N2o

A summary of the major N fluxes is presented in Table 12-2 and Fig. 12-8 shows a summary of the N fluxes for atmospheric NH3, N20, and NO,.

12.7 Human Impacts

Vitousek et al. (1997a,b,c) describe a range of concerns associated with the large anthropogenic perturbations to the nitrogen cycle. This includes groundwater contamination from N03 (agricultural runoff), eutrophication (agricultural runoff and atmospheric NOf deposition), radiative forcing of climate (N20 and tropospheric 03), stratospheric chemistry, photochemical ozone "smog," acid precipitation (HN03 deposition), changes in species diversity (due to N deposition), and N fertilization of the global carbon cycle. We provide a brief overview of each of these issues.

72.7.7 Groundwater/Eutrophication

As described previously, agricultural fertilizer contains large amounts of fixed nitrogen, mostly in the form of NH3, NH4, or N03 . Some of this N is utilized by the growing crops, but a significant amount is not taken up by the biomass and instead washes off the farm and into either ground or surface waters. This can cause two different problems, N03 toxicity and eutrophication.

Although N03 is not usually thought of as a "toxic" chemical, it does cause several health problems including methemoglobinemia in infants (blue-baby syndrome) and may also be linked to stomach cancer. Agricultural runoff can lead to significant, potentially harmful, concentrations of NO3 in ground or surface water.

Eutrophication can occur when N03 (or other nutrients such as P04 ) accumulate in lakes, ponds, or estuaries from agricultural runoff, sewage, or phosphate detergents. These nutrients will accelerate plant growth, often leading to algal blooms, oxygen depletion and sometimes mass fish death. Eutrophication can also lead to substantial impacts on the overall aquatic ecosystem. Impacts due to eutrophication are well known for the Baltic Sea, Black Sea, Chesapeake Bay, and other regions (Vitousek et al., 1997b). One example of the human influence on aquatic N03 is the results reported by Turner and Rabalais (1991). These authors have shown that N03~ concentrations in the Mississippi River have more than doubled (since 1965)

Table 12-2 Fluxes in the global nitrogen cycle (units are Tg N/yr)

Stedman and Shetter (1983)

Jaffe (1992)

Galloway et al. (1995)

Range

Terrestrial-atmospheric

1 Natural biological fixation (pre-agriculture)

2 Fixation due to planting of nitrogen fixing plants

3 Total biological fixation (1+2 above)

4 Industrial fixation (fertilizer production)

5 Natural denitrification (pre-agriculture)

6 Additional denitrification due to agriculture

7 Microbial NOx production

8 Microbial N20 production (natural)

9 Ammonia volatilization

10 Biomass burning N20 production

11 Biomass burning NO* production

12 Anthropogenic N20 production (all sources)

Oceanic-atmospheric

13 Biologic fixation

14 Denitrification

A tmospheric-atmospheric

15 NO, production by lightning

16 NO* production by industrial combustion

Terrestrial-oceanic

17 River run-off

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