Long term studies by a number of Long Term Ecological Research (LTER) sites were the basis for some of the 14 papers in a special issue of the journal Biogeochemistry entitled "Tracking evolution of urban biogeochemical cycles: past, present, and future." The issue encompasses work by urban ecologists in different cities across the U.S. including three LTER sites, Baltimore, Luquillo, and Plum Island.
Read a more detailed account of the studies.
A link to all the papers in the issue can be found at http://link.springer.com/journal/10533/121/1/page/1