I bought a young juniper from a garden center around mid-august and pruned and repotted it. I used the appropriate bonsai soil (akadama, lava rocks, pumice) and a reasonable sized pot. I also had to trim off some branches, but I used the cutters the wrong way by mistake, creating a vertical split which I later read was more difficult to heal. I used wound sealant where the branches had been cut. Here's a picture of the juniper before and after pruning and wiring.







The natural appearance of the foliage is a mix of green and greenish yellow.

It was fine for quite a long time so I'm pretty sure the initial pruning didn't do any harm, except for some of the greenish yellow needles turning brown, which could have possibly been an early sign of what's happening now.
A short while ago I noticed that the wires were starting to cut in a little in some places, so I removed them, which changed the appearance of the branches a bit.
But recently in the past couple of weeks it's been getting worse. This only started after two things happened.
1: I re-opened the pruning wounds so I could cut them the right way and re-applied the sealing paste. I noticed that after nights when it was cold and wet, the wound sealant would become wetter and lighter-colored, as temperatures have been dropping with the season, and that bothered me because I was worried it might not dry fully.
2: I was worried that it might need some food, but I couldn't find any bonsai-specific fertilizer, so I put a small amount of fertilizer for tomatoes and vegetables (4% nitrogen, 6% phosphate, 3% potash, 4.75% calcium, and some types of bacteria as well) around the root base. I only used a pinch because I wasn't sure if that would be right for it.

Now the needles have been turning brown, wilted and brittle, and I'm worried that it's dying. Here are some pictures of its current state. As you can see, it's much different from the previous images.





It's significantly worse. I removed the largest pieces of tomato fertilizer, and picked some fertilizing pellets out of a bag of standard potting soil and put those on top of the soil. Should I remove those too? Was the earlier incident of the yellow-green leaves turning brown a sign that this was going to happen eventually? Does anybody have an idea of what might be wrong, or ideas on how to fix it? Can I still save my bonsai?
Thanks in advance.

Your pictures are invisible, and i don't open random links. Please follow the how to post pictures explanations. Without pictures it is impossible to say anything.

It's a wild guess, but if your juniper is inside that is the reason it's dying or dead.

Thank you for that notice. The bonsai is not being kept inside.
I found out how to upload images, but now I can't edit my post. Are people unable to edit posts after they've been replied to?

Here is an edit which includes the images. Sorry, I'm not familiar with this forum.

In regards to any details I forgot: I always keep my bonsai outside and in full sunlight as directed by the label for the breed of juniper. It is not from a bonsai nursery, it started as an ordinary sapling in a 1-gallon pot. I put it under shelter whenever there's storm rain that may damage it or disrupt the roots.

Cece wrote:

I bought a young juniper from a garden center around mid-august and pruned and repotted it. I used the appropriate bonsai soil (akadama, lava rocks, pumice) and a reasonable sized pot. I also had to trim off some branches, but I used the cutters the wrong way by mistake, creating a vertical split which I later read was more difficult to heal. I used wound sealant where the branches had been cut. Here's a picture of the juniper before and after pruning and wiring.







The natural appearance of the foliage is a mix of green and greenish yellow.

It was fine for quite a long time so I'm pretty sure the initial pruning didn't do any harm, except for some of the greenish yellow needles turning brown, which could have possibly been an early sign of what's happening now.
A short while ago I noticed that the wires were starting to cut in a little in some places, so I removed them, which changed the appearance of the branches a bit.
But recently in the past couple of weeks it's been getting worse. This only started after two things happened.
1: I re-opened the pruning wounds so I could cut them the right way and re-applied the sealing paste. I noticed that after nights when it was cold and wet, the wound sealant would become wetter and lighter-colored, as temperatures have been dropping with the season, and that bothered me because I was worried it might not dry fully.
2: I was worried that it might need some food, but I couldn't find any bonsai-specific fertilizer, so I put a small amount of fertilizer for tomatoes and vegetables (4% nitrogen, 6% phosphate, 3% potash, 4.75% calcium, and some types of bacteria as well) around the root base. I only used a pinch because I wasn't sure if that would be right for it.

Now the needles have been turning brown, wilted and brittle, and I'm worried that it's dying. Here are some pictures of its current state. As you can see, it's much different from the previous images.





It's significantly worse. I removed the largest pieces of tomato fertilizer, and picked some fertilizing pellets out of a bag of standard potting soil and put those on top of the soil. Should I remove those too? Was the earlier incident of the yellow-green leaves turning brown a sign that this was going to happen eventually? Does anybody have an idea of what might be wrong, or ideas on how to fix it? Can I still save my bonsai?
Thanks in advance.

General suggestion with junipers and conifers in general is avoid pruning more than 30% of their foliage at once.
Here you probably left 1%, all while stressing the plant with a repotting, also done at the wrong time of the year (assuming you’re in the northern hemisphere).

I’d be more surprised if it survived, tbh.

Bofh is right, your plant is dead.
Way too much in one session.
One a side note: there is no need to bring plants under a porch when it rains, in nature trees don't go hiding when it rains either...