Oxygen and carbon dioxide in the subsurface

The extent to which oxidising sulphides affect the composition of the subsurface atmosphere will depend upon the rate of oxidation and the intensity of other activities that remove oxygen and generate carbon dioxide. The oxidation of pyrite was reviewed in the introduction to this chapter, and other sulphides more commonly of economic interest would be expected to behave in a similar manner. However, the stabilities of different metallic sulphides vary greatly in the secondary environment and consequently their oxidation rates differ. 

The rate of oxidation of pyrite has been studied in the greatest detail and may be summarised thus  

There are, of course, several processes that can frustrate the detection of such expressions. It is important to consider the rate at which gas enters and leaves the soil air. Baver (1972) quotes several authors and estimates that there would have to be a complete renewal of soil air every hour to a depth of 20 cm in a normal cropped soil in order to maintain its usual average composition and microbiological activity. If mineral deposits are to have adequate expression in the soil air, sulphide oxidation must clearly influence its composition at a rate commensurate with such rapid aeration. 

Relation between the daily soil respiration and mean soil temperature of a bare Rothamsted soil the two curves are the plot of RoQ ° for Q = 3 and Rq = 1.2 (upper curve) and Ro = 0.9 (lower curve) (from Russell, 1973). 

In a classic early study, Wollny (1881) reported that the CO2 content of the soil at 30 C increases about ten times as the soil moisture content changes from 6.8% to 26.8%. The CO2 content of soil air also increases with depth and the O2 content falls (Table 14-II). The gradient is influenced by the nature of the soil granular soils contain less than 

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