Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N.

From 2013.

The last time I looked at this was around 2011 or so, and I end up thinking we might be responsible for around a third of it; this paper suggests that my estimate was low.

But we'll all be fine if we can just stop burning fossil fuels, won't we?