A biochemical cycle is the transport and transformation of chemicals in ecosystems. These are strongly influenced by the unique hydrologic conditions in wetlands. These processes result in changes in the chemical forms of materials and also the movement of materials within wetlands. These, in turn, determine overall wetland productivity.
Materials cycle both within the wetland and between a wetland and its surroundings. Few of these processes are unique to wetlands, but some are more dominant in wetlands than in upland or aquatic ecosystems. For example, anaerobic conditions are the norm in wetlands, whereas they are unusual in both terrestrial and aquatic systems.
The cycling between the wetland and outside refers to the degree to which chemicals are transported to or from wetlands.
open System-abundant exchange of materials between the wetland and surrounding habitats (typical when the water moves)
closed System-little exchange of materials between the wetland and surrounding habitiats (typical when water is more stagnant)
When water fills the pore spaces in soil, the rate at which oxygen can diffuse through the soil is greatly reduced and anaerobic (or reduced as opposed to oxidized) conditions result in several hours to several days. This lack of oxygen prevents plants from carrying out normal aerobic root respiration and affects nutrient availability and the presence of toxic materials in the soil.
Usually on the surface of wetland soil there is a thin layer of oxidized soil. How thick this is depends on:
1. rate of oxygen transport across the atmosphere-surface interface
2. population of oxygen consuming organisms
3. photosynthetic oxygen production by algae in the water
4. surface mixing
This aerobic layer would tend to be reddish because of the ferric iron (Fe+3 = oxidized form) versus the gray-green soil beneath, the ferrous iron (Fe+2 = reduced form)
Note: In oxidation, oxygen is taken up or hydrogen is removed or the chemical gives up an electron (remember that an electron has a negative charge)
Fe++ ---> Fe+++ + e- or H2S ---> S-2 + 2H+
Reduction is the opposite where oxygen is given up, hydrogen is gained, or an electron is gained
Generally wetland soils are acidic while mineral soils tend to be neutral to alkaline.
Nitrogen is usually the limiting nutrient meaning it is the one in short supply so it limits plant growth. If there were more, plants would grow more.
Iron is oxidized to its ferric form in aerobic soils and this gives a characteristic red color. This process can be speeded up by microbial activity of so called iron bacteria. Iron is reduced to the ferrous form in anaerobic environments where it has a characteristic gray or gray-green color (gleying or gleyization). Interestingly, there are iron bacteria that can oxidize ferrous iron from anaerobic groundwater back to the ferric form. These "bog iron" deposits form the basis of the ore that is used in the iron and steel industries. Also, ferrous iron can be toxic to plants in high concentrations. It diffuses to the surface of the roots of wetland plants where it can be oxidized by oxygen leaking from root cells. The resulting iron oxide (rust, essentially) coats the roots and acts as a barrier to nutrient uptake.
Sulfur is very common in wetlands, and the hydrogen sulfide (H2S) characteristic of anaerobic wetland soils can be very toxic to plants and microbes. This is released when soils are disturbed and accounts for that rotten-egg wetland smell.
In wetland soil that contain high concentrations of ferrous iron, Fe2+, sulfides can combine with the iron to form ferrous sulfides which give the black color characteristic of many anaerobic wetland soils. One of the common mineral forms of this is pyrite, FeS2, which is commonly found in coal deposits.
Photosynthetic bacteria such as the purple sulfur bacteria found in salt marshes and mud flats can produce organic matter using sulfur and light:
CO2 + H2S + light ---> CH2O + S
Welands depend on decay of organic matter for nutient cycling (remember that the detrital food web is the main source of energy flow in most wetlands). Aerobic respiration is limited by anaerobic conditions but several anaerobic processes are at work:
a. fermentation: This is the breakdown of carbohydrates to alcohol.
C6H12O6 ---> 2CH3CH2OH (ethanol) + 2CO2
This is carried out by facultative or obligae anaerobes and is one of the major ways that high molecular weight (big) carbohydrates are broken down to low molecular weight (small) organic compounds which can then be taken up by the smallest microbes.
b. methanogenesis: Methanogenesis also breaks down carbohydrates. Methanogens are bacteria that use CO2 to produce CH4 (methane). This is then released when sediments are disturbed. The production of this has been extensively studied because of implications for global warming.
Phosphorus is one of the most important chemicals in ecosystems because it is vital for plant growth. It is often a limiting nutrient but wetlands can also be very good at retaining phosphorus. At any one time, most P in a wetland is tied up in organic litter and peat and inorganic sediments. Since it is a pollutant in high concentrations this ability of wetlands to retain it can be important and is one reason why wetlands can be good for wastewater treatment. Since P is a major ingredient in fertilizer, it is also a reason why it's good to leave a boundary of wetlands between farm fields and rivers, streams, or lakes.
There is no gaseous form of P so the cycle is sedimentary.
Chemical Transport in Wetlands
Materials enter wetlands via the same geologic, hydrologic, and biologic pathways typical of other ecosystems.
a. Geologic includes weathering and erosion
b. Biologic includes photosynthetic uptake of carbon, nitrogen fixation, and biotic transport of materials by mobile animals such as birds.
c. Except for gaseous exchanges like photosynthesis and nitrogen fixation, elemental inputs to wetlands are dominated by the hydrologic inputs:
Generally inputs from this are very dilute, but with increasing human inputs, such as those resulting from burning fossil fuels, there can be high levels of sulfates and nitrates (this would be "acid rain")
2. Rivers, Streams, and Groundwater
When precipitation reaches the ground in a watershed it infiltrates into the ground, passes back to the atmosphere through evapotranspiration, or flows along the surface as runoff. Flowing on or through the ground changes the content from the original precipitation. This is basically erosion, except in the case of human interference. For example, sewage effluent or runoff through farms or construction or logging sites can greatly increase concentrations of sediments, nutrients (fertilizers), herbicides, and pesticides.
Chemical Mass Balances of Wetlands
A mass balance of a habitat is basically the inputs versus the outputs.
Inputs include precipitation, surface water, groundwater, and tides. Outputs equal surface water, groundwater, tides, and deep sedimentation.
(from Mitsch and Gosselink, 1993)
Within the wetland there is intrasystem cycling among pools or standing stocks.
1. Wetlands can be sources, sinks, or transformers of chemicals
source = supplier of nutrients to other habitats
sink = "holder" of nutrients from other habitats
transformer = takes nutrients in one form and gives them up as another
2. Typically there are seasonal patterns. For example, during the temperate growing season, certain chemicals may be retained better in plant tissue.
3. Wetlands are often coupled to adjacent ecosystems through chemical exchanges that affect both systems. For example, wetlands can retain excess nutrients and this may benefit downstream aquatic systems. Or an upstream aquatic system may supply nutrients to a wetland.
4. Some wetlands are extremely productive and some aren't, depending on nutrient supply.
(from Mitsch and Gosselink, 1993)
5. Nutrient cycling in wetlands is different from both terrestrial and aquatic systems. More nutrients are tied up in sediments and peat in wetlands than in most terrestrial systems. Deepwater aquatic systems have autotrophic activity more dependent on nutrients in the water column than nutrients in the sediments.
6. Anthropogenic changes have led to considerable changes in chemical cycling in many wetlands.
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