Fellow bonsai enthusiasts, 

I'm relatively new to the bonsai world and I just joined bonsaistudygroup today. Since pines are my favorite bonsai subject(especially JWP), I started growing four 3" JWP seedlings, and purchased on-line one species(on its own roots). I'm relatively new to the bonsai world and since I hear JWP can be difficult, I'm scared that I'll kill it! Also, the information I find on the internet regarding this pine tends to be contradictory, or mainly applying to those living in warmer areas where JWP cultivation is more challenging. I live in New York City area(suburbs), the local climate is very similar to JWP's native habitat(central and southern Japan).
Is it safe to assume that JWP would feel at home in this area? 

Should I leave the pine exposed to full or partial sun? 

How often should I water it? 

What soil mix and fertilizer can you recommend? 

How does one care for JWP seedlings from their first year onward? 

Looking forward to reading everyone's comments! (will post photos after the tree arrives in several days)

Thanks John for the first comment and the welcome!

Here are different angles of the Japanese White Pine. I the photo I took outside is depicts the needle color more accurately. I think this tree has nebari potential.

I purchased it from an online catalog (wavecrest nursery in Michigan). It was the only place where I could find JWP for a student-friendly price. The manager is fantastic and she said since many of their customers are "bonsai people", she knew what I was looking for. The tree arrived 2 weeks ago(Sept 15th) with its root ball wrapped in plastic. It's a beautiful and healthy tree, with lots of small buds all along the branches and lower trunk, and many white tips on the roots. roughly 18 inches tall, 1.5" trunk, about 11 years old.

I know early spring is the time to repot, but I had no choice but to pot it that day. What I'm mainly concerned about is the roots. I removed about 25% of the root mass, mostly large dead roots, and took out roughly 50% of the old nursery soil.

It's been in a cool shady spot for 2 weeks now, in a 3 gallon nursery pot. I'm using a mixture of 75% course turface, 20% regular bonsai mix (gravel, akadama, grit, turface, bark bits, and ecto/endo mycorrhizae spores), 5% decomposed Japanese White Pine needles with some extra white spongy mycorrhizal fungus that I collected under large Eastern White Pines in my yard.

My local New York climate is very similar to Japanese White Pine's native habitat (central Japan).
I'm hoping the tree will naturally acclimatize.

I love this tree and I hope it can recover quickly!!!

I would think repotting now it should be fine, so no problem there, though you might want to move into full sun.  Your soil mix sounds a little complex, and the needles and added fungus is rather unnecessary.  Native beneficial fungus will find you, with good soil, water, and fertilizer.  Also, I would think it would fine for winter in your area, with some protection.

How new are you to bonsai?  Any local clubs?

Put in full sun, water when dry, repot in spring after you start going to you Bonsai Club. Lots of folks ( look for Mike Pollack), look at their trees before you take their advice.

John

Quick update:

It was successfully repotted that September. This wasn't a "slip-pot", but rather a thorough removal of dead roots and replacing garden center soil with an inorganic bonsai mix. I responded with healthy new growth the following spring but sadly died later that year due to my untimely wiring in late August - the loose cambium separated from the heartwood.

I now, however, have 2 JWP in the ground at my parents' upstate NY home and two recently purchased seedlings in plastic pots with me in MD - all are base species on their own roots, not cultivars.

The 2 growing in the ground were also previously in plastic pots. I found they did better in regular plastic pots than those that were growing in colanders, which I suspect had to do with the colanders being less stable on windy days than the flatter, broader bottomed pots. Also, the JWP in the colanders all died when being transplanted into the ground, this was done in autumn, so I cannot make a general recommendation for repotting JWPs then. The main cause of their demise is still unclear. The 2 JWP that were in plastic pots and were successfully transplanted (in early spring) in the ground are root pruned every 3-4 years. 

 recently relearned from Julian Adams (owner of Adams' Bonsai) the utmost importance of keeping JWP very stable in their pots. "Keep them thinking their in the ground," he said. JWP will grow poorly when their roots are disturbed by even the slightest rattle of the pot caused by wind, over-handling, and/or loosely anchored roots.