Elevated Atmospheric CO2 Impacts Carbon Dynamics in a C4-Sorghum-Soil Agroecosystem---An Application of Stable Carbon Isotopes (d13C) in Tracing the Fate of Carbon in the Atmosphere-Plant-Soil Ecosystem

Abstract

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) Mőench) crop was exposed to elevated CO2 ("FACE": ca. 560 mmol mol-1) and ambient CO2 ("Control": ca. 360 mmol 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 mmol m-2 s-1, i.e., 12.7% higher than the 2.9 mmol m-2 s-1 in 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.