Soil Carbon Fractions Based on their Degree of Oxidation, and the ...

Graeme J. Blair et al.
The key to sustained productivity of agricultural systems is the maintenance
of SOM levels and nutrient cycling. Both are closely related through the
microbially driven mobilization/immobilization processes (Duxbury et al. 1989).
Some researchers argue that measurement of the size of the soil microbial biomass
(SMB) is the key to understanding the turnover rate of the SOM (Ocio et al.
1991), whilst others consider SMB is a poor indicator of these changes because
factors such as the particular species of organisms comprizing the SMB and the
soil moisture can markedly affect the size of the SMB (Mazzarino et al. 1987).
Sanchez and Logan (1992) calculated from data of Greenland and Nye (1959)
that the generally higher SOM decomposition rates found in the humid tropics
are balanced by higher litter inputs, such that mean SOM contents vary little
between tropical and temperate soils (Sanchez et al. 1982). However, Buol et al.
(1990) found that at the same annual mean temperature, soils from tropical
areas, with isothermic regimes, had higher organic carbon (OC) contents than
those from temperate areas with non-isothermic regimes. The difference between
these two groups diminished as mean temperature increased.
Small changes in total SOM or C are difficult to detect because of the generally
high background levels and natural soil variability. For this reason many attempts
have been made to use subpools of SOM or C as more sensitive indicators
of changes in pool size. Jenkinson and Rayner (1977) identified five pools in
their organic matter cycling model ranging from a decomposable pool, with a
radiocarbon age of less than 1 year, through a biomass pool at 25.9 years to a
chemically stabilized pool with a radiocarbon age of 2565 years. Variations on
these pools have been used in other organic matter cycling models (Hunt 1977;
Paul and van Veen 1978; Smith 1979; Coughenour et al. 1980; McCaskill 1987;
Parton et al. 1989).
In addition to these major pools, SOM fractionation has been carried out on
the basis of extraction of humic substances (Schnitzer 1982), dissolved organic C
(Cook and Allan 1992), particle size (Christensen 1986), natural abundance
(Balesdent et al. 1990; Lefroy et al. 1993), microbial biomass C (Sparling 1992)
and ease of oxidation of C (Loginow et al. 1987). Although this latter method
does not provide quantitative data on C fractions, it does provide qualitative
characterization of soil C.
Changes in the lability of soil carbon have been proposed by Lefroy et al.
(1993) as a measure of sustainability. This procedure relies on the ease of
oxidation of the soil organic C by potassium permanganate. This paper reports
on the standardization of the KMn04 oxidation procedure of Loginow et al.
(1987) and on the development of a carbon management index based on changes
in the total C in the soil and its lability as determined by KMn04 oxidation.
Materials and Methods
The carbon fractionation method is based on that described by Loginow et al. (1987).
Potassium permanganate is a powerful oxidizing agent under neutral conditions, but it is
relatively unstable and thus cannot be used as a primary standard. This creates some problems
in obtaining repeatable and quantitative results. If the precautions outlined in this paper are
taken, however, this technique can be used as a quantitative measure of soil carbon fractions.
The original method of Loginow et al. (1987) relied on using three different concentrations
of the oxidizing agent to oxidize increasing proportions of the soil C within a fixed time
interval. Preliminary studies by Lefroy et al. (1993) found that the use of a single strength of