Unit 5.3 Wetlands and the Carbon Cycle [#]

Question 0

Outline how inundation leads to the unique characteristics of a wetland soil.

Answer 0

Wetland soils are defined in the International Soil Classification system as hybrid soils: ‘soils that formed under conditions of saturation, flooding or pounding long enough during the growing season to develop anaerobic conditions in the upper part’.

In organic soils (those containing at least 20% organic carbon) the decomposition rate is limited by the absence of oxygen, allowing this organic matter to build up in peatlands.

Wetland mineral soils are often distinguished from non-wetland mineral soils by the presence of characteristic features associated with inundation by water. These features include gleying or mottling.

In the absence of oxygen, soil organisms are forced to use oxidising agents other than O2, including NO3, SO4, long-chain organic molecules, and CO2. The organic carbon is cycled through a chain of microbial and chemical transformations, and reduced products such as nitrous oxide (N2O), hydrogen sulfide (H2S), ethanoic acid and methane (NH4) are formed.

Question 1

Using your own words, define ‘wetland’.

Answer 1

A wetland is an ecosystem that arises when inundation by water produces soils dominated by anaerobic processes and forces the biogas particularly rooted plants, to tolerate flooding.

Question 2

Describe gleying of hydric soils.

Answer 2

Gleying: bluish-grey clay-rich patches which may occur in a distinct horizon.

Gleying indicates the loss of more oxidised forms of iron, Fe(III), and manganese, Mn(IV) or Mn(III), which together give soil its typical red-brown colour. The more reduced forms of these metals (Fe2+ and Mn2+) generally form soluble compounds, and either leach out of the soil, leaving behind the natural colour of the parent material, or remain in the soil, leaving a characteristic blue-grey colour. Thus the colour of a gleyed horizon can derive from either the colour of reduced ions remaining in the soil, or the colour of the uncoated sand and silt particles from which iron and manganese have been removed.

Question 3

Describe mottling of hydric soils.

Answer 3

Mineral wetland soils may also show mottling: discrete areas of gleying occurring together in the soil profile with areas of red-brown oxidised soil. Mottling indicates that oxidising and reducing conditions have occurred intermittently, corresponding with periods of unsaturated and saturated conditions.

Question 5

Describe the overall adaptations of wetland biota to the major stresses of toxic compounds.

Answer 5

Some of the oxygen released to roots of plants may ‘leak’ out of the roots into the surrounding soil, producing local aerobic zones. Reduced compounds that build up in the wetland soils may be oxidised in these areas, effectively detoxifying them.

Some plants may accumulate and isolate toxic compounds in areas such as vacuoles, vascular support tissue or even senescing cells or tissues. Thus locked away, the compounds do not influence the metabolism of healthy cells and tissues.

Some wetland plants can biochemically convert dissolved forms of toxic compounds into gaseous forms that then diffuse out of the plant.

Question 6

Describe the overall adaptations of wetland biota to the major stresses of salinity.

Answer 6

• some cells, particularly close to the root-water interface, maintain high osmotic concentrations
• active pumping of salt out of cells
• developing resistant ‘barrier cells’ in areas such as the cortex where water passes through, thus preventing diffusion of salt into other cells
• active excreting of salt through leaves or roots
• enhanced physiological tolerance to high salt levels

Question 8

Describe the overall adaptations of wetland biota to the major stresses of nutrient limitations.

Answer 8

• thick leaves and woody stems, which reduce any leaching of nutrients out of the vegetation
• evergreenness, which reduces the loss of nutrients in shed leaves
• high root biomass and deep rooting to access a greater area of potential nutrients
• nutrient translocation, to move valuable nutrients into parts of the plant that are not shed in autumn or are not at high risk of being eaten, such as stems and roots
• ‘plasticity’ in nitrogen use: the ability to use different nitrogen species, including NO3, NH4 and perhaps organic nitrogen
• nitrogen-fixing bacteria on roots, which increase levels of nitrogen available to the plant

Question 9

Describe the overall adaptations of wetland biota to the major stresses of oxygen limitation.

Answer 9

Anaerobic bacteria are among the oldest life forms, having evolved when the atmosphere contained little oxygen.

Aerenchyma are tissues which contain air spaces, found in the cortex of stems and roots. These allow oxygen to diffuse from the stem into the roots (which would otherwise depend on oxygen dissolved in soil solution).

Aquatic animals may have physiological adaptations, such as mechanisms to increase the efficiency of oxygen collection, or reduced oxygen demand of tissues and organs.

Aquatic animals may also display behavioural adaptations, such as migration or going into a low-activity dormant phase when conditions are harsh.

As seeds and seedlings are especially vulnerable to O2 levels, plants may time seed production with low water levels. Many mangroves produce small ‘plantlets’, complete with leaves and tiny roots instead of seeds, giving the progeny a head start in establishing themselves.

