What you REALLY have is a Japanese Black Pine with Japanese White Pine folliage! AKA, a graft.
Which means you have Japanese Black Pine roots. There is a beneficial fungus, mycorrhizae, that aids JBP roots to absorb nutrients. It's a good thing. It will develop naturally. You can buy the stuff, but you don't need to. Sometimes, when I'm doing a repot and change a lot of soil out, I'll throw a little of a root I've cut off that has the fungus on it back into the pot to jump start it. But, it's really not necessary.
So, you're good.
By the way, the reason they graft the white pine scions on JBP stock is the JBP roots are more vigorous, and more forgiving of repotting. It also makes the JWP scion more vigorous and faster growing. The JBP trunk portion will always be thicker than the JWP truck as JBP puts on girth faster, and "barks up" faster than JWP. JWP will eventually "bark up" and lose that smooth grey appearance, and will "eventually" look like JBP bark, just thinner. It might take 10 to 15 years, however.
Meanwhile, your job as a bonsai artist is to hide that graft union as best you can so that the transition from JBP to JWP doesn't stick out like a sore thumb. (I'm not saying yours does.)
So, as you study and learn how to take care of this tree, treat the roots as you would JBP and treat the branches like JWP.
