Determination of Organic and Inorganic Carbon in
Calcareous Soils and Sediments
Q.R. WANG1*, Y.C. LI1, W.G. Harris2, and Y. Wang2
1 Tropical
Research and Education Center, IFAS, University of Florida, Homestead, FL., 33031-3314
2 Soil and Water Science Department, IFAS, University of Florida, Gainesville, FL 32611
™ Four methods on determination of organic and inorganic carbon (C) in calcareous soils and
Correlations between different methods
sediments were compared and evaluated. They were Walkley-Black procedure for organic C
associated with pressure-calcimeter determination for inorganic C; weight–loss-on-ignition for
organic and inorganic C; Thermogravimetric method for both organic and inorganic C, vario
Max CNS Autoanalyzer either reduced the reduction temperature to determine organic carbon
and subtracted inorganic carbon from the total obtained with the normal temperature, or
determine the total and the organic C after removed inorganic C by HCl fumigation,
respectively. The result displayed that Walkley-Black procedure provided an accurate
determination for organic carbon in soils but not in sediments; Weight-loss-on-ignition
provided an accurate estimation on organic carbon at 475 - 500 oC, and on inorganic carbon at
800 oC; Thermogravimetric method showed clear weight losses for both organic and inorganic
carbon on those samples but it needs some experiences on sample components to interpret the
data; CNS Autoanalyzer methods either removed inorganic C via HCl fumigation or reduced
the reduction temp. displayed a good estimation only on soil samples. With simplicity and
accuracy, weight-loss-on-ignition method at 500 oC for organic C and 800 oC for inorganic C
determinations is highly recommended for both soil and sediment samples in a calcareous
region.
Florida Bay
Fig. 1. Correlations between different methods on soil
organic C
INTRODUCTION
™
A simple, accurate, cost effective
and
environmental friendly method to determine organic
and inorganic carbon in calcareous soils and
sediments for the same batch of samples needs to be
developed. The conventional method to determine
organic C, dichromatic (Walkley-Black) method can
produce some bias results when samples contain
some reductive element (Cl-, Fe2+, Mn2+) or
carbonized materials (charcoal, graphite, coal, coke
and soot) since the method is based on the principle
of oxidation and reduction, and also the disposal of
waste H2SO4, Cr6+/Cr3+ is a major environmental
concern. Carbon autoanalyzer can only determine the
total C, which cannot provide enough information on
calcareous soils and sediments.
MATERIALS AND
METHODS
TGA
™Pressure-calcimeter method for inorganic C
determination: The volume of CO2 generated by
the reaction of samples with HCl was used to
calculate the inorganic C. In all those samples,
we assumed that the organic matter contains 58%
of C and inorganic matter contains 12% of C.
70
60
50
40
30
20
10
0
W105
W400
W425
8
3
4R
11
However, to determine the sample
organic C, the same sample has to run via
another acceptable method.
A good correlation between
pressure-calcimeter method
and both WLOI and TGA
methods was also observed in
sediment
inorganic
C
determination (Fig. 4A and
B).
No matter were the concentration high or
low, the highest losses for both organic and
inorganic C in both calcareous soils (Fig. 6)
and sediments (Fig. 7) were obviously
distinguished at 475-500 oC, and 800 oC,
respectively. The result also indicated that
CaCO3 under the experimental condition
was not destructed until over 600 oC (Fig. 6
and 7).
Fig. 7. Losses of mass for sediments with
WLOI method
For soil samples in this
experiment, the WalkleyBlack method provided a
good approach with a high
repeatability but the main
disadvantage is the disposal
of environmental unfriendly
waste, H2SO4, Cr6+ and Cr3+.
However, for those sediment
samples, the Walkley-Black
method was failed to provide
an accurate approach on
organic C determination.
Advantages of WLOI: It is simple, relatively easy to
operate, cost effective and accurate, and also can
eliminate the use of Cr6+ and conc. H2SO4. The more
important is that it can determine organic and inorganic
C with the same batch of samples for both soils and
sediments.
TGA method can display precise losses of mass with a
quantitative “fingerprint” because of a built-in
microbalance but it needs some knowledge on sample
components to correctly interpret the result. Samples
with simple components, it is easy to quantify the
organic and inorganic C (Fig. 5A), otherwise, it is
rather difficult (Fig. 5B), and also an expensive facility
makes the method widely unaccepted.
B
Carbon autoanalyzer method either
reducing the temp. or removing
inorganic C is simple but the sample
has to run twice to calculate the
inorganic C subtracted from the total.
Fig. 5. TGA on two different soil samples
6
2
5R
10
W600
W700
W800
W850
7
1
6R
Pure CaCO3
W900
5
12
7R
™The determination procedure
of WLOI is relatively simple
with an accurate result and less
laborious, without potential
hazard materials consumed or
generated, but it basically
requires a programmable muffle
furnace to combust samples, an
oven to remove sample
moisture, crucibles with high
temp. tolerance, a desiccator
with drierites to prevent
samples absorbing moisture
from the air, and an analytical
balance to obtain an accurate
weight.
CONCLUSION
™With consideration of procedure
simplicity, result accuracy and
reliability, cost effectiveness and
facility availability, the weightloss-on-ignition
method
is
obviously more acceptable for
most laboratories.
