Yours is a great question......and despite the fact I oversee a LOT of buckthorn control efforts, you've made me realize I do not have a good answer. Now very (very) generally, allelopathic chemicals, those substances produced by some plants which inhibit other plants, break down quite readily in the soil environment. But again, you're hot to trot and I don't want to steer you wrong. Here's something I just found:

http://www.botany.wisc.edu/waller/PDFs/KlionskyAmatWaller_2010_RestorEcol.pdf

If you read the results, you will see that, at least at the time this work was done, the very subject you present was still under investigation. Note too that this was in Wisconsin, and you appear to be in a roughly similar climate, etc. Happy reading!

+oM

Buckthorn tends to overwhelm other things in the garden when it's is allowed to grow but it's certainly not stunting/killing the competition...it's just a first class competitor.