January 1, 2010.
We used radiocarbon enriched leaf litter to quantify the transfer of carbon through a soil profile at an eastern deciduous forest in the United States, located at the University of Missouri's Baskett research area in the Ozark mountains. Mineral soil was sampled from five plots before (2007) and after (2008 and 2009). Radiocarbon enriched leaf litter was applied to the soil surface each year and samples of native litter-fall and mineral soil from 0-5 cm, 5-15 cm, and 15-30 cm depths were collected. Soil samples were first put through a 2mm sieve and the particles that passed through the sieve were dried at 70 degrees Celsius for several days until constant mass was reached. The samples were then sent to Lawrence Livermore National Laboratory to be graphitized and subsequently run on the accelerator mass spectrometer. The method for graphitizing the samples was to measure out an amount of soil based on its estimated percent carbon concentration that would yield 1mg of graphite for analysis. The first process was to combust the soil with copper oxide and silver catalyst under vacuum. The gases produced by combustion were put on a graphitization rig and the water vapor and other incondensibles were separated from the carbon dioxide based on each gases physical properties. The carbon dioxide was ultimately separated and sent to a reaction chamber where it was reduced in the presence of hydrogen gas and an iron catalyst that provided a surface for the graphite to adhere to during the reduction reaction. The iron also served as a binder and thermal conductor. The graphite was then pounded into targets and analyzed on the accelerator mass spectrometer. Measurements of the carbon-14 count to carbon-13 current were taken. This information was then used in the mixing model equation to calculate the fraction of carbon in the mineral soil that was transferred from the radiocarbon enriched leaf litter on the surface. The results show that 8% of the carbon from the labeled leaf litter transferred down to the 0-5cm mineral soil depth. The 0-5cm depth was the only depth that showed a statistically significant increase in radiocarbon after 2 years. t-tests also showed the mean differences in radiocarbon in the 2007 versus 2009 5-15cm and 15-30 cm respective depths did not have enough evidence to support a claim at a 25% level of certainty of an increase in radiocarbon. To determine whether there was an increase in radiocarbon with statistical certainty at these depths over time we would need more replicates and/or more time for the transfer to occur before the next sample is taken.
Lawrence Livermore National Laboratory (LLNL)