OXYGEN Soil atmosphere

Analyses of the soil atmosphere show that it suffers greater fluctuations in composition than ordinary air. As a rule it contains less oxygen, but nearly ten tunes as much carbon dioxide as the air, as shown by the following data 1... 

Usually the sum of these two gases m the soil atmosphere is only slightly less than that in the air although at periods when nitrates rapidly increase, and in water-logged soils, there is a perceptible reduction in oxygen. 

In addition to this free atmosphere there is a second soil atmosphere, consisting mainly of carbon dioxide and nitrogen, with practically no oxygen, which is dissolved in the water and colloids of the soil. The existence of this second atmosphere is important, in that it renders possible the existence of anaerobic organisms m the soil.2... 

At pH 7, this gives theoretical limits for the soil environment at Eh — approximately +0.8 V (oxidizing) and approximately —0.4 V (reducing). In practice soil atmospheres will not contain 1 atm of oxygen or hydrogen. 

Reduction is caused by a decrease in oxygen concentration in the soil atmosphere for example, by waterlogging or compaction. Oxygen... 

Soils vary greatly in composition and reactivity. Many complex and dynamic processes occur continuously in most soils composed primarily of mineral and organic matter, water, and air. The soil atmosphere is composed of oxygen, carbon dioxide, nitrogen, and several minor gases whereas the mineral fraction varies in amounts of sand, silt, and clay and in types and amounts of clay minerals. Moreover, hydration and base saturation of the clay minerals also vary considerably. The organic matter and mineral colloids present in the soil contribute directly and indirectly to the extremely active nature of pesticide-soil systems. Since soil water contains many soluble compounds, it serves as an essential medium for many chemical and physical processes. The extreme complexity of these soil systems has been the primary reason that so few fundamental studies have been undertaken involving the ultimate fate of pesticides in soils. 

Today there is little direct CH4 release to atmosphere from the sediment store, except in unusual events such as submarine landsUps or pockmark bursts. In an oxygen-rich atmosphere, methane is removed by OH, and to a minor extent by soil methanotrophs. In the sea, unless they are large, bubbles of methane released from... 

Nitrogen occurs in soils further in gaseous forms as elemental nitrogen N2, nitrous oxide N2O, nitric oxide NO, nitrogen dioxide NOj and ammonia NH3. N2 is a major component of the soil atmosphere. Removal of oxygen from nitrite or nitrate by soil microbes under anaerobic conditions leads to NjO this process is known as deni-... 

Nitrate reduction occurs when the soil atmosphere is completely stripped of oxygen. 

While the denitrification process is lamented by agronomists as a loss of plant-available N from the soH system, it is the only way that N is recycled back to the atmosphere as N2. Without this process, lower soil layers and groundwaters would become large reservoirs of NOs", and the oxygen-enriched atmosphere would support continual conflagrations and, thus, make life difficult. Therefore, the N cycle is just as important as the carbon and the hydrobgic cycles to the life support system of the earth,... 

In addition to its presence as the free element in the atmosphere and dissolved in surface waters, oxygen occurs in combined form both as water, and a constituent of most rocks, minerals, and soils. The estimated abundance of oxygen in the crustal rocks of the earth is 455 000 ppm (i.e. 45.5% by weight) see silicates, p. 347 aluminosilicates, p. 347 carbonates, p. 109 phosphates, p. 475, etc. 

Aqueous environments will range from very thin condensed films of moisture to bulk solutions, and will include natural environments such as the atmosphere, natural waters, soils, body fluids, etc. as well as chemicals and food products. However, since environments are dealt with fully in Chapter 2, this discussion will be confined to simple chemical solutions, whose behaviour can be more readily interpreted in terms of fundamental physicochemical principles, and additional factors will have to be considered in interpreting the behaviour of metals in more complex environments. For example, iron will corrode rapidly in oxygenated water, but only very slowly when oxygen is absent however, in an anaerobic water containing sulphate-reducing bacteria, rapid corrosion occurs, and the mechanism of the process clearly involves the specific action of the bacteria see Section 2.6). 

Bacterial activity often plays a major part in determining the corrosion of buried steel. This is particularly so in waterlogged clays and similar soils, where no atmospheric oxygen is present as such. If these soils contain sulphates, e.g. gypsum and the necessary traces of nutrients, corrosion can occur under anaerobic conditions in the presence of sulphate-reducing bacteria. One of the final products is iron sulphide, and the presence of this is characteristic of attack by sulphate-reducing bacteria, which are frequently present (see Section 2.6). 

The methyl parathion released to the atmosphere can be transported back to surface water and soil by wet deposition. Methyl parathion that is released to the atmosphere can also be transformed by indirect photolysis to its oxygen analog, methyl paraoxon, by oxidation with photochemically produced oxygen radicals. However, methyl parathion is not expected to undergo significant transformation to methyl paraoxon. 

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