> Can anyone say whether it is possible to have an accident where this becomes a runaway process like the Great Oxidation Event?

Probably not, due to denitrifying organisms [1]. The GOE occured because aerobic respiration, the biological balance to photosynthesis, didn't exist [2]. So when cyanobacteria began photosynthesising, the only counterbalance was ferrous iron dissolved into the oceans. Once that's reduced, that's it. No cycle. In contrast, more nitrogen fixation would prompt more denitrification, thereby balancing the system.

So to complete your disaster novel, you'd need to kill off denitrifying bacteria. Ironically, it appears they require "a very low oxygen concentration of less than 10%."

[1] https://en.wikipedia.org/wiki/Denitrification

[2] https://pubmed.ncbi.nlm.nih.gov/28360330/

[3] https://en.wikipedia.org/wiki/Oxygen_cycle

Just want to point out, the Great Oxidation Event took 400M years.

Given that we're talking about eukaryotes here, you can't just sprinkle some modified DNA on it and expect it to start executing that code (like you can with bacteria, see https://en.wikipedia.org/wiki/Transformation_(genetics) ). So patching the organism with the code for the new organelle requires some kind of vector, like a virus or CRISPR. If such a thing were to be surprisingly widely applicable across species and also escape the lap, I suppose it could update more than just the desired plant.

Given that some other nutrient would then become the bottleneck, I think this would be ok though. You'd just have upgraded plant growth, which would mean locking carbon into biomass at an increased rate, which might be kinda helpful around now.

Would it have unintended consequences? Probably. Would they be worse than the unintended consequences of industry? Maybe not.

I suspect what stops this from being a universal feature of plants is the energy required to fix nitrogen. A plant is better off not trying to do that. Especially since if nitrogen is a limit then other nutrients are probably close behind.

Once nitrogen is not a limiting factor for growth anymore, other elements will be, like phosphorus and sulphur.

I don't know, but if it was possible surely it would already have happened with the legumes?