Question 10

Explain what is meant by a ‘perched’ wetland.

Answer 10

Accumulated organic matter in these wetlands can serve as a kind of plug, impending downward percolation of water and creating a ‘local water-table’ above the regional water-table.

Water flow in perched wetlands can be very slow.

Question 14

Explain how the amount of water (especially the height of the water table above or below the surface) affects the chemistry and biology of wetlands.

Answer 14

In wetlands, the level of the water table roughly marks the transition between an oxygen-rich environment and an oxygen-depleted environment.

Question 15

Explain the differences between bogs and fens.

Answer 15

Both bogs and fens are peatlands.

• Bogs are isolated from groundwater and receive little throughflow water; their main source of water is precipitation. Bogs have low pH, low nutrients and relatively low species diversity.
• Fens receive a significant proportion of groundwater or throughflow water, but not so much that the accumulation of peat is prevented. The inputs of nutrients and basic cations from water flowing through fens leads to a higher pH and greater overall nutrient levels than in bogs. Consequently species diversity in fens is usually higher.

Question 16

Explain how the rate of flow of water can affect the chemistry and biology of wetlands.

Answer 16

Since external sources of water are often fully oxygenated, rapidly flowing water can allow oxygen to reach plant roots and often permeate deep into wetland soils.

Flowing water can also flush out the products of anaerobic decomposition, which can build up to toxic levels.

Question 18

Explain how the water quality can affect the chemistry and biology of wetlands.

Answer 18

In addition to oxygen, water flowing through wetland soils can carry dissolved organic carbon and organic nitrogen, major ions such as calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+), nitrate (NO3/-) and sulfate (SO4/2-), and more complex organic compounds. Many of these ‘dissolved solids’ are important plant nutrients, and different sources of water typically contain different concentrations of dissolved material.

• precipitation generally has low levels of dissolved solids, with a relatively low pH (usually around 5 or 6)…
• because groundwater and throughflow water generally are richer in basic cations and nutrients than is precipitation, plant productivity is generally higher in wetlands receiving a high proportion of subsurface flow…
• throughflow or groundwater that passes through soils affected by human activities can also, however, bring an excess of nutrients, pesticides and herbicides, or trace metal that may potentially be toxic to wetland biota. Water flowing through estuaries also brings salts that wetland biota must be adapted to in order to survive…

Question 19

Describe how the British NVC classification system and the North American classification systems differ with regard to wetlands.

Answer 19

Confusingly, the British scheme uses the same terms as other classifications, such as ‘swamp’, to mean something quite different. In the North American system a ‘swamp’ denotes a woodland, whereas in Britain it is a marsh community without trees!

North American ‘bog’: mire (M), which are nutrient poor, with the water source usually dominated by precipitation, and vegetation often dominated by Sphagnum mosses.

N. A. ‘marsh’ and ‘fen’: swamp (S), which have some water from throughflow or groundwater and so are richer in nutrients than mires.

N. A. ‘swamp’: wet woodland (W1 - W7) of alder, willow and poplar.

Question 20

Give the three major classifications of wetlands and describe the differences between them based on their hydrology, vegetation and soils.

Answer 20

North American classification.

Peatland: wetlands that accumulate significant amounts of partially decayed plant material, or peat. Peat formation requires significant organic matter production, in excess of decomposition, and restricted or impeded drainage,which limits the removal of dissolved or particulate carbon. [bogs and fens]

Marshes: a wetland community that is dominated by non-woody, vascular plants, and does not show a deep accumulation of peat. Marshes are characterised by a significant flow of subsurface water and often have some contact with the regional groundwater-table. This often leads to a rich nutrient status. [freshwater, saltwater]

Swamps: wetland communities that are dominated by trees, with waterlogged but generally non-peaty soils. They are often flooded, sometimes deeply and for long periods, although most swamps show alternating periods of flooded and non-flooded conditions. [freshwater, salt water including mangroves]

Question 21

Outline the major vegetation adaptations to the nutrient-poor peatland environment.

Answer 21

Lose fewer nutrients: evergreenness

Gain additional nutrients: carnivorous (e.g. sundew, pitcher plant)

Question 22

Describe the two major ways in which peatland form.

Answer 22

Terrestrialisation: occurs when shallow lakes are gradually filled by the accumulation of organic matter deposited on the lake floor and from the encroachment of peatland plant vegetation from the sides. The organic matter comes from internal lake production, such as algae, from material that has fallen into the lake from the shore (leaves etc), and from the organic matter or sediment carried into streams and rivers that feed the lake. Herbaceous plants such as reeds and sedges, and mosses such as Sphagnum, can form a mat floating on the lake surface, which gradually thickens as these plants die and others grow on top of them. The thick mat can eventually become colonised by other plants, including shrubs and trees, forming a quaking bog. These are so called because the surface undulates, or quakes, when walked upon. Eventually the lake fills from both above and below. Lakes are temporary landscape features; most have a lifespan of only a few thousand years before they are completely filled.

