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BIOGEOCHEMICAL CYCLES

The basic elements that occur in living organisms move through the environment in a series of naturally occurring physical, chemical and biological processes known as biogeochemical cycles. The cycle generally describes the physical state, chemical form, and biogeochemical processes affecting the substance at each point in the cycle in an undisturbed ecosystem. Many of these processes are influenced by microbial populations that are naturally adapted to life in either aerobic, oxygenated, or anaerobic, oxygen free, conditions. Because both of these conditions are readily created by varied and fluctuating water levels, wetlands support a greater variety of these processes than other ecosystems.

There is usually a physical state, chemical form, and location in the cycle in which nature stores the bulk of the various chemical elements. Pollution occurs when the cycle is sufficiently disturbed that an element is caused to accumulate at some point in the cycle in an inappropriate physical state, chemical form, location or amount disrupting environmental balance.

In the nitrogen cycle, for example, the bulk nitrogen is stored as nitrogen gas in the atmosphere. The nitrogen cycling process in wetlands involves both aerobic and anaerobic conditions. Nitrogen in the form of ammonium (NH 4) is released from decaying plant and animal matter under both aerobic and anaerobic conditions in a process known as ammonification. The ammonium then moves to the aerobic layer where it is converted to nitrate (N03). p30pic2Nitrate not taken up by plants or immobilized by adsorption onto soil particles can leach downward with percolating water to reach the groundwater supply or move with surface and subsurface flow. Nitrate can also move back to the anaerobic layer where it may be converted to nitrogen gas by denitrification, a bacterial process, and subsequently returned to the atmosphere.

If both aerobic and anaerobic conditions were not available, some of the cycle processes would cease and pollutants could accumulate. In wetlands, anaerobic condition are amply provided by flooding and by saturated soils. However, the oxygen requiring processes take place in a thin oxidized zone usually existing at the soil surface. This layer may be only a fraction of an inch thick and is present even when the wetland is submerged. In many wetlands the water table fluctuates 12 to 18 inches each year with the summer level averaging between 4 and 18 inches below the surface of the soil. This zone of aeration is often called the active layer or in Russian soil terminology, where it was first used, the acrotelm.

Phosphorus, sulfur, iron, manganese and carbon also move through the wetland ecosystem in complex cycles. Sulfur and carbon, like nitrogen, have gaseous cycles. As a result of the biogeochemical cycle processes, sulfides and methane are released into the atmosphere attended by the smell of rotten eggs and swamp gas respectively. Phosphorus, however, has a sediment cycle with excess phosphorus being tied up in sediments, peat in organic wetlands and clay particles in mineral wetlands. However, although phosphorus retention is an important attribute of wetlands, sediment attached phosphorus is subject to resuspension and movement with water when sediments are disturbed.
Nitogen cycling in wetlands
The cycles are similar in that they provide storage for excess elements and require a certain amount of time to complete the chemical processes. The cycle processes also require the varying environments provided by aerobic and anaerobic conditions. In closed systems, the processes take place within the wetland. In open systems, like riparian wetlands, many elements can be imported from or exported to adjacent systems with surface and groundwater flows or flooding.

To avoid changing the natural biogeochemical function, it is important that the hydrology of the wetland, the inflow, outflow and residence time of the water, remain relatively undisturbed. It is also necessary to minimize disturbance to the aerobic zone of saturated soils. However, even with minimal disturbance, wetlands will continue to function as net receptors (sinks) or net exporters (sources) of various elements primarily due to seasonal and other natural fluctuations in the biogeochemical cycle processes.

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