New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 78

fuels and cultivation of legumes. These three anthropogenic sources of nitrogen are
estimated to have added more nitrogen (187 Tg N/year in 2005; Galloway et al.,
2008) into the terrestrial environment during the past few decades than natural
sources (110 Tg N/year; see Figure 2.24; Gruber and Galloway, 2008). In addition,
anthropogenic emissions of nitrous oxide (N 2 O, a greenhouse gas) directly contribute
to stratospheric ozone depletion and tropospheric N 2 O accumulation (Ravishankara et
al., 2009), while emissions of nitrogen oxides (NO x ) indirectly contribute to
tropospheric ozone and aerosol formation (Arneth et al., 2010).
Figure 2.24 Natural (blue) and anthropogenic (orange) nitrogen fluxes for the
terrestrial ( left ) and marine ( right ) nitrogen cycles. Illustrates major sources, sinks,
and processes associated with production of reactive nitrogen and the coupling of the
nitrogen cycles with those of carbon and phosphorus. Values are for the 1990s in Tg
N/year. SOURCE: Gruber and Galloway (2008). Reprinted by permission from
Macmillan Publishers Ltd.
Understanding how all of this additional nitrogen will affect climate,
terrestrial ecosystems, and carbon cycling is essential as we attempt to anticipate
future environmental change and possible mitigation strategies. For example, recent
modeling studies indicate that nitrogen feedbacks represent an important control on
changes in terrestrial carbon storage driven by increases in atmospheric carbon
dioxide, though the nature of this control varies between tropical, temperate, and
high-latitude ecosystems and the magnitude of the effect remains uncertain (e.g.,
Zaehle et al., 2010). Nitrogen-related changes in carbon storage feed back into
climate by regulating atmospheric carbon dioxide levels. In addition, any changes in
the C:N ratio of terrestrial plants and/or changes in rates or the geographic
distribution of biological nitrogen fixation and denitrification would alter regional and
global carbon cycles (Gruber and Galloway, 2008). Redistribution of nitrogen,