Project Abstract: RUI: Fine root production and architecture in a loblolly pine forest: interactive effects of atmospheric carbon dioxide-enrichment with soil nitrogen availability.
Seth Pritchard (PI) and Allan Strand (Co-PI), College of Charleston
Efforts to characterize carbon cycling between the atmosphere, forest canopies, and the soil are hindered by a poor understanding of how long tree roots live. The influence of elevated atmospheric carbon dioxide (CO2) concentrations and fertilization on the smallest roots of loblolly pine trees has been studied since 1998 in the Duke University Forest. The lifespans of the smallest tree roots are being documented by periodically inserting a miniature camera into clear tubes installed permanently in soil. Using sequential digital images to measure the lifespan of roots of different sizes will be combined with analysis of root architecture and chemical analysis to better understand how root biology will be affected by elevated atmospheric CO2 concentrations.
Carbon dioxide emitted from burning fossil fuels and deforestation is accumulating in the atmosphere. Some unknown quantity of this extra CO2 will be removed from the atmosphere by plants during photosynthesis and then moved by the plants into the soil where some of it may remain for centuries. Understanding how elevated CO2 will influence the lifespan of the smallest and most ephemeral roots is particularly important because these roots absorb most of the nutrients and water acquired by trees and when they die they transfer a large amount of carbon into the soil. A better understanding of root biology is needed to predict the potential for carbon storage in the soil.
The amount of carbon dioxide (CO2) in the atmosphere is increasing due to clearing of forests for agriculture and burning of fossil fuels. A long-term experiment was established in the Duke Forest in North Carolina in 1996 in which forest plots were fumigated with elevated carbon dioxide to evaluate the potential for trees to absorb some of this extra CO2 by growing faster and maintaining larger shoot and root systems. We collected blocks of soil at the termination of the experiment in 2010 to determine if trees growing in CO2-enriched atmospheres would 1) produce more roots or contribute to greater soil fungal proliferation; 2) change the lifespan or shape of the smallest diameter root branching systems that absorb the majority of nutrients into trees; 3) alter the chemical composition of roots. Results indicated that long-term exposure to elevated carbon dioxide increased the total length of root systems in the soil and also changed root shape in ways that would make them more efficient in acquiring nutrients from the soil. In general, root systems were composed of individual roots that were smaller in diameter and appeared capable of exploring larger volumes of soil to harvest more nutrients. Adjustments in the size and shape of tree roots in the future have implications for tree nutrition and cycling of nutrients at the forest, landscape, and global scales. We also quantified the effects of elevated carbon dioxide on root systems by taking repeated measurements (approximately monthly) over the final six years of the experiment with an underground camera. This analysis indicated that the symbiotic relationship between tree roots and soil fungi (illustrated in Figure 1), called mycorrhizas, was enhanced by exposure to elevated CO2, although not as much as expected based on previous experiments conducted on potted plants growing in greenhouses. The lifespan of mycorrhizal root tips was also extended in CO2-enriched plots (but only under unfertilized conditions). We also determined that symbiotic relationships between roots and soil fungi were suppressed during years with warmer temperatures during late spring and were also suppressed during years with cooler mid-winter temperatures. These results suggest that ongoing climate change is likely to influence the interactions between roots and soil organisms. Further research will be required to determine how temperature changes will alter forest growth patterns and soil biodiversity and how the effects of elevated atmospheric CO2 will modify tree response to climate change. It is important to note that although we found enhanced root and soil fungal growth in forest plots fumigated with elevated CO2, other results from this experiment have indicated no additional storage of carbon in pools of dead or decaying organic matter in soil. It is likely that rising CO2 will stimulate the growth of trees, including roots, but stimulation of soil organisms that decompose organic matter, particularly soil fungi, may preclude the possibility of significant sequestration of anthropogenic carbon within soil organic matter in forests. This award resulted in the publication of 6 articles in peer reviewed scientific journals and the results of this project were presented at 2 national Ecological Society of America meetings and 2 US DOE workshops. Several students received training through this project at both graduate and undergraduate levels.
