IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis
Figure 7.16

Figure 7.16. (a) Changes in the emissions of fuel combustion NOx and atmospheric N2O mixing ratios since 1750. Mixing ratios of N2O provide the atmospheric measurement constraint on global changes in the N cycle. (b) Changes in the indices of the global agricultural N cycle since 1850: the production of manure, fertilizer and estimates of crop N fixation. For data sources see http://www-eosdis.ornl.gov/ (Holland et al., 2005b) and http://www.cmdl.noaa.gov/. Figure adapted from Holland et al. (2005c).