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inter-instrument comparison of calculated humification indices was possible but required
application of an appropriate correction curve specific to the compared instruments. This
correction curve would require running the same set of standards on each instrument. They
also found the index would be highly prone to inner-filter effects if samples were too con-
centrated, resulting in artificially high estimates of the degree of aromaticity (or conjugated
ring structures) from the humification index. A sample concentration of approximately 10
mg L
-1
DOC was recommended to minimize concentration and inner-filter effects.
Figure 9.1
demonstrates that the humification index of Kalbitz et al. (
1999
) is essentially
measuring the intensity of fluorescence associated with what is now referred to as peak C.
Rather than tracking the change in maximum fluorescence of the peak, the chosen wave-
length pairs are at longer excitation wavelengths. By capturing the fluorescence at these
longer wavelengths, the index identifies changes in the shape of the peak which may relate
to aromaticity and other bulk properties of the sample.
9.2.2 Zsolnay's Humification Index to Identify Soil Organic
Matter Properties (HIX
EM
)
Zsolnay et al. (
1999
) proposed a “humification index” (HIX
EM
) around the same time as
Kalbitz et al. (
1999
). Zsolnay et al. (
1999
) investigated the effects of drying on soil OM,
seeking to identify the source of the observed increase of extractable DOM due to drying
processes. Aqueous extracts (using 4 m
M
CaCl
2
) of DOM from multiple soil types were
separated into fulvic acid and material released by cell lysis. The fluorescence signature
of the DOM and of each fraction was analyzed using a fixed excitation wavelength of
254 nm and usable emission was measured between 300 and 480 nm. Samples were run
at a pH of 2 to negate pH effects in comparisons and diluted to an absorbance no greater
than 0.1 cm
-1
.
The HIX
EM
was determined by dividing the area of fluorescence intensity between 435
and 480 nm by that between 300 and 345 nm (mistakenly given as 300-445 nm in the
original text, but correct in
Figure 9.1
in the paper,
Figure 9.3
in this chapter). Zsolnay
et al. (
1999
) used theoretical concepts similar to those of Kalbitz et al. (
1999
), proposing
that an increase in the HIX
EM
is associated with an increase in emission wavelength due
to increased polycondensation (lower H/C ratios). Because humification is also associated
with a decrease in the H/C ratio, they suggested the index represented the degree of humifi-
cation of the organic matter.
Figure 9.4
presents results showing the effects of air drying,
oven drying, and fumigation on two contrasting soil OM samples in comparison to a fulvic
acid standard, showing how oven drying decreased the HIX
EM
and standard relative fluores-
cence (fluorescence per gram of carbon in quinine sulphate dihydrate equivalents). Because
humified material is generally more condensed than its precursor material, and because
the HIX
EM
decreased during oven drying, Zsolnay et al. (
1999
) concluded that most of the
organic material released is from non-humified sources such as cell lysis.
Ohno (
2002a
) investigated concentration and inner filter effects on the humification
index. Field corn residue extract, water-extractable soil organic matter, and soil fulvic acid
