Page 19 - 1_Environmental analysis

Sum parameter – Total Organic Carbon
Determination of microbial biomass in soils -
TOC in soil science
No. SCA-130-109
Fertile soils contain a multitude of micro-
organisms. They are responsible for the
degradation of organic substances and
protection of the nutrient cycle. To evaluate
soils in terms of their biodegradability and their
fertility, the microbial biomass of the soil, i.e.
the organic carbon fraction that is bound to
these tiniest organisms, is determined.
The fumigation-extraction method is a
commonly used method for the determination
of microbial biomass in soils. The microbial
biomass is described regarding extractable
organic carbon compounds before and after
killing off the microorganisms.
In the fumigation-extraction method, a
subsample of the soil is treated with
chloroform gas in a suitable apparatus (e.g. a
desiccator) over an extended period of time of
at least 24 hours. This destroys the cell walls
and kills off the microorganisms. After
fumigation, the chloroform remaining in the
soil is removed.
■ DOC – Determination
Subsequent to fumigation, each fumigated
subsample and one non-fumigated soil
sample is mixed with a 0.5 M (mol/L)
potassium sulfate solution and then shaken.
After filtration of the eluate, the DOC
(dissolved organic carbon) of the extracts is
determined. Since experience has shown, not
all cells are destroyed and extracted, and an
empirical correction factor is additionally
applied. (An exact description of the
fumigation-extraction method is found in the
EN ISO 14240-2:2011 standard).
■ Kit for high salt loads
The kit for high salt loads was used for these
measurement because the extraction solution
alone has a salt load of approximately 87 g/L.
High salt loads can lead to clogging of the
catalysts and the combustion tube. This
special kit consists of a combustion tube
featuring a specific geometry and a unique
catalyst mixture. This particular catalyst can
handle up to 12 times more salt than a
conventional catalyst.