I have been told by friends who had this same problem that applying molasses to the spot takes care of it. I have kept a spray bottle with a molasses/water mix handy for the last few years to speed the decomposition of feces and that works great.
Just a spritz and it's gone within 48 hours.
Another suggestion was to use a low acidity vinegar (5%) solution (half vinegar half water) and then the molasses as a followup. The vinegar begins to pull the nitrogen down and the molasses bumps up the microbe level so they speed the process. It makes sense in its simplicity.
Either way will be both inexpensive and easy so it's worth a try!
At least we know it won't hurt.
Lastly, I have been told by animal people that a dog whose urine burns badly eats too much protein. My dogs' urine has never been much of a problem, and the little dog I have now doesn't put out enough to bother anything.
Hope that helps!
Kathe