1
Measuring organic carbon in calcarosols: understanding the pitfalls and
2
complications
3
4
Running head: Measuring organic carbon in calcarosols
5
6
Aaron SchmidtA, Ronald J. SmernikA,C, and Therese M. McBeathA,B
7
8
A
9
University of Adelaide, Waite Campus, Urrbrae, SA, 5064, Australia.
School of Agriculture, Food and Wine and Waite Research Institute, The
10
B
11
Glen Osmond, SA 5064, Australia
12
C
CSIRO Sustainable Agriculture Flagship, CSIRO Ecosystem Sciences, PMB 2,
Corresponding author. E-mail: [email protected]
13
14
1
15
Abstract
16
The measurement of soil organic carbon (OC) is important for assessing soil
17
condition and improving land management systems as OC has an important role
18
in the physical, chemical and biological fertility of soil. The OC contents of
19
calcarosols often appear high compared to other Australian soil types with
20
similar fertility. This may indicate either systematic overestimation of OC in
21
calcarosols or the existence of a mechanism of OC stabilisation specific to
22
carbonate-rich soils. This study compares three dry combustion techniques: dry
23
combustion with correction for carbonate-carbon (carbonate-C) determined
24
separately, dry combustion following sulfurous acid treatment and dry
25
combustion following treatment with hydrofluoric acid (HF-treatment) and two
26
wet oxidation techniques: Walkley-Black and Heanes for the measurement of
27
soil OC content, to determine which method is best for calcarosols. Eighteen
28
soils were analysed: nine calcareous and nine non-calcareous. Of these methods,
29
dry combustion with carbonate-C correction and dry combustion following
30
sulfurous acid pre-treatment were found to be unsuitable for highly calcareous
31
soils. Dry combustion with carbonate-C correction was unsuccessful primarily
32
due to incomplete conversion of carbonate to CO2 under the combustion
33
conditions used. However, even if this problem could be overcome, the method
34
would still not be suitable for highly calcareous soils, as it would involve the
35
measurement of a relatively small value (organic C) as the differences of two
36
much larger values (total C and carbonate-C). Sulfurous acid pre-treatment was
37
unsuitable because it did not remove 100% of carbonate present. Although the
38
remaining dry combustion technique (i.e. following HF-treatment) did not have
39
such problems, it did give very different (and much lower) OC estimations than
40
the two wet oxidation techniques for the highly calcareous soils. These results
2
41
are consistent with carbonate minerals interacting with and stabilising a
42
substantial quantity of soluble OC. This has implications for the way OC levels
43
should be measured and interpreted in calcarosols, both in terms of fertility and
44
C stabilisation and sequestration.
45
46
Additional keywords: calcareous, stabilisation, dry combustion, wet oxidation,
47
Walkley-Black, HF-treatment
48
49
Introduction
50
51
Calcarosols are defined under The Australian Soil Classification (Isbell 2002) as
52
soils that are calcareous through the soil profile. They are widely distributed
53
across south-eastern Australia, mostly in the drier areas. The calcarosol
54
classification corresponds approximately with the calcid suborder of aridosols
55
under the Soil Taxonomy classification (Soil Survey Staff 1999). They
56
reportedly cover 3.7% of the world’s ice-free land area, mostly in arid areas of
57
the temperate climatic zone (Soil Survey Staff 1999). Some Australian
58
calcarosols, e.g. those from the Eyre Peninsula of South Australia, have
59
extremely high carbonate contents of up to 87% of soil mass (Bertrand et al.
60
2003) and this impacts strongly on their chemical fertility, and in particular on
61
their ability to provide nutrients to plants (Holloway et al. 2001; McBeath et al.
62
2005).
63
64
Despite their limited fertility, high carbonate calcarosols are often reported to
65
have relatively high organic carbon (OC) contents. For example, Bertrand et al.,
66
(2003) reported that of six alkaline soil groups from southern Australia, it was
3
67
the grey calcarosols, which had low clay contents but very high calcium
68
carbonate contents (average 59%), that had the highest average OC content.
69
Since OC content is associated with soil fertility in agricultural systems (Manlay
70
et al. 2007) and is also sensitive to land management (Battle-Aguilar et al.
71
2011), it is important to be able to accurately gauge and interpret OC levels. For
72
calcarosols in particular, it is important to know whether (i) specific methods are
73
required to accurately determine OC in the presence of carbonate; and (ii)
74
specific guidelines of “acceptable” OC contents are required, which may differ
75
from those developed for non-calcareous soils.
76
77
Soil OC levels are often measured by dry combustion (Nelson and Sommers
78
1996; Skjemstad et al. 1998; Chatterjee et al. 2009); however, dry combustion
79
does not provide a direct measurement of OC in soils that contain substantial
80
quantities of carbonate. This is because carbonate is unstable at the temperatures
81
required to efficiently combust organic matter and convert OC to CO2. Thus dry
82
combustion, in which carbon dioxide evolved from both combustion of organic
83
matter and decomposition of carbonate is measured by gas chromatography or
84
infrared analysis, only provides a measure of total carbon. Determination of OC
85
contents of carbonate-rich soils requires either correction for carbonate C by
86
subtracting a separately measured carbonate-C value from total C or removal of
87
carbonate prior to combustion (Nelson and Sommers 1996; Skjemstad et al.
88
1998; Chatterjee et al. 2009).
