The issue is most likely that high radiation levels are inhibiting the growth of microorganisms. So new microorganisms would probably just die.

Note that this is not particularly unexpected. We do this on purpose (in a more controlled manner) to prevent spoilage of food and reduce food borne illnesses.

http://en.wikipedia.org/wiki/Food_irradiation

"To confirm their hunch, they created around 600 small mesh bags and stuffed them each with leaves, collected at an uncontaminated site..."

They used uncontaminated samples yet the environment killed the microorganisms and fungi they brought in. I think this doesn't work as long as the radiation level stays this high in the area.

That will be happening every time it rains, wind blows, river flows, something like an animal comes in etc. Repopulating it probably isn't hard and wouldn't take long as it's happening naturally. The tricky part is trying not to have every organism killed after arriving.