Environmental Engineering Reference
In-Depth Information
Site Controls
Much of the variation in N cycling among terrestrial ecosystems can be understood
by considering how inherent site resources control interactions between water, plant com-
munity composition, and N dynamics (e.g., Pastor et al. 1984; Pastor and Post 1986 ). Within
any given region, soil texture exerts strong control over water availability as coarse-textured
(sandy) soils hold less water than more fine-textured soils. Water stress favors the develop-
ment of plant communities with leaves containing compounds (e.g., waxes) that contribute
to the retention of water. These compounds increase the C:N ratio of the leaves, which shifts
the balance between N mineralization and immobilization toward immobilization, leading
to low inorganic N availability for plant uptake or hydrologic losses. The decrease in
available N creates positive feedback toward plant species with high C:N ratios, leading to
further decreases in N availability ( Figure 7.7 ). Thus over time, marked patterns in plant
community composition and N cycling across the landscape develop and persist. A major
CO 2
Litter
Decomposition
Wood
Leaves
and
roots
Death
Sunlight
Nitrogen
availability
Geomorphology,
soil texture
and depth
Net
primary
production
Canopy
Water
availability
Light
availability
Climate
Birth
of
trees
Temperature
Geological processes
Microbial processes
Demographic processes
Ecosystem feedback loops
FIGURE 7.7 Conceptual model showing how site characteristics (geomorphology, soil texture, and depth)
influence water availability, which in turn influences litter quality and nitrogen availability in temperate forests.
(From Pastor and Post 1986 .)
 
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