CO 2 concentration profiles, and carbon and oxygen isotopes in C 3 and C 4 crop canopies

Abstract

CO 2 concentrations ([CO 2]), as well as carbon and oxygen isotope ratios (δ 13C, δ 18O) were measured within alfalfa (C 3) and corn (C 4) 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 CO 2 exceeded soil respiration to such an extent that daytime [CO 2] values were depleted 15 to 50 ppm below tropospheric values; δ 13C values of canopy air reached a maximum of 3‰ heavier than the tropospheric baseline values. Highly significant relationships were observed between δ 13C and δ 18O 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/[CO 2] and δ 13C, as well as 1/[CO 2] and δ 18O of canopy air did not differ between the two crop species. Thus, ecosystem respiration had an average δ 13C ratio of −21.6‰ and a δ 18O ratio of 29‰. The δ 13C values of soil-respired CO 2 were similar in both C 3 and C 4 crop stands (approximately −22.6‰). Ecosystem-level carbon isotope discrimination ( Δ e) estimates were indistinguishable between both crops (13.8‰ for alfalfa, and 13.2‰ for corn). Thus, the Δ e estimates, as well as the δ 13C values of soil organic carbon and soil-respired CO 2 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.