We added a carbon mixture that contained inorganic
salts and amino acids (not shown here) to each soil microcosm where the
only difference between treatments was the $^{13}$C-labeled isotope (in red). At days
1, 3, 7, 14, and 30 replicate microcosms were destructively harvested for
downstream molecular applications. DNA from each treatment and time (n = 14)
was subjected to CsCl density gradient centrifugation and density gradients
were fractionated (orange tubes wherein each arrow represents a fraction from
the density gradient). SSU rRNA genes from each gradient fraction were PCR
amplified and sequenced. In addition, SSU rRNA genes were also PCR amplified
and sequenced from non-fractionated DNA to represent the soil microbial
community.