In paludification, terrestrial vegetation is blanketed by peatland vegetation due to a change in the site hydrology. This can occur if the climate changes, for instance becoming cooler and/or wetter (thereby reducing the rate of evapotranspiration, a major mechanism for water removal). Paludification can also occur if the local hydrology is altered such that water permanently floods previously dry land, for example if a river course is changed through flood scouring, or if sediments of a river are dammed due to activities of humans or beaver. Logging can lead to paludification since trees remove large amounts of water through evapotranspiration.

Question 23

Define (a) a bottomland forest, and (b) a vernal pool.

Answer 23

A bottomland forest is a floodplain forest that is only intermittently flooded. Such forests can grade into swamps or marshes if inundation increases. Common moisture-loving tree species in bottomland forests include willow, alder and poplar.

Vernal pools are small depressions (about 50-5000m2), found particularly in northern latitude forests, that only exist as pools for a few weeks each year. They form when a large snowpack melts and saturates the soil in early spring. With low rates of evapotranspiration because of cool temperatures, and restricted runoff, the meltwater accumulates in small ponds dotted through the landscape. Vernal pools disappear by late spring, when increasing temperatures and growing vegetation increase evapotranspiration, or water percolates into the soil and drains away.

Question 25

List some of the environmental stresses that face marsh vegetation, and describe the major plant adaptations shown by the stresses.

Answer 25

Marsh plants are exposed to perhaps the widest variety of conditions of all wetland plants. Since marshes are often alternately flooded and dry, plants must be able to tolerate deficits in oxygen, but must also tolerate oxidation and drying of soil, herbivory, competition and fire when water levels are low, and freezing. Saltmarsh plants must also tolerate extremes in salinity, waves, tides, tidal floods etc.

Marsh plants have adapted to these conditions by strategies including producing thick roots to act as food stores, or dense roots to anchor the plant and deter the establishment of other plants.

Question 26

Describe the adaptations shown by swamp trees to a waterlogged environment.

Answer 26

Pneumatophores are root adaptions, which bring oxygen directly to the roots from the atmosphere.

Prop roots are roots connected to the main stem of the tree, but which arch out over the surface of the water before descending underground. They are common in many mangrove species, and can provide both physical support and oxygen to the tree.

In some species, the main function of the prop root (or ‘buttress roots’) is support and stabilisation.

Question 29

Outline how carbon enters wetland ecosystems, and how carbon is transferred from wetland vegetation into wetland soils.

Answer 29

The major source of carbon to the wetland ecosystem is the fixation of atmospheric CO2 by vegetation. Carbon can also enter the wetland from the surrounding environment as DIC, DOC and POC.

• dissolved inorganic carbon (DIC) is the product of respiration, released through roots. This is dissolved CO2 which, depending on pH may be CO2, HCO3 or CO3.
• dissolved organic carbon (DOC) are often waste products of plant metabolism and are known as root exudates.
• particulate organic carbon (POC) are, simply put, parts of dead leaves or roots which enter the soil.

Question 30

Describe five major pathways of microbial respiration of organic carbon in wetland soils, and indicate the circumstances under which each pathway occurs.

Answer 30

The organic carbon from respiration that is incorporated in wetland soils as POC and DOC is decomposed by anaerobic organisms, fermenting bacteria and fungi, denitrifying bacteria, sulfate-reducing bacteria and methanogenic bacteria. In the process, more complex carbon compounds are broken down into simpler compounds, and the soil solution is progressively depleted of O2, NO3 and SO4.

• aerobic respiration occurs in soil above the water table, transforming DOC into DIC and atmospheric carbon..
• fermentation of POC and DOC into simpler compounds: in reduced environments, the more oxidised forms of carbon are effectively used to oxidise more reduced forms of carbon in the same compound. The simpler compounds are then available for other anaerobic bacteria to act upon..
• POC and DOC may be oxidised through denitrification (the reduction of NO3 into N2 or the greenhouse gas N2O). NO3 is chiefly found in runoff from fertilised agricultural fields, as it is the product of oxidisation of NH4. Except in wetlands receiving runoff that is relatively high in NO3, nitrification is not usually a major pathway of carbon breakdown in wetlands…
• DOC can be oxidised through sulfate reduction, in which SO4 is reduced by microbes. Inefficient, but common in saltwater wetlands, where seawater brings a relatively high and constant supply of SO4…
• methanogenesis: of which there are two types. In the first, ethanoic acid is reduced to produce CO2 + CH4. In the second, CO2 is reduced to produce CH4…

Question 31

Outline how carbon is removed from wetland ecosystems.

