Carbon isotopes and carbon turnover in cotton and wheat FACE experiments

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The Maricopa cotton and wheat FACE (free-air CO2 enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO2 (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW% baaSqabeaacaaIXaGaaG4maaaakiaaboeacqGHijYUcqGHsislcaaI% ZaGaaG4naiaacwcaliaad+gaaaa!3FCB!\[\delta ^{13} {\text{C}} \approx - 37\% o\]) used to elevate CO2 concentration in field plots provided a strongly 13C-depleted tracer. Soil CO2 and δ 13C 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 δ 13C 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 δ 13C 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.