Differences persist between ex-arable and ancient woodland even after 25 years, study suggests - Forest Research
(www.forestresearch.gov.uk)
This isn't really surprising. 25 years after planting the trees are still young. Some won't have even had their first seeding season. Few if any will have fallen to rot and contribute to soil quality. Come back in another 75 years and I suspect they'll still be a measurable difference.