You are a forester charged with increasing productivity in a South American forest newly planted with pines from Oregon. You believe that the southern forest lacks the fungal diversity needed by the North American pines and that this lack of fungi is affecting the pines' productivity, but you have no evidence to support your ideas. Which of the following would be the best approach to assess the diversity of fungi likely contributing to tree productivity in the Oregon forest?
a. Count all the plant species and multiply by 6, as David Hawksworth did when determining the ratio of fungal to plant species in England.
b. Collect all the fruiting structures (mushrooms, morels, etc.) found aboveground.
c. Do direct sequencing on representative soil samples from across the forest.
d. Expose the trees to radiolabeled carbon dioxide and then collect the soil samples with the greatest radioactivity and do direct sequencing.