Wetlands soil oxygen

The presence of iron oxyhydroxide coatings (i.e., Fe plaque, often dominated by ferrihydrite) on the surface of wetland plant roots is visual evidence that subsurface iron oxidation is occurring in otherwise anoxic wetland soils and sediments. Oxygen delivered via radial O2 loss may react with reduced iron in soil pore spaces to form oxidized iron that can be deposited on the plant roots as Fe plaque. Despite a long history of observing Fe plaque on wetland plant roots and understanding the basics of plaque formation [i.e., reaction of plant-transported O2 with Fe(II) in soils and sediments], it was largely assumed that plaque formation is predominately an abiotic (i.e., chemical) process because the kinetics of chemical oxidation can be extremely rapid (Mendelssohn et al., 1995). However, recent evidence has demonstrated that populations of lithotrophic FeOB are associated with Fe plaque and may play a role in plaque deposition. 

Besides restricting the supply of oxygen, excess water causes other important effects in wetland soils. Some of these effects are ... 

Upland soils can be transformed into wetland soils as a result of excessive rainfall, poor drainage, and high oxygen demand in the soil. Under these conditions, oxidized forms are reduced as a result of the respiratory requirements of anaerobic bacteria. Similarly, when wetland soils are drained, they function as upland soils, and under these conditions many of the reduced compounds are oxidized by either chemical or biochemical reactions. 

In upland/drained soils, oxidized forms of chemical species dominate the system, whereas in wetland soils reduced forms dominate the system (Figure 3.6). During flooding or in the absence of molecular oxygen, the oxidized forms are converted into reduced forms, through several microbially mediated catabolic processes. Under drained conditions, many of the reduced forms are converted into oxidized forms through chemical and biological processes. Presence of reduced forms indicates soil wetness or anaerobic soil conditions, which are used as indicators of hydric soil identification. 

Fungi, which are active in upland environments, cease to exist in wetland soils. This is primarily due to the absence of oxygen and alteration in soil pH (acid to neutral) under anaerobic conditions. Overall, microbial biomass decreases under saturated soil conditions. The metabolic activities of anaerobic bacteria depend on alternate electron acceptors, such as oxidized forms of nitrogen, iron, manganese, and sulfur. Under wetland soil conditions, rates of many microbially mediated reactions decline, and some reactions may be eliminated and replaced by new ones. New microbial reactions are involved in the reduction of oxidized compounds during respiratory processes, resulting in the production of reduced compounds. 

When oxygen is limited, as is the case in wetland soils, unique conditions are set in motion that differentiate wetlands from uplands in snch a way as to increase organic matter in the soil, which may even resnlt in the formation of thick layers of peat, and a change in the distribntion of microorganisms (with anaerobic bacteria being more active) and chemical properties of wetland soil. 

Plants have adapted to the harsh anaerobic conditions of wetland soils. Development of aerenchyma tissnes permit oxygen pumping to the roots, to support root respiration and aerobic bacteria in the root zone. 

In addition to restricting oxygen supply, what other important effects does excess water have on wetland soils ... 

List the ways in which oxygen is introduced into wetland soils. 

What are the two important purposes for which oxygen is needed in soils Which electron acceptors can be used by microorganisms in wetland soils ... 

Wetland soils are usually limited by electron acceptors and have abundant supply of electron donors. Upland soils are usually limited by electron donors and have abundant supply of electron acceptors (primarily oxygen) ... 

Oxygen supply in wetlands is restricted to the water column and to a thin layer of surface soil. Oxygen is also transported by wetland macrophytes to their root zone, resulting in the creation of aerobic conditions on root snrfaces (see Chapters 3 and 6 for a detailed discussion on aerobic-anaerobic interfaces in wetlands). 

In wetland soils, organic matter decomposition is frequently limited by electron acceptor availability, rather than carbon availability as in upland ecosystems. The concentration and type of electron acceptors available in soils determine the types of microbial communities involved and the rate of decomposition process. Much of the detrital matter produced in wetlands is deposited on the soil surface. It is unlikely that there is enough oxygen in this matrix to decompose this material. Therefore, the decomposition of detrital matter is also dependent on the activity of anaerobic microorganisms using alternate electron acceptors. Similarly, the rate of organic matter decomposition in soils is dependent on the availability of electron acceptors (see for discussion in Chapters 3 and 4). 

It is evident that oxygen-, nitrate-, sulfate-reducing and methanogenic conditions have a profound effect on various biogeochemical properties regulating organic matter decomposition in wetland soils (Table 5.14). A review on the comparison of microbial dynamics in marine and freshwater system as influenced by the availability of electron acceptors is presented by Capone and Kiene... 

Soil oxygen or the aeration status of soils can be determined by various techniques, which can be used to characterize wetland soils undergoing seasonal hydrologic fluctuations and to delineate wetlands from uplands. These techniques are as follows ... 

Field measurements of Eh are significantly correlated with soil oxygen (Figure 6.6), suggesting that Eh measurements can provide a reasonable indication of soil aeration status (Megonigal et al., 1993 Faulkner et al., 1989). Soil Eh and oxygen levels also respond to water table fluctuations in wetlands (Figure 6.7). 

Flooding soils for either short-term (irrigation or rainfall) or long-term (wetlands or paddy fields) results in displacement of soil oxygen. Dissolved oxygen present in the pore water is rapidly consumed by aerobic bacteria during their respiratory activities, and depletion of oxygen results in... 

FIGURE 6.15 Schematic showing oxygen diffusion into a wetland soil (a) saturated soil conditions and (b) water table below the soil surface. 

FIGURE 6.17 Aerobic-anaerobic interfaces in a wetland soil dissolved oxygen concentration in soil and water column. (D Angelo, E. M., and Reddy, K. R., unpublished results.)... 

FIGURE 6.18 Aerobic-anaerobic interfaces in a wetland soil dissolved oxygen concentration in a saturated... 

The thickness of the aerobic layer varies from <1 mm to 3 cm. In relation to anaerobic soil volume, the aerobic soil volume at the soil-floodwater interface is small. However, this thin aerobic interface in the proximity of anaerobic soil is key to many unique biogeochemical processes functioning in wetlands. The differentiation of a wetland soil or sediment into two distinct zones as a result of limited oxygen penetration into the soil was first described by Pearsall and Mortimer (1939) and Mortimer (1941). 

FIGURE 6.26 Relationship between soil oxygen demand (SOD) and heterotrophic microbial respiration by different wetland soils. Each data point represents soil from a different locations in the United States (D Angelo and Reddy, 1999). 

In biological systems, oxygen serves as an electron acceptor during respiration by bacteria and as a reactant in certain biochemical reactions. In addition, oxygen can be involved in chemical oxidation of reduced species in wetland soils. 

In flooded and waterlogged soils, the pores are filled with water and any dissolved oxygen is rapidly consumed. Under these conditions, oxygen is introduced into the wetland soil profiles by diffusion and mass flow through the floodwater and plants. The oxygen concentration in the soil pore space is lower than that in the atmosphere. In wetland soils, the net movement is restricted by the presence of water in the pore space. The diffusion of oxygen in water is 10,000 times slower than that in air. 

The diffusion of oxygen in air can be described by Pick s first law. Redox potential, which measures electron activity in soils, can be nsed as an indicator of soil aeration status. Aerated soils have Eh greater than +300 mV. Redox potential or Eh below +300 mV indicates little or no presence of oxygen. In addition to redox potential measurement, ODR and soil oxygen content can be used to determine the aeration status of wetland soils. 

SOD is used as an indicator of oxygen in wetland soils. The SOD is regulated by soil organic matter content and reduced substances in the soil profile. 

---