Environmental Engineering Reference
In-Depth Information
Extra reactive N in the atmosphere also cascades through the environment creating a
series of untoward effects. Some forms of excess N in the atmosphere are precursors to tro-
pospheric ozone and particulate matter (PM) that are detrimental to human health. Other
forms of atmospheric N contribute to the greenhouse effect and to the destruction of ozone
in the stratosphere. Some of the reactive N added to the atmosphere is deposited on terres-
trial ecosystems, where it can increase the productivity of vegetation, but can also cause N
saturation (extra N beyond what the biology can use) of ecosystems that developed under
low N conditions ( Aber et al. 1989, 1998 ). N saturation can lead to changes in plant com-
munity composition, nutrient imbalances (e.g., with phosphorus or base cations), and
delivery of excess N to lakes, rivers, and estuaries.
In summary, two concepts that are helpful to keep in mind as we think about the envi-
ronmental impacts of N are (1) the idea that N added to one place will often move to other
THE GLOBAL NITROGEN BUDGET IN 1860 AND MID-1990s, TgN/yr
5
6
NO y
N 2
NH x
6
9
8
120
6
7
0.3
15
11
8
27
5
6
NO y
N 2
NH x
16
33
23
26
18
21
25
25
110
100
39
48
N 2 + 3H 2
2NH 3
FIGURE 7.2 Global nitrogen budget for 1860 and mid-1990s. Data in Tg N/yr. The emissions to the NOy box
from the coal reflect fossil-fuel combustion. Those from the vegetation include agricultural and natural soil emis-
sions, and combustion of biofuel, biomass (savannah and forests), and agricultural waste. The emissions to the
NH x box from the agricultural field include emissions from agricultural land and combustion of biofuel, biomass
(savannah and forests), and agricultural waste. The NH x emissions from the cow and feedlot reflect emissions
from animal waste. (From Galloway and Cowling 2002.)
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