™ With
the
weight-loss-onignition method, the organic and
inorganic C in either the soil or
sediment samples in a calcareous
region can be determined with the
same batch of samples, and a
highly comparable result can be
obtained with setup of the temp. at
500 oC for organic C and 800 oC
for inorganic C for 12 hrs.,
respectively.
Fig. 4. Correlations
between different methods
on sediment inorganic C
Method evaluation
W500
Fig. 6. Losses of mass for soil samples with WLOI method
Losses of mass for samples of the soil and
sediment via the WLOI method
™Major losses occurred at 400 oC, however,
for both soil and sediment samples, some
losses were still apparent after this critical
temp. (Fig. 6 and 7).
Fig. 3. Correlations between
different methods on soil
inorganic C
W475
o
Fig. 2. Correlations between different methods on sediment
inorganic C
A
W450
Temperature ( C)
10
4
1R
8R
Inorganic C: The inorganic C determined with pressure-calcimeter method
correlated closely with each of other methods on soil samples (Fig. 3) but at the low
concentration of inorganic C, e.g., <4%, some errors might be produced for the
method of subtracting organic C to obtain the inorganic C using CNS-RIC (Fig. 3D).
OBJECTIVE
™ The objective of the experiment was to
compare and evaluate those four different
methods in order to develop a simple, cost
™ Soil samples were collected from 5 calcareous effective, accurate and environmental friendly
and 2 non-calcareous soils in the State of Florida one for the determination of organic and
,
containing 0-85% of CaCO3 pH (water) 5.4-8.2,
inorganic C in calcareous soils and sediments.
and organic C from 0.4 (Red Bay) to 319
(Lauderhill) g kg-1. Sediments containing 3-81%
of CaCO3, pH 2.8-7.9, EC 0.2-38 Ms cm-1, and
™CNS Auto-analyzer
organic C 1-750 g kg-1 were from Lake
method with normal and
Okeechobee, middle Everglades, St. John River,
reducing the reduction
Florida Bay and St. Lucie Estuary, respectively.
temperature (CNS-RT):
™Walkley-Black method: An excessive volume
Subsamples were run at
of K2Cr2O7 plus H2SO4 to oxidize organic C,
the normal temp. to
titrated the unreacted Cr2O72- with FeSO4.
obtain the total C, and
™Weight-loss-on-ignition (WLOI) method:
reduced the reduction
samples were preheated at 105 oC for 12 hrs. to
temp. from 830 oC to
remove the moisture, combusted the samples in a
650 oC to run the other
programmable muffle furnace for 12 hrs at 400,
subsamples to obtain the
425, 450, 475, 500, 600, 700, 800, 850, and 900
organic C, and the
oC, respectively, to obtain the weight losses based
difference between the
on the dry weight after preheated at 105 oC.
total and the organic C
™Thermogravimetric method (TGA): Samples
Pressurewas inorganic C.
were heated from 25 to 1000 oC using a computer
calcimeter
controlled thermogravimetric analyzer, losses of
mass were determined within an appropriate
temperature range as indicated by the weight-loss ™CNS Auto-analyzer method after removal of
inorganic C (CNS-RIC): Subsamples were run at
curve.
the normal temp. to obtain the total C. The other
subsamples were fumigated with conc. HCl for
over 24 hrs. to remove the inorganic C before
they were run on the CNS with the normal temp.
to obtain the particulate organic C. Subtracted the
organic C to obtain the inorganic C content.
Muffle furnace
However, there was a close
correlation on measurements
between WLOI and each of
sampling
other
dryCanalcombustion
methods (Fig. 2B-D) in
sediment samples.
80
Sediment weight losses on ignition (%)
CNS Autoanalyzer
Organic C: A highly close
correlation was observed
between measurements with
Walkley-Black and each of
other methods on organic C
of soil samples (Fig. 1) but
not of sediments (Fig. 2A).
™ The
pressurecalcimeter
method
for determining the
inorganic C provided
a simple and accurate
approach,
an
excellent
linear
2
relationship
(R ≥
0.999) between the
rate
of
reagent
CaCO3 and the CO2
volume can usually
be obtained.
Weight lost (%)
RESULTS AND DISCUSSION
ABSTRACT
REFERENCES
™ Allison L.E., 1960, Wet-combustion apparatus and procedure for organic
and inorganic carbon in soil: Soil Science Society of American Proceedings,
v. 24, p. 36-40.
™ David M.B, 1998, Use of loss-on-ignition to assess soil organic carbon in
forest soils: Communications in Soil Science and Plant Analysis, v. 19, p.
1593-1599.
™ Konen M.E., Jacobs P.M., Burras C.L., Talaga B.J. and Mason J.A., 2002,
Equations for predicting soil organic carbon using loss-on-ignition for north
central U.S. soils: Soil Science Society of American Journal, v. 66, p. 18781881.
™ Sherrod L.A., Dunn G., Peterson A. and Kolberg R.L., 2002, Inorganic
carbon analysis by modified pressure-calcimeter method: Soil Science
Society of American Journal, v. 66, p. 299-305.
ACKNOWLEDGEMENT
The authors are deeply grateful to Dr. Y. Qian for CNS autoanalyses; to Mrs. L.
Rosado for pressure-calcimeter analyses; to Drs. Z. He at IRREC, University
of Florida, and W. Perry at Everglades and Dry Tortugas National Parks,
Florida, and Mr. N. Campbell for their invaluable assistance in sediment
sampling.