Personally, I'd be a little surprised if the PS was adding bubbles to the substrate. While PS are notorious for adding bubbles, they are typically free floating.
My guess on the bubbles in the sand would not immediately be nitrogen, as the conversion of elements and organic compounds into nitrogen is a pretty slow process and is more likely to take place at the surface of the tank where there is ample O2
for the NO2
to form. Naturally, I may be wrong and Dewey may have hit it right on the head - only a guess.
If it was not nitrogen, my second guess would be that it is trapped air bubbles. How long has the Sand been in the tank?. How did you lay the substrate? Since the particles of sand are likely very small they may take considerable air down with them as they break the surface tension of the water. Also, since they can combine to form tiny pockets - air can get trapped there as well. It may actually remain at the bottom of the tank for quite some time until some unexpecting creature stirs it up a bit and then Voila - a bunch of bubbles.
I'd certainly agree on the Aloe as this is likely to increase surface tension and make any existing bubbles have a greater resiliency.
Just thinking out loud.