Answer 31

Since the concentration of CO2 in soil solution is higher than in the atmosphere, most of the CO2 ultimately diffuses out of the soil solution and into the atmosphere:

• if the rate of CH4 is high enough, tiny bubbles of gas form that grow as they rise through the soil and collect more gas, including CO2, to eventually break the surface in a process called ebullition.
• methanotrophic bacteria may convert some CH4 into CO2, as it diffuses upwards towards the surface.
• the aerenchyma tissue of stems and roots of vascular wetland plants can provide a shortcut for CH4 and CO2 to bypass the soil and pass directly into the atmosphere.

The carbon fixed by NNP may also be removed from wetlands in leachate water, mainly as DIC, DOC or POC:

• when the leached water reaches the surface in streamflow or in a lake, some of the DIC may be released back to the atmosphere as CO2, and some may remain dissolved, potentially available for photosynthesis by aquatic vegetation.
• unlike DOC, the net loss of CH4 in leachate is generally low - it is only poorly dissolved and most of that is oxidised rapidly upon contact with throughflow or groundwater.

Question 32

Describe the major similarities and differences between carbon accumulation in (a) peatlands and (b) marshes and swamps.

Answer 32

Net carbon accumulation in wetlands is the difference between the carbon gained through primary production and in water entering the wetland, and carbon lost as CO2 and CH4 through decomposition and emission to the atmosphere, or simply physical removal of carbon in water flowing out of the wetland.

Both peatlands and swamps/marshes accumulate carbon, but due to a different balance of processes. In general, swamps and marshes accumulate carbon because they have vey high rates of carbon input (and high, but not as high, rates of carbon output). Bogs and fens, on the other hand, generally accumulate carbon because they have very low rates of carbon output.

Question 33

Outline some of the major environmental factors that influence the carbon balance in wetlands.

Answer 33

Temperature: influences rate at which plants grow, and also the rate of bacterial activity…

CO2 levels: influence rate at which plants grow, as CO2 is an ingredient in photosynthesis…

Precipitation and water level: the balance between aerobic and anaerobic conditions…

Nutrient levels: influence plant growth…

Fire: releases carbon…

Question 34

Describe the importance of wetlands in producing the greenhouse gas methane and explain the conditions in a wetland that would favour the conversion of methane to CO2 by methanotrophic bacteria.

Answer 34

Wetlands are a major source of atmospheric CH4, with natural wetlands and rice agriculture together contributing about 33% of the global total.

Methanotrophic bacteria in soils can oxidise methane wherever they come into contact with oxygen. When the water-table is below the surface, methanotrophic bacteria colonise the aerobic zone and oxidise CH4 diffusing through on its way to the surface. The proportion of CH4 removed by oxidation in wetlands is thus related to the depth of the water-table. If the water-table is well below the surface, much of the methane diffusing to the surface is oxidised to CO2 and little CH4 is released. If the water-table is close to the surface, much of the methane escapes oxidation and is released to the atmosphere.

Question 35

Compare (without quoting actual values) the amount of carbon stored in wetland soils to that stored in other major reservoirs of the global carbon cycle, such as rocks, non-wetland soils, and the atmosphere.

Answer 35

The amount of carbon stored in rock is over 30,000 times more than the amount stored in soil, but the amount of carbon that moves into or out of soil each year is about 240 times greater.

The amount of carbon stored in natural wetlands soils is about as much as in the atmosphere. It is also about as much carbon as is stored in non-wetland soils, despite only covering 5% of the area.

Question 36

Explain why the carbon stored in wetland soil is highly important in the global carbon cycle.

Answer 36

Wetland soil is a globally important reservoir because it can respond so rapidly to changes in climate.

Question 38

Explain the significance of rice paddies in the global carbon cycle, and how management can be improved.

Answer 38

Rice agriculture is a major source of CH4, accounting for about 11% of the global CH4 emissions budget. Rice paddies cover an area only about 25% the size that natural wetlands cover, but even so, they release in total about half as much methane. Methane emission per unit area from a rice paddy is thus about two times higher than the mean methane emission from a natural wetland.

Managing rice plantations to mitigate CH4 release can have economic benefits. Advantageous management strategies include reducing the amount of irrigation water (therefore increasing oxidation of CH4 into CO2), reducing the intensity of compaction, and adding sulfate fertiliser (allowing sulfur-reducing bacteria to outcompete methanogenesis).

Question 39

Given appropriate information, perform basic calculations such as unit conversion and ‘scaling-up’ from the local scale to a regional or global scale.