Just a quick note to say that Kendra's paper on reconstructing N availability was just published.
It's an amazing synthesis.
What makes it amazing is the ability to look back in time at an index of terrestrial N availability and see that for thousands of years during the Pleistocene-Holocene transition.
As interesting, there just wasn't any evidence of N availability increasing over the past 500 years.
Whether N availability--the supply of N relative to biotic demand--is increasing at the global scale is one of the most fascinating debates that isn't occurring.
McLauchlan, K. K., J. J. Williams, J. M. Craine, and E. S. Jeffers. 2013. Global nitrogen cycling changed during the Holocene as terrestrial ecosystems accumulated carbon. Nature 495:352-355.