Wetlands

January 18 , 2005

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What's the big deal about wet soil?
If you can squeeze a fist full of soil into a ball, you've got soil with no room for much oxygen. And that's bad news for most plants, because oxygen is an essential part of their respiration. They absorb the oxygen from the soil through their roots. These oxygen starved (anaerobic) soils often smell like sulfur or rotten eggs due to the bacteria that thrive under anaerobic conditions. The dampness also causes chemical reactions in the elements of the soil; for example, iron will oxidize and mottle the soil with orange.

Scientists recognize two major types of wetland soils: organic and mineral.

  • Organic soil has an obvious amount of decomposing plants; this kind of soil is often black or dark brown.
  • Mineral soil contains few decomposing plants; instead, it is comprised of materials such as clay, sand, or silt. Wetland mineral soils may be gray, greenish, or bluish gray; they also might be mottled.

Lucky for scientists, color charts exist that key soil color with the amount of water in the soil. The U.S. Natural Resource Conservation Service also publishes a list of hydric soils.

Soil content also determines the speed of draining. For example, water seeps through sandy soils faster than clay soils. Sand particles are large and irregularly shaped; they have more air pockets through which water can move. Clay particles are smaller and they can compress when wet; their smaller air pockets fill more quickly and completely.

Identifying Wetlands....Wet Roots
Plants that live in wet soil must adapt to the lack of oxygen. Reeds and sedges, found in freshwater wetlands, have hollow structures that enable the little oxygen they obtain to travel quickly through the plant. Mangrove trees, found in saltwater (marine) wetlands, have a tangle of roots that are exposed periodically to the atmosphere as the tides ebb and flow.

Cypress trees, which grow in freshwater swamps, have knobs or "knees" of root material that emerge from the water; scientists speculate that these knees absorb oxygen. The roots of floating plants, such as duckweed or lilies, dangle into water and absorb oxygen. Wetlands plants also must be efficient at absorbing other nutrients such as nitrogen and phosphorus.

For example, in wetlands, nitrogen is most often available as ammonia (NH4+), thus many wetlands plants have become super-efficient absorbers of ammonia.

Sometimes Wetlands Are Hard to Identify
OK, you're looking at a spot that someone insists is not a wetland. By looking at the plants, this person might be right—no wetlands plants grow here. But look again. This might be what is sometimes called a "problem wetland," one that is hard to identify due to its brief season or because it appears only during wet years. These types of wetlands include playas, potholes, vernal pools, and ephemeral ponds.

So take a look at the location: is it on high ground or a depression, slough, or other low ground? Sample the soil and study its color and composition. For example, some prairie potholes in the Dakotas may remain dry during a prolonged drought. When the rains and snow return, though, they reappear and continue to provide excellent habitat for ducks and other creatures.

Soil alone won't identify these potholes, though, because even dry grasslands contain soils that resemble hydric soils. To confirm the presence of a pothole during a drought, you need to look for a layer of decomposed vegetation in the soil, lots of mottling, and the remains of aquatic insects and other invertebrates near the surface.

Learn about How Scientists Mark Wetlands

Identifying Wetlands |
Marking Wetlands |
What Lives in Wetlands
|
Why Do We Need Wetlands? |

Types of Wetlands |

Common Names of Wetlands |
Five Subsystems of Wetlands |

What is a Wetland? | Where are all the Wetlands? |
Inland Wetlands | Wetlands for the Future | People and Wetlands |
Classroom Activities | Wetland Facts | Wetland Links |

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