Carbon isotopes and carbon turnover in cotton and wheat FACE experiments

Abstract

The Maricopa cotton and wheat FACE (free-air CO2 enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO2 (6 13C ~ -37 %o) used to elevate CO2 concentration in field plots provided a strongly laC-depleted tracer. Soil CO2 and 613C 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 ~I3C between FACE and Control plots) persisted at the same level (1.9 %0) 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 dl3C 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.i FAX

Changes in stable isotopic signatures of soil nitrogen and carbon during 40 years of forest development

“…The similar soil d 13 C and d 15 N values at all depths early in this forest's development reflect the historical agricultural use of this land. Soil supporting cotton frequently exhibits d 13 C values between À23.9 and À21& (Conteh et al 1998;Leavitt et al 1996). When planted with cotton, these soils were regularly cultivated and well mixed throughout the plow layer (presumed to be up to 20-cm depth), with crop residues incorporated into the surface horizons; data from deeper horizons reflect decomposing root biomass and illuviated material.…”

Global food insecurity. Treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields

“…However, analysis of isotopic composition across FACE plots shows a remarkable uniformity. This suggests that although transient gradients occur, when averaged over growing seasons, these gradients are not detectable (Leavitt et al 1996).…”

Photosynthesis in a CO2-Rich Atmosphere

“…The center of a plot may be close to the target [CO 2 ], while the upwind side may be above the target and the downwind side below the target. However, isotopic analysis of the carbon composition of vegetation exposed to 13 C and 14 C showed remarkable uniformity, suggesting that over growing seasons, dilution gradients in [CO 2 ] are not significant (Leavitt et al, 1996). A second potential disadvantage is that fumigation is compromised during still conditions.…”