Abstract
CO2 concentrations ([CO2]), as well as carbon and oxygen isotope ratios (delta(13)C, delta(18)O) 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 CO2 exceeded soil respiration to such an extent that daytime [CO2] values were depleted 15 to 50 ppm below tropospheric values; delta(13)C values of canopy air reached a maximum of 3 parts per thousand heavier than the tropospheric baseline values. Highly significant relationships were observed between delta(13)C and delta(18)O ratios of canopy air in both crop canopies. Leaf carbon isotope discrimination was significantly different between species, 20 parts per thousand (alfalfa) vs. 4 parts per thousand (corn). However, the relationships between 1/[CO2] and delta(13)C, as well as 1/[CO2] and delta(18)O of canopy air did not differ between the two crop species. Thus, ecosystem respiration had an average delta(13)C ratio of -21.6 parts per thousand and a delta(18)O ratio of 29 parts per thousand. The delta(13)C values of soil-respired CO2 were similar in both C-3 and C-4 crop stands (approximately -22.6 parts per thousand). Ecosystem-level carbon isotope discrimination (Delta(e)) estimates were indistinguishable between both crops (13.8 parts per thousand for alfalfa, and 13.2 parts per thousand for corn). Thus, the Delta(e) estimates, as well as the delta(13)C values of soil organic carbon and soil-respired CO2 integrate C-13 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. (C) 1998 Elsevier Science B.V.
