Aww this is a good one.
1) When you baked the soil this killed the bulk of the little critters in the soil.
2) The the amount of N in the microbial biomass can be large. For example crops need between 150-300lbs/acre of N to grow. The amount of N locked up in the soil microbes can be as much as 130lbs N/acre.
Microbial Biomass | Fact Sheets | soilquality.org.au
3) Depending upon the makeup of the soil and its CEC (cation exchange capacity) the minerals in the soil can lock up even more N. Even if you never fertilized it the nitrogen fixing organisms are slowly adding nitrogen to the soil from the air. This is then locked up into the minerals of the soil.
4) When organic material is decomposing the carbon to nitrogen ratio is very important to understand.
High carbon & low nitrogen means that nitrogen will be deficient until all of the food is broken down then released rapidly when microbes doing the decomposition run out of food and die.
Balanced Carbon and Nitrogen will lead to a more steady release of N during the decomposition process.
Low carbon and high nitrogen will lead to a very fast initial release of N followed by a slower more steady release.
So now what in blue blazes is happening in your tank.
Hypothesis: (Gratuitous scientific word used correctly).
After baking the soil you added it into the tank with the established filter and plants. This inoculated the tank with lots of bacteria, algae, protists, rotifers, fungus, etc. The whole kit and caboodle of microbial aquatic world. These quickly found all of the yummy killed microbes from the baking and quickly started gobbling them up. Since these killed microbes have about the same C:N ratio as the live ones they grew rapidly. However when all of the yummy killed microbes are eaten up, the large population of living microbes start starving. They slowly start to die off and as their corpses are eaten by remaining living ones they steadily release ammonia into the water.
The mineral side of the soil when immersed in water started releasing some of the nitrogen it had locked up. So you likely had a steady amount of ammonia in the tank from the beginning. Likely much more than the filter or plants could readily handle.
Then you popped in your adorable little carnivorous betta. He dines on a premium high protein feed and produces a steady stream of ammonia.
Like a good owner you test your water frequently to ensure your adorable little carnivorous betta has the best conditions. YIKES! Ammonia is there. So you do an immediate water change. The next day you test again. YIKES! more ammonia. Another water change. Repeat for 3 weeks and you are left with low Nitrates and a steady stream of ammonia entering the water.
Between the water changes and hungry hungry hippos of plants you detect the steady incoming stream of NH3 but no real changes to the other ratios.
Solution for next time: Don't add in fish for 3-4 weeks after starting a baked soil substrate.
Ta Da - BTW most hypothesis are proven wrong.....
Bump: No drinking was involved in the previous post but it is a boring Friday afternoon at the office.