Final Report on "Rising CO2 and Long-term Carbon Storage in Terrestrial Ecosystems: An Empirical Carbon Budget Validation"

Abstract

funded through the TECO program. However, this project has a longer history because DOE also funded this study from its inception in 1985 through 1997. The original grant was focused on plant responses to elevated CO 2 in an intact ecosystem, while the latter grant was focused on belowground responses. Here we summarize the major findings across the 25 years this study has operated, and note that the experiment will continue to run through 2020 with NSF support. The major conclusions of the study to date are: Elevated CO 2 stimulated plant productivity in the C3 plant community by ~30% during the 25 year study. The magnitude of the increase in productivity varied interannually and was sometime absent altogether. There is some evidence of down-regulation at the ecosystem level across the 25 year record that may be due to interactions with other factors such as sea-level rise or long-term changes in N supply. Elevated CO 2 stimulated C 4 productivity by <10%, perhaps due to more efficient water use, but C 3 plants at elevated CO 2 did not displace C 4 plants as predicted. Increased primary production caused a general stimulation of microbial processes, but there were both increases and decreases in activity depending on the specific organisms considered. An increase in methanogenesis and methane emissions implies elevated CO 2 may amplify radiative forcing in the case of wetland ecosystems. Elevated CO 2 stimulated soil carbon sequestration in the form of an increase in elevation. The increase in elevation is 50-100% of the increase in net ecosystem production caused by elevated CO 2 (still under analysis). The increase in soil elevation suggests the elevated CO 2 may have a positive outcome for the ability of coastal wetlands to persist despite accelerated sea level rise. Crossing elevated CO 2 with elevated N causes the elevated CO 2 effect to diminish, with consequences for change in soil elevation. I. RationaleThe possibility that terrestrial ecosystems may be able to slow the rate of CO 2 rise is both intriguing and important to pursue as the world struggles to address the challenges of climatic change. When this work began in 1986, the pressing question was whether rising CO 2 would stimulate CO 2 assimilation and carbon sequestration in terrestrial ecosystems through direct effects on photosynthesis. Later we tackled the far more difficult question of whether this increase in CO 2 assimilation would ripple through the ecosystem carbon cycle to cause a longterm increase in carbon sequestration. As the DOE funding of this project ends, there are now thousands of studies that have demonstrated that elevated CO 2 elicits a sustained increase in photosynthesis (Ainsworth and Long, 2005), and we have evidence that a substantial portion of this carbon is sequestered as soil organic matter in our tidal wetland ecosystem.There are myriad feedbacks on carbon cycling that involve plant and microbial physiology, biogeochemistry and other physical and ecological interactions. As a result, increases in photosynthesis can...