Bacteria don't create nitrogen. It only comes as a byproduct of their activity. Some are nitrogen fixers, pulling atmospheric N gas from the environment and converting it into nitrates (usually in conjunction with a host), nitrifying bacteria that convert ammonia into nitrates and decomposing bacteria that convert nitrogen waste - organic matter - into ammonia.

Since nothing is growing in this area, we can safely eliminate the first two. If this area is the source of the smell, then it is likely due to organic matter already present in the soil. Adding more OM will just create more bacterial activity.

I'd leave the area alone. Once all the OM as been digested and decomposed, the smell will dissipate.

I've sown grass there countless time, but nothing ever grows but weeds. I always thought that was due to the lack of sun (even when I used grass designed for shade), but now I'm thinking it could be because of that nitrogen.

This is where I'd read to till in wood chips:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/ammonia-odors-in-garden.htm

It's definitely the location of the smell, if not the actual source. People walk by it and immediately ask, "what's that smell?" :-/ But after eliminating the septic lines and the weeds, the only thing left is bacteria.

I like your plan of ignoring it and let it go away, I can afford that! LOL

You missed an opportunity to fix the drainage (toward the garage) when you had the yard dug up. The yard should slant DOWN out to the grass line and then start to slant upwards again if you want. That would have required the removal of many, many cubic yards of soil.

Anyway, back when we were not speaking to the USSR, they were conducting a lot of agricultural research. One of the things they learned was that dry leaves and chipped wood absorbs ammonia gas., When the leaves get wet (morning dew) the ammonia gas, which has a high affinity for water, joins up with the dew and drips back down into the soil to provide fertility. I cannot remember the book or author, but if you go back to about 2010 in the Organic Gardening forum, you'll find a guy named Valerie (in Russia that's vuh-LARRY, not VAL-er-ee) talking about it. So with that in mind, a dense covering of chipped wood mulch should work for you. No tilling needed. This is the reason you can cover a smelly compost pile with dry leaves to take the smell away.

You missed an opportunity to fix the drainage (toward the garage) when you had the yard dug up. The yard should slant DOWN out to the grass line and then start to slant upwards again if you want. That would have required the removal of many, many cubic yards of soil.

I'm not sure that I follow you there, @dchall_san_antonio :-O

I hired a waterproofing specialist company to handle it for me. There's a hill on the left that was sloped to the garage on the right, and where I'm standing when taking the photo goes downhill until the bottom of the sidewalk. So all of the rainwater poured towards the garage.

What they did was add a slope from the garage that aims everything towards those catch basins. Then the 4 downspouts from the back yard connect to a 6" drain pipe, and that pipe is on a downward slope for a little over 100' to where it shoots out into the woods.

Here's the same pic, with some drawing so you can see what I mean.

I spent about $5k on it, so I darn well hope it's done right!! I haven't had a major storm yet to find out, though.

So with that in mind, a dense covering of chipped wood mulch should work for you. No tilling needed.

Lucky for me, @dchall_san_antonio, I have a HUUUUUUGE pile of chipped wood mulch!!

Any suggestions on how long I would need to leave said mulch? I'm not going to be able to do anything with this ground until late September, anyway, so if I could cover it with mulch and then rake it back up in September? That'd be awesome!! Absorb ammonia and stop weeds at the same time :-D But if we're talking more like years and years, that's a whole 'nother story.