AusBonsai

Hi, been reading this platform a lot but only just signed up
I’ve recently acquired a 25 year old Japanese black pine that I thought had a lot of potential.
I did notice it was pretty root bound, as it was pultruding above the pot quite abit.
The needles alot lighter shade then my other JBP I’ve had for quite a while now
When watering it was very hard for the water to pass through the pot..

Upon research I thought I needed to do an emergency repot as I didn’t want to risk needing to do it later in spring when the pine was even weaker

I repotted it two weeks ago (late September)
The pine had a thick mat of roots at the bottom of the pot which I took a centimetre off or so.
I didn’t remove too much soil or roots, just removed abit, pulled them apart abit loser and removed most of the mat at the bottom.
Also just carefully removed abit of the fine dark soil around the top of the roots without damaging any roots, trying to do as little damage till I can repot again once it’s more healthy..

The was no new needle growth on the candles before repotting but as soon as I’ve repotted it there’s been a lot of needles sprouting out candles all over the tree.
But there has a lot been so yellowing of the older needles, I’ll notice more daily.

I’m just wondering if it is a good sign that the tree is pushing more needles even though some of the needles are yellowing? Can the tree still be dying/dead even if it’s sprouting needle growth?

I noticed today there’s more of a grouping of yellowing instead of a couple odd yellow needles spread across the tree not in concentrated areas

Really hope I’m not going to lose the tree, any opinion or advice would be really helpful

Firstly, welcome to the forum ... always nice to have a new contributor on board!
Secondly, there are a million questions here in order to even begin figuring out what (or if) there is a problem with your tree, and the first of those will be ...
Where are you located?
Your climate and the position of the tree in your garden (or balcony possiblly by the looks) gives a lot of info. I would suggest you fill in your location as part of your profile so it comes up each time you post. This will save you having to repeat where you are and save responders having to repeat the location question. It doesn't need to be exact, just a nearby city/town will help with a reference.

Then more specifically in relation to the tree :
Do you know when it was last repotted?
Do you know what the bonsai mix was and how broken down was it when you repotted?
Were the roots healthy (white tips, no smell etc) when you took it out of the pot?
Do you know what kind of space it was kept in before you bought it? (i.e: full sun, part shade, in the open, under a bigger tree or a verandah, morning sun but full shade in the afternoon etc etc...)
How do those conditions above correlate with where you have it now (mainly in terms of sunlight, wind exposure, aspect etc)?
How long have you had it (is it months, weeks or days since you bought it)?
What soil/mix did you use to replace around the roots you did cut?
Do things seem to have improved with percolation since your root work (is water effectively being taken up by the tree)?

The fact you say new candles are extending/needles developing sounds pretty positive to me but see how you go with answers for the above and we might be able to give you a better idea of what to do from here.

The tree's fine. Just let it dry out before you water it for a while and keep it in full sun.

Looks good to me too.
Your repot appears to be at a good stage of growth for the tree - just before new growth happening so you could have gone harder but probably OK to leave that for next spring. It is also good to repot a bit earlier when the candles are much smaller.
It is normal for JBP to grow candles then open needles as they reach full size which is probably around now.

Just be wary that water actually soaks into the original soil after a repot like this. Water tends to run through the new mix around the edges and not soak into the main root ball. An occasional soak in a tub might help maintain hydration through summer but if you have plenty of soaking rain that may not be necessary.
A few scattered yellow needles is not a problem. Might just be that the roots got a bit dry at some stage - probably weeks ago. It takes conifers quite a while to show symptoms after stress.
Yellowing can also be a sign of nutrient deficiency - N or maybe iron of Mg. I find it is common in trees that need repotting. Liquid fert is quick acting but it needs to soak into the root zone to be effective.

Good luck with the tree. Keep posting as things change - for better or worse.