Environmental Engineering Reference
In-Depth Information
TABLE 7.3
Pools and fluxes of N in an aggrading (55-year-old) northern hardwood forest (Hubbard Brook,
New Hampshire) and an ungrazed tallgrass prairie (Konza Prairie, Kansas)
Hubbard Brook
1
Konza Prairie
2
Pools (kg N/ha)
Soil organic matter
4700
6250
Inorganic N pool in soil
26
2
6
Plant biomass
532
60
250
Fluxes (kg N ha
2
1
y
2
1
)
Atmospheric deposition
10
10
20
N fixation
1
1
5
Net mineralization
70
10
40
Net nitrification
15
?
Hydrologic losses
4
0.1
0.3
Denitrification
0
10
0
10
Plant uptake
80
40
50
Litterfall
54
5
20
1
Values from
Bormann et al. (1977)
,
Likens and Bormann (1995)
, and
Melillo (1977)
.
2
From
Blair et al. (1998)
.
terrestrial systems, especially in systems with woody biomass (e.g., tree trunks) that repre-
sent another relatively large pool of N that is not actively cycling.
The dominant cycling fluxes in intact native terrestrial ecosystems are plant uptake, lit-
terfall, and mineralization, which roughly balance each other in an ecosystem that is not
growing (adding new biomass). Inputs in fixation and atmospheric deposition are low
(roughly 10-fold less) relative to these internal fluxes, even though deposition has been
roughly doubled by anthropogenic additions of reactive N to the atmosphere near many
densely populated regions. Hydrologic and gaseous outputs are also low relative to inter-
nal fluxes, reflecting the intense demand for N as a critical resource by plants and
microbes.
Ecosystem Development
A useful way to think about N cycling in terrestrial ecosystems is to think about
changes that occur during ecosystem development or plant succession following a distur-
bance that removes most of the living plant biomass, like clear-cutting a forest. Following
such a disturbance, plant uptake is reduced and microbial mineralization and nitrification
often increase as the loss of plant biomass (and uptake) leads to increases in soil moisture
