Where do anaerobic bacteria get nitrogen to fix?

A large number of free-living bacteria of anaerobic environments have the ability to fix nitrogen, including the genus Clostridium. Fewer genera of aerobic bacteria have this ability.

This seems to be standard information online.

Where would anaerobic bacteria get nitrogen gas to fix? Wouldn't conditions that keep out oxygen also keep out any significant amount of nitrogen to replace that being fixed?

Nitrogen that we think of and want in the soil mostly comes from ammonium compounds which are found in good quantities in anaerobic environments.
Also there are many free style bacteria that go merrily along all on their own, these guys can operate as either aerobic or anaerobic organisms, thus they can utilize a good number of different compounds to get their carbon and nitrogen from.
most acidic soil is made that way sulfur compounds or nitrogen compounds which release nitric acid or sulfuric acid as a by product of these bacteria using those compounds as a food source.

Redhawk