They stated that three boundaries have already been exceeded. The rate of climate change and biodiversity loss were the first two. The third was biogeochemical flows, specifically the N cycle.
Direct and indirect N fixation can have a host of unintended consequences from acidification to biodiversity loss to enhanced warming to hypoxia. Clearly, too much N is bad from local to global scales.
The authors set their limit as 35 MT N y-1. In the longform version, they state this value is " ~25% of the total amount of N2 fixed per annum naturally by terrestrial ecosystems" Estimates are that we've exceeded this by a factor of 4.
Basically, they suggest reducing N fixation by 75%.
The authors state that this value is "is a first guess only. Much more research and synthesis of information is required to enable a more informed boundary to be determined."
If you take the negative consequences on N fixation one by one, the key question to ask is what have the trends in these states been over time at the global scale.
Not easy to do.
One of the more tractable is to ask whether N availability to plants, a key component of terrestrial eutrophication and acidification, has actually been increasing or not.
One thing ignored in the paper is that CO2 concentrations in the atmosphere have also been rising. If N2 fixation were drastically reduced, could N availability actually decline as plants receive more C than N?
To answer those, we need better monitoring of N availability to plants at a global scale.
And there is nothing in place to try to answer those questions.