Bearded dragon burrowing substrate

Question
Good morning :), I'm writing to inquire about bearded dragon habitat (ie: enclosures# and gravid females, I'll start off with the enclosure question first #but they are both related questions#, recently my gravid female #Wolf# started digging around her enclosure making a mess everywhere trying to find a suitable nesting site with not much luck as the substrate is made up of small pebbles, I then decided that she needed a exclusive tank for burrowing and 1/2 filled a tank with soil #here comes the problem# with the light/heat on the tank the soil now has a fury kind of mold #which I assume would not be any good for my BD# and was wondering what the best substrate for digging would be that wouldn't go moldy under heat lights? Also I read that it is bad to breed young Bd's as this could be problematic for both the male and female, my question is why this would be as a unmated female will still lay eggs, and what problems could this case to the male? #curious about this one).

Thank you for your time and response, Wolf.

Answer
Hi,
I'll have to say "good Evening" as it is 8PM here!!
Gravel(pebbles)  really should not be used as substrate for many reasons.. BD's do like to burrow at times..and enjoy digging and the pebbles, even though you think they may not be able to eat them, they can and will.  These, in most cases will not pass and will cause internal blockages.
Childrens Play sand can be used if your dragons don't eat the sand. Its safer to  provide a "sandbox" for them and use newspaper for a substrate.
Many people that breed beardies or have females that choose to lay eggs without a male, use vermiculite. You want it moist but not soaked.  Here is a good link for more info on it.
http://www.bio.miami.edu/ktosney/file/BDeggs.html
Using a sterile potting soil such as sphagnum moss prepared the same way can also work. This mix is generally put into a large tote type container that will hold about 15-18 gallons...no..you don't fill it with the soil!!  You only fill it part way.
As to the mold....it may be because its too warm or it can be that the soil isn't sterile. You may be able to salvage that soil by baking it at a low temperature for about 2 hours.
With breeding at young age..for the male, it can cause hemipene prolapse.  Its also very stressful to both sexes. Its the stress and the aggression that is very hard on them. Males and females should not be housed together other than for a short period of time during breeding season.
When the female is not bred, the stress factor of breeding isn't there nor the "being beat up" by the male.