Geoscience Reference
In-Depth Information
10
Environmental cycles
and feedback
• Wetland productivity varies from quite high
to rather low depending on availability of
nutrients.
• Storage and cycling of nutrients in wetlands
are different from drylands or deep-water
environments.
• Human activities have greatly altered chemi-
cal cycles and nutrients in many wetlands.
10.1 Biogeochemical cycles
10.1.1 Wetland elements
Biogeochemical cycles involve all elements and
their chemical compounds at the Earth's surface.
These elements are contained in various por-
tions of the biosphere, atmosphere, hydro-
sphere, cryosphere, and lithosphere (Fig. 10-1).
Their residence times vary from minutes to mil-
lions of years, and their movements and trans-
formations involve energy. All these aspects
interface in wetlands, which support high bio-
diversity and are important storage points for
water and carbon. The chemistry of wetlands
depends upon many factors including the inl u-
ence of bedrock and soil, inl ow and outl ow of
surface and ground water, climate and vegeta-
tion, characteristics of surrounding terrain, and
human impacts. Complex and often poorly
understood feedback relationships are involved
in all these cycles. Mitsch and Gosselink (2007)
summarized the roles wetlands have in biogeo-
chemical cycles:
Key wetland elements include nitrogen, potas-
sium, iron and manganese (see chapters 4.5 and
5.3), sulfur, phosphorus, and carbon. The chemi-
cal status and transformations of these elements
depend primarily on the presence of oxygen
within wetland water and soil. Three zones are
typically developed from the surface downward:
aerobic, facultative, and anaerobic (Fig. 10-2).
The vertical position of these zones may l uctu-
ate seasonally, depending on the hydroperiod,
and over longer time frames due to changes in
drainage, vegetation cover, climate, and other
factors.
10.1.2 Nitrogen
• Wetlands may be sources, sinks, and trans-
formers of elements and their compounds.
• Chemical reactions may display strong sea-
sonal patterns related to hydroperiods, tem-
perature, and organic activity.
• Wetlands are often coupled with adjacent
environments through chemical exchanges
and feedback relationships.
Nitrogen is a major nutrient and is often a limit-
ing factor in l ooded wetlands or peatlands. The
ammonium ion (NH
4
+
) is the common reduced
form of nitrogen, called mineralized nitrogen,
which is biologically produced by decay and
decomposition of organic matter. Once ammo-
nium is formed, it may be used directly by
plants or anaerobic microbes, which convert it
