To narrow it down, I would say less than 50 million, as that is roughly the amount resulting in certain deadly radiation dose of ~5Sv.

So better make sure to stay below 5 million bananas to be on the safe side…

Sums up and confirms my impression of this.

If you have a look at the figures in the study, only adding N and P did not significantly increase CO2 emissions.
Only when also adding carbon to the soil (which has been a starved kind of soil to begin with) emissions were increased.
Which is kinda… obvious??
Here is the link to the relevant figures:
https://link.springer.com/article/10.1007/s10533-025-01265-0/figures/1

So, is this a good or a bad message for climate change?
On the one hand, it could point to man-made CO2 currently emitted being even higher than we think (as less is contributed by the warmed up soil), which would be bad.
On the other hand, this might reduce the risk of yet another runaway scenario where higher temperatures lead to higher emissions, again leading to higher temperatures (like there is for soils in cold climates and methane emissions).
So, although I am just now not sure how to interpret the results, still a very interesting study!