Agriculture Reference
In-Depth Information
Selecting indicators to assess soil organic matter change
Measurement of total organic matter concentration in soil at any one time is
relatively straightforward; however, in many cases it is difficult to measure
significant change in soil organic matter concentration, even if soil samples
are obtained over time. Small, but important changes can be difficult to
detect against the relatively large background concentration of soil organic
matter.
To resolve this dilemma, some of the above-described soil organic
matter fractions can be chosen that are more sensitive to change than total
organic matter. For example, soil microbial biomass within specific limits
can serve as an indicator of organic matter change (Carter et al ., 1999).
Other labile soil organic matter fractions, such as macroorganic matter and
the light fraction, are highly responsive to changes in carbon inputs to the
soil and will provide a measurable change prior to any such change in the
total organic matter (Gregorich and Janzen, 1996). Campbell et al . (1997)
and Bolinder et al . (1999) illustrated the sensitivity of several soil organic
matter fractions to changes in carbon inputs. However, the validity of this
approach to indicate the direction of soil organic matter change is restricted
under conditions where climate impedes adequate carbon inputs (Mele and
Carter, 1993) or suppresses the rate of decomposition (Janzen et al ., 1998).
In these latter situations, the change is characterized by a temporary
increase in labile carbon and does not indicate a concomitant change in
total soil organic matter.
Critical values of soil organic matter
Critical values involve establishing a critical concentration or range for the
property in question that is needed to ensure that dependent soil processes
and function are not restricted or adversely affected (Larson and Pierce,
1994). Analogous to the 'critical loads concept' used to assess the
influence of various pollutants in the environment (HMSO, 1996), the
concept of 'critical values' is used commonly in soil pollution studies
(Nortcliff, 1997).
Little attempt has been made to characterize critical values for soil
organic matter. Limiting values or thresholds for fractions and components
of soil organic matter are generally not known (Carter et al ., 1999). The
non-uniform and multi-functional nature of organic matter ensures that the
task of selecting a specific organic matter-mediated function and attempt-
ing to set a critical value would be problematic. However, some studies
have been successful in this regard for total organic matter. For example,
Feller et al . (1996) linked critical values of soil organic matter for both
soil fertility and erodibility in tropical 'kaolinitic' (e.g. Alfisols, Ultisols and
 
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