Agriculture Reference
In-Depth Information
represented up to 0.83 % of the soil mass in three of four Australian soils (two mollisols‚
an oxisol and a vertisol) and formed a substantial proportion of total soil carbon.
Charcoal consists of condensed or cross-linked aromatic rings resistant to microbial and
probably chemical decomposition (Skjemstad et al .‚ 1996). It comprises a substantial
proportion of the aromatic material less than 53 in size and may be confused with
partially-decomposed lignin and humic substances. Charcoal is measured as organic
carbon in conventional wet chemical estimations and this has considerable implications
for studies of carbon turnover in soils.
Carbon comprises ca. 43 % of the dry biomass of fresh plant residues although con-
centrations decline as decomposition proceeds. The terminal products of organic matter
decomposition include humic and fulvic acids‚ a range of carbon-rich organic polymers
with high cation exchange capacities and affinities for trace metals. In addition‚ large
quantities of polysaccharides are produced in the rhizosphere although these may have
relatively short turnover times. Considerable amounts of carbon are included in the living
biomass of the soil external to the roots; estimates of microbial biomass are in the order
of 1 to 5 per cent. of organic carbon (see‚ for example‚ Brookes et al .‚ 1984; Sparling‚ 1992).
However‚ this excludes the biomass carbon of the mesofauna and the macrofauna‚
particularly the earthworms‚ termites and ants‚ which may represent 10-20 % of the
microbial biomass.
As considered earlier‚ carbon gases‚ notably carbon dioxide and methane‚ may occur
in the soil environment at concentrations considerably higher than atmospheric. In the
liquid phase‚ dissolved gases and other soluble carbon compounds play important
biological and pedogenetic roles in chelation and complexation reactions and thus the
transport of metals. Carbonate weathering and leaching will lead to the movement of
bicarbonate ions in the groundwaters of environments where these minerals occur in
the soil.
The chemical form of the dissolved organic carbon in soil interstitial waters is
poorly known. Fulvic acids are generally present in greater concentrations than humic
acids while a large proportion of the dissolved organic carbon consists of low-molecular-
weight organic acids. Some compositional differences occur between drainage classes
and the humic and fulvic acids of the interstitial waters differ from those of bulk soil.
Current estimates of the concentrations of dissolved organic carbon in soil interstitial
waters lie in the range 5-1000 mg C (Malcolm‚ 1993).
Carbon concentrations in soils differ widely reflecting climatic conditions‚ local
ecosystem productivity‚ the types and amounts of the clay and other minerals present
(notably carbonates)‚ internal and external environmental conditions and cultural
practices. The organic horizons of peat soils (histosols) may have carbon concentrations
similar to that of plant tissues (up to ca. 43 %) while those of mineral soils are much
lower. Figure I.32a presents the carbon concentrations of the A horizons of 1975 soils
from throughout Australia. It has been estimated that more than 75 % of the Australian
land surface is covered with soils having less than 1 % organic carbon while soils
having high levels of organic carbon are very limited in extent (Spain et al .‚ 1983a).
Other less-arid continents have greater proportions of their soils and landscapes in
higher concentration classes.
Search WWH ::




Custom Search