Infodoc : Sciences, Techniques et Culture

CO2 concentrations ([CO2]), as well as carbon and oxygen isotope ratios (δ13C, δ18O) were measured within alfalfa (C3) and corn (C4) crop canopies (leaf area indices of 4.6 and 2.5, respectively). Daily fluctuations were observed within the canopy and extended into the canopy boundary layer (at heights 2 to 3 times higher than the maximum plant height). Photosynthetic demand for canopy CO2 exceeded soil respiration to such an extent that daytime [CO2] values were depleted 15 to 50 ppm below tropospheric values; δ13C values of canopy air reached a maximum of 3‰ heavier than the tropospheric baseline values. Highly significant relationships were observed between δ13C and δ18O ratios of canopy air in both crop canopies. Leaf carbon isotope discrimination was significantly different betwe[...]

CO2 concentrations ([CO2]), as well as carbon and oxygen isotope ratios (δ13C, δ18O) were measured within alfalfa (C3) and corn (C4) crop canopies (leaf area indices of 4.6 and 2.5, respectively). Daily fluctuations were observed within the canopy and extended into the canopy boundary layer (at heights 2 to 3 times higher than the maximum plant height). Photosynthetic demand for canopy CO2 exceeded soil respiration to such an extent that daytime [CO2] values were depleted 15 to 50 ppm below tropospheric values; δ13C values of canopy air reached a maximum of 3‰ heavier than the tropospheric baseline values. Highly significant relationships were observed between δ13C and δ18O ratios of canopy air in both crop canopies. Leaf carbon isotope discrimination was significantly different between species, 20‰ (alfalfa) vs. 4‰ (corn). However, the relationships between 1/[CO2] and δ13C, as well as 1/[CO2] and δ18O of canopy air did not differ between the two crop species. Thus, ecosystem respiration had an average δ13C ratio of −21.6‰ and a δ18O ratio of 29‰. The δ13C values of soil-respired CO2 were similar in both C3 and C4 crop stands (approximately −22.6‰). Ecosystem-level carbon isotope discrimination (Δe) estimates were indistinguishable between both crops (13.8‰ for alfalfa, and 13.2‰ for corn). Thus, the Δe estimates, as well as the δ13C values of soil organic carbon and soil-respired CO2 integrate 13C contributions from the current standing plant cover, as well as from crops of previous years in this crop rotation system. Furthermore, this study clearly indicated that the carbon isotope ratios of carbon fixed and carbon released were not near the equilibrium values expected for the current crop at each site. The implications of this isotopic disequilibrium of a crop rotation agricultural system are discussed with respect to scaling canopy-level observations to global models for identifying C sinks.