But I've also seen a few studies that were concluding that the Douglas fir had a (much?) more positive effect on top soil and humus creation than the European s

But I've also seen a few studies that were concluding that the Douglas fir had a (much?) more positive effect on top soil and humus creation than the European spruces, firs and pines that are also used in fast-growth forests (and that are often not indigenous either), with better nutrient composition. Their acidifying potential was also significantly lower. An example I could find: https://www.agriculturejournals.cz/pdfs/jfs/2013/09/02.pdf https://www.agriculturejournals.cz/pdfs/jfs/2013/09/02.pdf