(Table 4.7), and is therefore utilized first by aerobic organisms. Once oxygen has
been used up other electron acceptors are used in order of energy efficiency
(Table 4.7), by nitrate, manganese, iron and sulphate reducers. This order of elec-
tron acceptor usage is common to most biotic Earth surface environments (see
Sections 5.5 & 6.2.4), although the relative importance of each electron accep-
tor depends on its concentration in specific environments. Sulphate reduction,
for example, is not a common process in most soils because sulphate is in low
concentrations in most continental waters except in some coastal areas (see
Section 5.5). Methanogenesis can, however, be an important microbial reaction
in waterlogged soils, especially paddy fields and marshes.
Soil microorganisms (fungi, bacteria and actinomycetes) play a major role
in the degradation of organic matter, ultimately releasing nutrient elements—
about 98% nitrogen, 5–60% phosphorus and 10–80% sulphur to the soil nutrient
pool—along with micronutrients such as boron and molybdenum, into the soil
for reuse by plants and animals. The role of biotic soils as sources of N and CH 4
The Chemistry of Continental Solids 103Table 4.7Order of bacterial reactions during microbial respiration of organic matter based
on energy yield. Modified from Berner (1980), reprinted by permission of Princeton
University Press.
Bacterial reaction DG° (kJ mol-^1 of CH 2 O)
Aerobic respiration: important in all oxygenated Earth
surface environments
CH 2 O+O 2 ÆCO 2 +H 2 O - 475
Denitrification: most important in terrestrial and marine
environments impacted by anthropogenic inputs from
fertilizers
5CH 2 O+4NO 3 - Æ2N 2 +4HCO 3 - +CO 2 +3H 2 O - 448
Manganese reduction: minor reaction important in some
marine sediments
CH 2 O+3CO 2 +H 2 O+2MnO 2 Æ2Mn^2 ++4HCO 3 - 349
Iron reduction: can be significant in some soils and marine
sediments with high iron contents from contamination or
weathering flux (e.g. Amazon Delta)
CH 2 O+7CO 2 +4Fe(OH) 3 Æ4Fe^2 ++8HCO 3 - +3H 2 O - 114
Sulphate reduction: major process in anaerobic marine
sediments, especially on continental shelves
2CH 2 O+SO 42 - ÆH 2 S+2HCO 3 - - 77
Methanogenesis: important process in freshwater wetlands,
waterlogged soils and in deeply buried low-sulphate marine
sediments
2CH 2 O ÆCH 4 +CO 2 - 58
Note: Free energy value for organic matter (CH 2 O) is that of sucrose.