The limited activity of soil microbes is caused by cold climates, and as the local climate in that forest warms up, we would expect microbes to decompose all th

The limited activity of soil microbes is caused by cold climates, and as the local climate in that forest warms up, we would expect microbes to decompose all that organic material better, improving the soil. Regarding soil acidity, wood ash is highly basic, so large quantities of ash from a major fire would help reduce the acidity. In essence, with warming climate and mass wood fires we would expect also the soil in these areas to change in ways that are better for agriculture; the only question is about the speed of that change. Also, the whole point of slash-and-burn agriculture in northern europe was that it was infertile and lacking nutrients otherwise - it was not a way of increasing the quantity your fields, you would abandon your fields after a couple of years to overgrow with forest, and slash-and-burn a new one for that temporary influx of nutrients in soil that is otherwise lacking them.