Bubbles from my DSB

A few weeks ago I moved an established FOWLR into a new 55g tank, and
put down about 5" of Southdown sand.

Before long I noticed bubbles under the surface of the sand, and now
it's giving off bubbles like crazy. This activity has increased as I
have increased my light. Now there are small bubbles all over the
surface of the sand bed.

Does nitrate reduction result in free nitrogen? Is that what these
bubbles are likely to be? Looking through the acrylic under the
sandbed, there doesn't appear to be any organic material down there
that is rotting, so I don't know what else it could be.

Are the nitrate-reducing bacteria light dependent?

Nitrates are currently high, at 50 ppm, but I'm hoping to get them low
enough to start keeping some soft corals. I just added plants to my
refugium and I'm hoping that will help, too.

Thanks...

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