Geology Reference
In-Depth Information
introduced as a result of biological activity. Much of this organic material
is plant-derived litter and the amount added to soil per year depends very
much on the dominant vegetation type (Table 7). These figures do not
include inputs from roots, which may be significant but dicult to
quantify, or more resistant inputs, such as wood in forests. In addition,
animal and microbial excretion and decomposition must be taken into
account. The values in Table 7 do, however, give a reasonable estimate of
what may be described as degradable carbon entering soils per year.
The organic materials added to soil have varying degrees of suscept-
ibility to degradation. Simple compounds, such as sugars and amino
acids, are rapidly decomposed by soil microorganisms, whereas complex
material, such as lignin and hemicellulose, tends to persist in an unde-
graded or partially degraded form. About 70% of the carbon added to
soil is lost as carbon dioxide, produced mainly by microbial respiration.
The other 30% is incorporated into the microbial biomass and soil
humus, a resistant material, which degrades only very slowly. Figure 8
shows typical values for the annual turnover of organic matter in soil.
Humus formation proceeds broadly along two pathways (Figure 9).
Readily decomposed material releases low molecular weight com-
pounds, especially phenolic and amino compounds, which react and
polymerize to form humic material. Alternatively, the more resistant
organic material can undergo partial degradation, and this partially
degraded material may also react with the low molecular weight
compounds released into the soil, again leading to the formation of
humus. It is likely that both of these processes operate in soils.
The terms humus and humic substances are used interchangeably to
refer to the resistant organic material that persists in soil over relatively
long timescales (100s-1000s of years). Although the humic substances
may be chemically diverse, being dependent on the dominant type of
plant tissue input, they do have some overall similarities. They are
amorphous, colloidal polymers built up mainly from aromatic units.
There is a wide range of molecular weights, from 100s to 100,000s Da.
Table 7
Inputs of carbon to soil by various types of land use
Tonnes carbon added to soil ha
1
year
1
Dominant land use
Alpine and arctic forest
0.1-0.4
Arable agriculture
1-2
Coniferous forest
1.5-3
Deciduous forest
1.5-4
Temperate grassland
2-4
Tropical rain forest
4-10
Source: adapted from White, Principles and Practice of Soil Science, Blackwell Science, 1997.
