ScienceDaily (Apr. 2, 2012) — University of California, Berkeley, chemists have found a smoking gun proving that increased fertilizer use over the past 50 years is responsible for a dramatic rise in atmospheric nitrous oxide, which is a major greenhouse gas contributing to global climate change.

Boering was able to trace the source of N2O because bacteria in a nitrogen-rich environment, such as a freshly fertilized field, prefer to use nitrogen-14 (14N), the most common isotope, instead of nitrogen-15 (15N).
http://www.sciencedaily.com/releases...0402144930.htm