It is also a simple fact that CO2 concentrations have been rising, which likely should be causing N to become progressively more limiting.
It is also a simple fact that no one has taken the time to comprehensively address whether N availability has been increasing or decreasing in the ecosystems of the world. There are almost no time series of direct measurements of N supplies or availability to test whether N availability is going up or down.
As a result, it is unresolved as to whether N availability is increasing or decreasing in ecosystems across the world.
Andrew Elmore and Dave Nelson (with a little help from me) report in the latest issue of Nature Plants new data that looks at whether N availability is increasing or decreasing in US eastern deciduous forests.
Short answer: N availability looks to be decreasing.
Using ratios of N isotopes in wood as a proxy for N availability, Elmore et al. show that N availability has been declining in the forests they examined for some time.
That's a pretty big result.
Not only do they show this, but they also show that the declines are tied to spring phenology. Years with warmer springs have the lowest N availability.
Mechanistically, one link between phenology in N availability is that years with warmer springs have greater increases in plant demand for N than any increases in N supplies, leading to declines in N availability.
One question that arises from this work...if N availability is declining in these forests, how sure are we that we have crossed a planetary boundary for N? Are the world's terrestrial ecosystems really eutrophying?
Elmore, A. J., D. M. Nelson, and J. M. Craine. 2016. Earlier springs are causing reduced nitrogen availability in North American eastern deciduous forests. Nat Plants 2:16133.