That's a noteworthy point about fresh Biochar (or char). When it's fresh, it will have anionic attraction. So fresh char in your soil could suck up free N in the soil. Activating fresh char with urine is a quick fix. Now the biochar has N locked up, so it won't rob the soil, and as time goes on, the biochar polarity changes and it will shed the N and other anions in favor of Cation sequestration.
Biochar is easy to make and incredibly useful for soil building, BTW. It's one of the topics I think we should co-op. Have a class, get some materials, go home and do it. Learn the skills, then spread the skills. We all benefit.