Introduction - The Biogeochemistry of Submerged Soils - page 4

nutrients and carbon, whereas uplands tend to be sources and aquatic systems
sinks. Three types of wetland are distinguished based on hydrology (Figure 1.2):
a. fluxial , which receive water wholly or in part from surface flow, such as in
runoff or streams;
b. phreatic , which receive water from groundwater that rises to the soil surface
for at least part of the year; and
c. pluvial , which receive water entirely from rainfall.
In fluxial wetlands water flowing in from neighbouring upland brings with it
sediment and nutrients which are only slowly lost to deepwater areas downslope,
and may be supplemented by seasonal inflow from deepwater areas. Because of
the net inflow of nutrients, the abundance of water, and beneficial changes in the
soil resulting from chemical reduction under anoxia, fluxial wetlands are among
the most productive ecosystems on Earth. By contrast pluvial wetlands rely on
nutrients brought in by rainfall or fixed biologically from the atmosphere, and they
therefore tend to be much less productive. Phreatic wetlands are intermediate.
As sources, sinks and transformers of matter and energy, wetlands have impor-
tant roles in element cycles at local, regional and global scales. They contribute
to the global stability of carbon dioxide, methane and sulfur in the atmosphere
and of available nitrogen and phosphorus in surface waters, and they are impor-
tant regionally as sinks for organic and inorganic pollutants released into them
accidentally or otherwise. These topics are introduced in the following sections;
all are returned to in greater detail later in the topic.
Carbon Balances in Wetlands
Table 1.3 shows the net primary production of different wetlands compared with
upland and aquatic ecosystems. The generally greater productivity of wetlands is
evident. Net primary production (NPP) is the gross rate of carbon fixation in pho-
tosynthesis less the rate of loss in plant respiration. The chief factors governing
NPP are radiation, temperature, water, nutrients and toxins. Hence for a given type
of wetland, NPP tends to increase from polar to tropical regions as incident radi-
ation and day length increase; correspondingly nutrients and temperature become
increasingly limiting. There are of course interactions between these changes. For
example, the greater productivity of temperate compared with tropical ricefields
on a per crop basis shown in Table 1.3 arises because of interactions between
radiation and temperature: in temperate rice areas with high radiation during the
growing season, low night-time temperatures result in lower respiratory losses
compared with tropical areas and hence greater net productivity.
Because of their often high biological productivity and low rates of decompo-
sition under anoxia, wetlands are one of the largest terrestrial sinks for carbon.
They account for about a third of the soil carbon globally (Table 1.4). However
there are large differences between wetland types. Organic wetland soils tend