When pine trees live in a harsh or new environment, they tend to have closer relationships with soil microorganisms (such as mycorrhizal fungi) in order to receive sufficient nutrients. I think that's the reason for more microorganism activity in dune-covered pine forest.

I found some article for you. In this article, activity of microorganisms at interdune was higher than at playa because soil moisture and organic C at interdune were much lower than playa. Moreover, active fungal and bacterial density at interdune were much higher than playa. This environment is more extreme than your comparison, but I think these reasons can help your question.

You can find more in this article: Article Soil microbial metabolic profiles in two geomorphological un...

Yes, mycorrhizal fungi

Andrew Paul McKenzie Pegman Thank you for your confirmation.