Upland oxygen

The effects of upland acidification of freshwaters can be dramatic. Between 1930 and 1975 the median pH of lakes in the Adirondack Mountains of northeastern USA decreased from 6.7 to 5.1, caused by progressively lower pH in rainwater (Fig. 5.7). The acidified lakewater killed fish and other animals by several mechanisms. The problem for fish is that the dissolved Al3+ in the acidic water precipitates as an insoluble Al(OH)3 gel on the less acidic gill tissues, preventing normal uptake of oxygen and suffocating the animal. Similar problems have occurred in Scandinavia and Scotland. In addition to problems in freshwaters, the loss of forests in high-altitude areas has been linked to acid leaching, which leads to impoverishment of soils coupled with direct loss of cations from plant leaves. 

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. 

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) ... 

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). 

The concentration of oxygen in soil pore waters is generally lower than that in the atmosphere above the soil, so that the net movement is downward, that is, from the atmosphere to soil. In wetlands, this net movement is prevented by the presence of floodwater within the pore space. However, when the water table is below the soil surface, the oxygen movement into soil is similar to uplands. 

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 ... 

Redox potential that measures electron activity in soils is used as an indicator of soil aeration status in upland environments. Changes in Eh values are small if sufficient oxygen is present in the soil pores. Aerated soils have characteristic Eh values in the range of +300 to +500 mV and air-filled porosity of 0-60% (Figure 6.5). The low concentration of redox couples in aerated soils reduces... 

Continuous study of oxyacetamide chemistry shifted research from the paddy herbicide to an upland herbicide with increasing water solubility that is suitable for such upland use. To this end, benzanellated analogues such as the benzothia-zole moiety of mefenacet were changed to simple five-membered heterocycles that contain at least one nitrogen atom to increase water solubility, and sulfur or oxygen atom to decrease lipophilicity, for instance thiazoles, thiadiazoles, oxazoles and oxadiazoles (Fig. 8.2). Consequently, many patent applications of the new class of heteroxyacetamide herbicides were disclosed [17, 18]. 

The fast-flowing mountain and upland streams the oxygen minima for the quality grades II to IV are frequently higher than the figures quoted in the table. 

Uplands When lead shot falls on dry land shooting ranges, it falls in relatively aerobic environments - pellets are exposed to oxygen in the atmosphere and readily corrode. Soil contamination and therefore leaching from the topsoil downward into the water table is high to extreme. ... 

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