Elevated levels of carbon dioxide in the atmosphere can increase plant productivity and carbon storage in ecosystems. This study examines how excess carbon is processed in a loblolly pine forest in North Carolina over 12 years. Soil samples from plots with normal and elevated carbon dioxide levels were analyzed to study carbon and nitrogen cycling. Previous results did not find significant carbon sequestration, instead increased carbon inputs sped up carbon turnover. The smallest particles in soil had the lowest carbon-to-nitrogen ratios, indicating this pool is most stable. Some evidence suggests plants and fungi extract nitrogen from these pools to meet demands from elevated carbon dioxide.

1. Soil carbon and nitrogen sequestration and turnover after 12 years of elevated carbon dioxide
Shirley Wu, Class of 2011
The purpose of Duke FACE is to study ecosystem response to elevated atmospheric carbon dioxide in the face of current
increasing anthropogenic carbon emissions. Anthropogenic CO2 emissions have altered the global carbon cycle and the
additional inputs have been found to increase plant productivity and ecosystem carbon storage even with nutrient
limitations. It is believed that if terrestrial ecosystems take in more atmospheric carbon than what is put out by human
activity, we could make up for increased carbon emissions. Our project looks at how the excess carbon is processed in an
aggraded loblolly pine forest in North Carolina and whether or not it is sequestered in soil, thus removing carbon from the
active cycle and restoring the carbon cycle. In addition, we look at how elevated CO2 affects the nitrogen cycle and plant
productivity by tracking the 13
C signature from the added carbon.
Soil samples were taken from the Duke Forest in 2008, three samples per quadrant per forest plot. Of the eight
total plots, four were treated with carbon dioxide mixed with air and four were ambient control plots which only received
air. Previously, we processed and analyzed the entire 15cm core soil samples. This summer, we looked at the top 5cm of
soil where 80% of carbon is stored. Each soil sample was ground and treated with a polytungstate solution to separate the
heavy fraction of soil from the light fraction. Each fraction was then washed and filtered to remove the solution and dried
in an oven. The heavy fraction was treated with a dispersing agent and sieved to separate the different soil pools by size.
All samples were then ground up with mortar and pestle and sent to a lab for analysis.
We are still waiting for data, but data from the previous summer did not show significant evidence for carbon
sequestration. Increased carbon inputs to soil translated to increased carbon output and turnover, speeding up the carbon
cycle, which implies a faster nitrogen cycle as well. We have been looking to see where the nitrogen is coming from and
our previous results show that the smallest size fraction of soil (<53 um) has the lowest C:N ratio and the greatest amount
of nitrogen and is therefore the most stable soil pool. Some evidence shows that allegedly stable soil C pools with low
C:N ratios can be mined to meet the N demand put on by elevated CO2. We propose that this nitrogen mining occurs in
addition to plant allocation of energy towards root growth and mycorrhyzal fungi connections. The smaller the size
fraction, the lower the C:N ratio, demonstrating decomposition effects. We expected to see an inverse relationship
between C:N and mean residence time (MRT), but we did not see that. We also looked at the effects of nitrogen
fertilization in combination with elevated CO2 in ambient versus elevated plots. We found that without N fertilization, the
13
C signature was lower, indicating higher turnover and evidence of N mining. Inconsistencies within our results may be
due to site disturbance in the past or changing microbial communities in response to altered conditions that resemble those
of the pre-industrial period, which are hypotheses for further study.
Faculty Mentor: John Lichter
Funded by a faculty grant from the NSF