before and after two growing seasons showed increasing C-13-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.