Figure 7.16. (a) Changes in the emissions of fuel combustion NO_x 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).