CARBON ISOTOPES AND CARBON TURNOVER IN COTTON AND WHEAT FACE EXPERIMENTS

The Maricopa cotton and wheat FACE (free-air CO2 enrichment) experimen ts offer propitious opportunity to quantify carbon turnover. The comme rcial CO2 (delta(13)C approximate to-37 parts per thousand) used to el evate CO2 concentration in field plots provided a strongly C-12-deplet ed tracer. Soil CO2 and delta(13)C of soil organic carbon (SOC) in CO2 -enriched and Control plots were measured between the final cotton FAC E project (October 1991) and the end of the second wheat experiment (J une 1994). The initial C-13-depletion in SOC of cotton FACE plots (mea sured 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 t he experiment ended. A similar depletion was observed in soil CO2 evol ved in the same plots, indicating ongoing decomposition of the new SOC . The SOC delta(13)C of wheat plots before and after two growing seaso ns showed increasing C-13-depletion in FACE relative to Control. Isoto pic mass balance was consistent with 5-6% new carbon input from the tw o wheat crops. This is lower than the 12-13% calculated for FACE cotto n 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 whea t.