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-400 -300 -200 -100 0 100 200 300 400 500 600 Eh (mV) at pH 7

FIGURE 2.7 Global warming potential (GWP) contribution of N2O, CH4, and CO2 as a function of soil Eh. All eight soils showed the same pattern of (i) N2O, (ii) CH4, and (iii) CO2 dynamics with soil Eh change from high to low. A logarithmic scale is used to cover a wide range of values. Global warming potential contributions below 1 mg CO2 equivalent kg-1 h-1 were considered insignificant and not illustrated for clarity (from Kewei and Patrick, 2004.)

-400 -300 -200 -100 0 100 200 300 400 500 600 Eh (mV) at pH 7

FIGURE 2.7 Global warming potential (GWP) contribution of N2O, CH4, and CO2 as a function of soil Eh. All eight soils showed the same pattern of (i) N2O, (ii) CH4, and (iii) CO2 dynamics with soil Eh change from high to low. A logarithmic scale is used to cover a wide range of values. Global warming potential contributions below 1 mg CO2 equivalent kg-1 h-1 were considered insignificant and not illustrated for clarity (from Kewei and Patrick, 2004.)

SOIL pH

Acidic protons supplied to the soil from atmospheric and organic sources react with bases represented by aluminosilicates, carbonates, and other mineralogical and humic constituents. In a humid climate with excess precipitation, and given sufficient time, basic cations (Na+, K+, Ca2+, Mg2+) will be exchanged from mineral and organic constituents by H+ and leached from the surface soil. The presence of clay minerals such as smectites, which are saturated with basic cations, retards the progress of acidification, and calcite buffers the soil between pH 7 and 8 until it is completely dissolved. Continued hydrolysis results in the formation of a residuum made up of such minerals as kaolinite, gibbsite, and goethite, together with resistant minerals, of which quartz is common, that are buffered between pH 3.5 and 5. Semiarid and arid conditions lead to an opposite trend, a soil solution buffered at an alkaline pH and in the presence of sodium.

Soil pH influences a number of factors affecting microbial activity, like solubility and ionization of inorganic and organic soil solution constituents, and these will in turn affect soil enzyme activity. There are a large number of both organic and inorganic acids found in soils; the majority of these acids are relatively weak. Traditionally soil pH is measured in a soil paste prepared by the addition of a dilute CaCl2 solution with an appropriate electrode. While achieving a pH measurement of the soil is relatively easy, interpretation of its affect on microbial processes is difficult. This is because concentrations of cations sorbed to the surfaces of the negatively charged soil colloids are 10-100 times higher than those of the soil solution. The pH at the colloid surface will be much more acidic than that of the measured pH of the bulk soil solution. For soil exoenzymes sorbed to colloid surfaces, their apparent pH optimum would be 1-2 pH units higher than if measured free in solution. An example of this is soil urease activity, which has an apparent pH optimum of 8.5-9.0 that is about 2 pH units greater than optimal ure-ase activity measured in solution.

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