Elevated atmospheric carbon dioxide impacts carbon dynamics in a C(4)-sorghum-soil agroecosystem: An application of stable carbon isotopes (delta carbon-13) in tracing the fate of carbon in the atmosphere-plant-soil ecosystem

Although a strong inter-dependence exists between atmospheric carbon dioxide (CO₂) and the terrestrial carbon (C) cycle, the response of plant-soil ecosystems to the rapid increase in atmospheric CO₂ is not well understood. My dissertation research focused on the impacts of elevated CO₂ on the carbon dynamics of plant-soil ecosystems, which were a major part of the overall C₄-sorghum Free-Air CO₂ Enrichment (FACE) experiment conducted by the University of Arizona and USDA at the Maricopa Agriculture Center, Arizona, USA, in 1998 and 1999. In the experiment, sorghum (Sorghum bicolor (L) Moench) crop was exposed to elevated CO₂ (“FACE”: ca. 560 µmol mol⁻¹) and ambient CO₂ (“Control”: ca. 360 µmol mol⁻¹) interacting with well-watered and water-stressed treatments. The results from my study showed that the seasonal mean soil respiration rate measured in elevated CO₂ plots over two growing seasons was 3.3 µmol m⁻²s⁻¹, i.e., 12.7% higher than the 2.9 µmol m⁻²s⁻¹in ambient CO₂ plots. The increased respiration mainly resulted from the stimulated root respiration under elevated CO₂, which increased 36.1% compared to that under ambient CO₂. Measured changes in sorghum residue biochemistry caused by CO₂ were detected, with decrease of amino acids and hemicellulose carbohydrates by 7% and 8%, respectively, and increase of cellulose carbohydrates and lignin by 49% and 5%, respectively. Phenolics were only significantly higher in FACE roots. The C:N ratio of sorghum tissues was not affected by elevated CO₂, but was substantially lower under water stress. The laboratory incubation showed that an average of 7.3% significantly less respired CO₂ was released from the FACE-tissue-amended soil than the Control-tissues-amended soil over the full 79-d incubation period. Non-lignin phenolics (r² = 0.93, p = 0.002), and lignin (r² = 0.89, p = 0.004) were found to be the most important factors related to the sorghum tissue decomposition. Highly stable residues of FACE sorghum input to the soil resulted in the increase of the recalcitrant C pool and the decrease of the labile C pool. As a result, mean residence time of SOC in FACE field plot increased compared to that in Control plot, suggesting that the SOC under elevated CO₂ was more stable against decomposition.