I'm not sure. The only thing I know is that when I mare is in heat or going to be, she urinates a lot more often, at least my mare did, other than that I'm not sure what to tell you. I'd say put as much bedding in there as you can, and try clean it morning, afternoon and night.
The obvious thing to do would be to turn her out except for nights or yucky days. I can't think of anything to be honest to really soak it up except for what is already mentioned.