Environmental Engineering Reference
In-Depth Information
the system, as with changes in latent heat as water moves among solid, liquid, and gas
phases. Within a system, repeated changes that involve both abiotic and biotic binding are
often thought of as cycling or spiraling—that is, the repeated transformation from inor-
ganic to organic and from organic back to inorganic ( Figures 5.1 and 5.3 ; Box 5.2 ).
To illustrate move, stick, and change, we use an example of (simplified) sulfur cycling
in a northern hardwood watershed ecosystem, from atmospheric deposition through
leaching loss to stream water ( Figure 5.6 ). Sulfur, more specifically, sulfate (SO 4 2 2 ), has
been used as a tracer of atmospheric deposition (e.g., Weathers et al. 2006 ), because it
moves from atmosphere to the forest floor, and into ground and stream water with little
sticking, especially where acidic deposition is prevalent. However, the largest long-term
pools of sulfur in these northeastern ecosystems are in the biomass and soils. Hence, when
a forest is our frame of reference, we can see sulfur moving as sulfate from atmosphere to
Atmosphere Land
Move: physical (wind, air)
Change: chemical (oxidation,
exchange reactions)
Stick: (physical, biological)
Canopy Soil
Move: physical (leaf drop, water)
Change: chemical (oxidation,
exchange reactions)
Stick: (minimal)
Deposition
SO 2
H 2 SO 4
(NH 4 ) 2 SO 4
Gases, particles
rain, snow,
cloud, or fog
Soil ←→ Plant
SO 4 2- ,SO 2
+ associated
cations
H 2 SO 4
K 2 SO 4
MgSO 4
Move: physical (water, leaf drop)
Change: chemical (biologically mediated
inorganic ←→ organic S,
exchange reactions)
Stick: physical, biological
(adsorption, uptake,
recalcitrant SOM)
Throughfall
Adsorption
in soil
SO 4 2-
Org S
Soil solution Stream
Move: physical (water)
SO 4 2-
Uptake
Decomposition
SO 4 2-
+ associated cations
FIGURE 5.6 Sulfur moves, sticks, and changes as it cycles through a northern hardwood ecosystem. Sulfur may
enter a forest through atmospheric deposition of, for example, gaseous sulfur dioxide (SO 2 ), dissolved sulfuric
acid (H 2 SO 4 ), or dissolved or particulate ammonium sulfate ((NH 4 ) 2 SO 4 ). In the canopy, ions such as potassium
(K 1 ) and magnesium (Mg 2 1 ) may be more abundant than in the atmosphere, so these may be picked up and
become associated with sulfate (SO 2 2 ) in solution as water moves through the canopy. Sulfate moves readily
from atmosphere to forest, soil, and stream water with little sticking and usually few chemical changes. Sulfur
sticks when taken up by plants and adsorbed in soils, and this may or may not involve a change in form. Sulfur
can change between abiotic and biotic forms when it is taken up by plants or later when organic material is min-
eralized from organic to inorganic forms during decomposition (SOM
soil organic matter). The loss of sulfur
from a forested ecosystem is most often as sulfate in solution, although in anoxic soils sulfate reduction can lead
to the production of hydrogen sulfide gas (appendix).
5
 
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