Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 240

Nutrient cycles have general features that can be used to build a con-
ceptual framework of the way that materials move through the environ-
ment. Some of this framework is built on the requirements of organisms
and some on the constraints that redox exerts on chemical reactions. The
general features make it easier for students to understand nutrient cycles
and ultimately to link the cycles together in a larger, more comprehensive
view of aquatic ecosystems.
All organisms require nutrients, and they
can acquire them as dissolved inorganic
forms, organic forms, or both. For example,
people obtain carbon in the form of organic
molecules, but plants can obtain carbon from
either CO 2 or organic carbon sources. This
acquisition of nutrients is called assimilation
and generally occurs regardless of the redox
state of the environment. Likewise, organisms
excrete inorganic nutrients; the general name
for this is remineralization or regeneration .
The remineralization of organic C to CO 2 is
called respiration or fermentation in the car-
bon cycle. When reduced inorganic chemicals
are in an oxidized environment (e.g., an oxic
region), microbes can oxidize them, and meta-
bolic energy can be gained. Energy is gained
because the flux goes with potential energy.
Similarly, oxidized compounds can release en-
ergy when reduced in an anoxic environment.
Here, I introduce a method for dia-
gramming nutrient cycles to assist the reader
in understanding and remembering complete
nutrient cycles. It is important to keep in
mind the idea of potential energy in envi-
ronments of differing redox (see Chapter
11). A generalized nutrient cycle is presented
to illustrate the point (Fig. 12.5). In this di-
agramming method, the oxidized inorganic
forms are listed from right to left, oxic
processes are placed in the top half of the di-
agram, and anoxic processes are placed in
the bottom half of the diagram. This bound-
ary can be considered similar to the dividing
line between an oxic epilimnion and an
anoxic hypolimnion, the difference between
an oxic sediment surface and an anoxic por-
tion deeper in the sediments, or the differ-
ence between the oxic habitat outside a de-
caying leaf and the anoxic habitat inside.
Assimilation and heterotrophy generally oc-
cur regardless of the presence of O 2 , so they
are placed across the oxic/anoxic boundary.

Sidebar 12.3.
Global Emission of Methane
Related to Wetlands

Global methane concentrations have more than
doubled in the past 200 years. The absolute level
is less than that of CO 2 in the atmosphere, but
the rate of increase is greater. Methane con-
centration increases cause concern because
methane absorbs heat and acts as a green-
house gas. The exact causes of the increase
are not well established (Schlesinger, 1997), but
it is clear that human activities put a significant
amount of methane into the atmosphere. Such
human activities include burning, gas and coal
production, and ruminant production.
Wetlands are predominant sources of
methane, even more so when rice paddies
(agricultural wetlands) are considered (Table
12.2). Wetlands and rice paddies make up 54%
of the total methane produced globally.
Methane escapes wetlands because anoxic
processes lead to methanogenesis. Most of
the methane produced is intercepted and oxi-
dized by methanotrophic bacteria and this is
not accounted for in the budget in Table 12.2.
However, the stems of plants form a conduit
from anoxic sediments to the atmosphere, by-
passing the populations of methanotrophs
found at the oxic-anoxic interface in the sedi-
ments (Joabsson et al., 1999).
Understanding carbon cycling in wetlands
will aid future efforts to predict atmospheric
methane concentrations. For example, it is un-
known what will happen to methane produc-
tion in high latitudes if the permafrost thaws
and significant numbers of new wetlands are
formed. It has also been demonstrated that in-
creased CO2 can stimulate methanogenesis in
wetlands (Megonigal and Schlesinger, 1997),
complicating the relationships among climate
change, CO 2 , and CH 4 .