Soil Solution Chemistry

An understanding of the chemistry of the soil solution providing an environment for soil organisms needs to take account of the nature and quantity of its major components: water, dissolved organic matter and inorganic constituents, and O2 and CO2. The biogeochemistry of the soil solution is mainly determined by acid-base and redox reactions. Consequently the thermodynamic activities of protons and electrons in soil solution define the chemical environment that controls microbial activity. Both can be considered as flowing from regions of high concentration to regions of low concentration, and soil microbial activity has a profound effect on regulating this flow.

The most reduced material in the biosphere is the organic matter contained in living biomass. Organic matter in soils ranges from total dominance, as in peat-lands, to the minor amounts found in young soils or at depth in the vadose zone. Soil organisms generate electrons during the metabolic oxidation of organic matter, and these electrons must be transferred to an electron acceptor, the largest of which is atmospheric O2 in freely drained, aerobic soils. The O2 trapped in the soil or present in the water can be consumed within hours by soil microbes and is replenished by O2 diffusion. If O2 diffusion into the soil is impeded, for reasons of waterlogging, restricted pore sizes due to clay texture or to soil compaction, the resultant soil becomes practically devoid of O2. When microbial activity uses up all of the available dissolved O2, the soil solution as a whole changes from aerobic (oxic) to anaerobic (anoxic). Microbial activity will then be controlled by the movement of electrons to alternative electron acceptors.

Development of anaerobic conditions results in a shift in the activity of the soil microbial populations, with the activity of aerobic and facultative organisms, which dominate well-drained soils, decreasing and the activity of obligate anaerobic and fermentative organisms increasing. This switch in electron acceptors promotes the reduction of several important elements in soil, including nitrogen, manganese, iron, and sulfur, in a process known as anaerobic respiration and carbon dioxide by methanogenisis.

Redox potential (EH) measurements provide an indication of the soil aeration status. They are a measure of electron availability occurring as a result of electron transfer between oxidized (chemical species that have lost electrons) and reduced (chemical species that have gained electrons) chemical species. The measurements

TABLE 2.3 The Most Important Redox Pairs and the Approximate Eh Values at the Occurrence of Transitions at the Reference Soil pH of 7.0

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