I think you have the nitrogen thing backwards: lupine are nitrogen-fixers, meaning they pull nitrogen from the air and put it in the ground where non-nitrogen-fixing plants can access it.* However you're correct that they're invasive in Europe, and multiply quickly and aggressively (a single plant will produce thousands if not tens of thousands of seeds every year). I've heard that despite this they were intentionally introduced to Iceland to help revegetate the barren landscape after many failed attempts at reforestation (despite being known for its wide open spaces, Iceland used to be a giant forest... before humans cut down all the trees). The idea is for the lupine, which grow splendidly there, to be the first line of revegetation, to be later replaced by native species, kind of like cover-cropping in agriculture. We'll have to wait and see if this Hail Mary approach works out...

*Nitrogen-fixing is way more complicated than I'm describing here, especially in a non-agricultural setting such as the wild. I know that in agriculture, to get the most benefit from your nitrogen fixers it's best to not let them go to seed, something that obviously happens all the time in the wild.