Principles of terrestrial ecosystem ecology.pdf

214 9. Terrestrial Nutrient Cycling
Solution Losses
Nitrate and dissolved organic nitrogen account
for most of the solution loss of nitrogen from
ecosystems. Undisturbed ecosystems that
receive low atmospheric inputs generally lose
relatively little nitrogen, and these small losses
occur primarily as dissolved organic nitrogen
(Hedin et al. 1995). Although nitrate is also
highly mobile, plants and microbes absorb most
nitrate before it leaches below the rooting zone
of intact ecosystems. Disturbance, however,
often improves the environment for mineralization
by increasing soil moisture and temperature
and reduces the biomass of vegetation
available to absorb nutrients (see Chapter 12).
At the Hubbard Brook Forest in the northeastern
United States, for example, all vegetation
was removed from an experimental
watershed to examine the consequences of
devegetation.There were large losses of nitrate,
calcium, and potassium to the groundwater and
streams when vegetation regrowth was prevented
(Bormann and Likens 1979) (Fig. 9.8).
Nutrient loss (g m -2 )
9
6
3
0
3
2
1
0
60
40
20
0
40
30
20
10
0
60 yr old
forest
Reforestation
prevented
Deforested
Control
Recovery
Calcium
Potassium
Nitrate
Particulate matter
1964 1966 1968 1970 1972 1974 1976
Once vegetation began to regrow, however, the
accumulating plant biomass absorbed most of
the mineralized nutrients, and stream nutrient
concentrations returned to their preharvest
levels. Additions of fertilizer nitrogen or nitrogen
deposition that exceed plant and microbial
nitrogen demands also increase nitrate leaching.
Increased nitrate leaching is one of the
characteristics of nitrogen saturation, the
changes that occur in ecosystem functioning
when anthropogenic nitrogen additions relieve
nitrogen limitation to plants and microbes
(Aber et al. 1998). Anthropogenic nitrogen
inputs are generally correlated with nitrogen
outputs via leaching (Tietema and Beier 1995,
Fenn et al. 1998) (Fig. 9.9).
Nitrate loss to groundwater can have important
consequences for human health and for
the ecological integrity of aquatic ecosystems.
Nitrite, which forms from nitrate under reducing
conditions, can reduce the capacity of
hemoglobin in animals to transport oxygen,
producing anemia, especially in infants.
Groundwater in areas of intensive agriculture
Figure 9.8. Losses of calcium, potassium,
nitrate, and particulate organic matter in
stream water before and after deforestation
of an experimental watershed at Hubbard
Brook Forest in the northeastern United
States. The shaded area shows the time interval
during which vegetation was absent due to
cutting of trees and herbicide application.
(Redrawn with permission from Springer-
Verlag; Bormann and Likens 1979.)