Environmental Engineering Reference
In-Depth Information
respiration can be fueled by reduced inorganic compounds (e.g., iron, sulfur) that can be
common in the subsurface (see the Appendix). These compounds are often important
sources of energy for subsurface microbes because organic carbon (the energy source for
heterotrophs) is often very low in the subsurface.
NITROGEN BALANCES: THE ENIGMA OF
MISSING NITROGEN
It is generally assumed that the N cycle is balanced globally by equal movements of N
out of the dominant atmospheric pool by fixation (both natural and anthropogenic) and
into the atmospheric pool by denitrification. This assumption is supported by the fact that
the concentration of N
2
in the atmosphere is relatively stable (although small changes in
the concentration of this dominant gas are hard to detect). Mechanistically, the balance is
facilitated by the movement of excess anthropogenic N from land to the ocean, which has
a vast potential for denitrification.
In addition to the global balance, it is also useful to consider N mass balances at smaller
(1 to
1,000,000 ha) scales, comparing inputs from atmospheric deposition and fertilizer to
hydrologic outputs in stream flow, allowing for the calculation of ecosystem “N reten-
tion.” These balances are useful tools for addressing questions about the fate of anthropo-
genic N, eutrophication of coastal waters, and changes in atmospheric chemistry.
A common feature of nearly all N mass balances is a high “retention” (calculated as
inputs minus hydrologic outputs;
Figure 7.9
). Given that the balances do not specify just
.
1600
N Sea
1400
NW European Coast
1200
NE US
1000
800
Mississippi
SE US
600
St. Lawrence
Baltic
y = 102.5 + 0.2x
r
2
= 0.73
p = 0.002
400
SW European Coast
200
N Canada
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Net anthropogenic N inputs per area (kg km
-2
yr
-1
)
FIGURE 7.9
Riverine N export versus net anthropogenic N inputs for nine regions. The regression suggests
that 75% of input is “retained” in soils or vegetation or lost as gas somewhere within these large watersheds.
(From
Howarth et al. 1996
.)