89
90
An alternate method of OC determination is wet oxidation. This involves the
91
addition of excess oxidant to soil. Organic C is determined indirectly by
92
measuring the amount of oxidant remaining by redox titration or photometry
4
93
(Nelson and Sommers 1996; Skjemstad et al. 1998; Chatterjee et al. 2009). The
94
advantage of wet oxidation for calcarosols is that it is not influenced by the
95
presence of carbonate. However, in any soil, organic matter may not be
96
completely oxidised and therefore a correction factor may be required to provide
97
the total OC concentration (Nelson and Sommers 1996; Skjemstad et al. 1998;
98
Chatterjee et al. 2009). A second problem is that wet oxidation assumes that the
99
oxidation state of OC is the same as that of elemental C. In reality, OC usually
100
contains components that are both more and less oxidised than elemental C
101
(Hockaday et al. 2009) and unless these are in balance, this will result in a
102
biased OC value. A third problem is that overestimations of OC can occur if
103
other oxidisable species, such as chloride and ferrous iron, are present (Nelson
104
and Sommers 1996; Skjemstad et al. 1998; Chatterjee et al. 2009).
105
106
A number of previous studies have shown good agreement between dry
107
combustion and wet oxidation techniques for OC determination for soils in
108
general, but most studies either include no calcareous soils (Kowalenko 2001;
109
Brye and Slaton 2003; Mikhailova et al. 2003; Lettens et al. 2007) or few
110
calcareous soils (Soon and Aboud 1991). Santi et al. (2006) found good
111
agreement between dry combustion and wet oxidation techniques for carbonate-
112
containing soils, but their soils contained no more than 15% calcium carbonate,
113
and in most cases considerably less than that. Chichester and Chaison (1992)
114
reported similar findings, but again for soils with <10% calcium carbonate.
115
116
In this study, variants of both dry combustion and wet oxidation are used to
117
determine the OC content of nine calcarosols, including soils containing up to
118
85% calcium carbonate. The primary purpose was to identify which method or
5
119
methods are best suited to highly calcareous soils and to investigate whether the
120
anomalously high OC contents (given their low inherent fertility) of such soils
121
are real or caused by incomplete removal of, or correction for, carbonate.
122
Organic C contents of nine non-calcareous soils are also determined using each
123
method in order to confirm any differences are due to the presence of carbonate.
124
125
Materials and methods
126
127
Soils Collection and Properties
128
Eighteen soils were analysed in this study: nine calcarosols and nine non-
129
calcareous soils; all are topsoils collected from the top 10 cm. Following
130
collection, soils were dried at 40°C and sieved to <2 mm. Sampling locations,
131
climate, management and basic properties (clay content and pH) are shown in
132
Table 1.
133
134
Carbonate Determination
135
The carbonate content of each soil was determined on duplicate samples using a
136
volumetric calcimeter (Allison and Moodie 1965). A third replicate was
137
analysed if the evolved carbon dioxide varied by more than 0.4 mL. Two
138
different quantities of soil were used depending upon the expected carbonate
139
content. For non-calcareous soils, 2 g of soil was used and for calcarosols 0.2 g
140
of soil was used to ensure that the CO2 evolved was well within the detection
141
limit of the method. This was reacted with 20 mL of 4.0 M HCl. Carbonate-C
142
was determined by multiplying the calcium carbonate content by the mass
143
proportion of C in calcium carbonate (0.12).
144
6
145
Dry Combustion
146
A Leco CNS 2000 automated analyser was used to determine OC in by dry
147
combustion following the procedure of Merry and Spouncer (1988). Duplicate
148
samples were heated to 1350ºC and evolved carbon dioxide (CO2) was measured
149
using an infrared detector cell.
150
151
Dry combustion was also carried out following two acid pre-treatments, one
152
using sulfurous acid (H2SO3) and the other using hydrofluoric acid (HF). Both of
153
these involve treatment with a weak acid at relatively low concentrations in
154
order to avoid highly acidic conditions that would result in rapid hydrolysis of
155
acid-susceptible species including proteins and carbohydrates. For the sulfurous
156
acid treatment, 2 g of soil was weighed into a 50 mL centrifuge tube and treated
157
with 40 mL of 6% sulfur dioxide solution (~1M sulfurous acid). The acid was
158
added slowly to prevent excess effervescence that could cause loss of soil over
159
the top of the centrifuge tube. The acid solution was then mixed with a vortex
160
mixer for 10 s, six times at 10 min intervals. The centrifuge tubes were then
161
shaken end-over-end for two hours. For the first hour, shaking was stopped
162
every 10 min to release evolved CO2 by opening the lids of the centrifuge tubes.
163
The samples were mixed continuously for the last hour. The samples were
164
centrifuged for 20 min at 740 × g to separate the soil from the supernatant. The
165
supernatant was discarded and the residue rinsed three times with water, and
166
then freeze-dried.
167
168
For the calcarosols, HF treatment was preceded by treatment with sulfurous acid
169
as described above up until the rinsing stage, in order to remove the majority of
170
carbonate. Thereafter, the procedure of Skjemstad et al. (1994) was followed.
7
171
This involved treatment of 3 g aliquots of soil with nine consecutive 50 mL
172
aliquots of 2% (1 M) HF, with reaction times of 1 h (five times), 16 h (three
173
times) and 64 h (once). The supernatant was discarded after each step and the
174
final residue rinsed and freeze-dried, as described above.
175
176
Wet Oxidation
177
The Walkley-Black (6A1) and Heanes (6B1) methods are two Australian Soils
178
and Plant Analysis Council Inc. (ASPAC) accredited wet oxidation techniques
179
that were used in this study to determine OC (Rayment and Higginson 1992).
180
The Walkley-Black method is the most commonly used wet oxidation technique
181
in Australian commercial soil laboratories. However, the Walkley-Black method
182
generally results in incomplete oxidation of OC (Walkley and Black 1934). The
183
Heanes method is a slight modification of the Walkley-Black method in which
184
external heating is applied in order to increase the proportion of OC oxidised
185
(Heanes 1984).
186
187
The Walkley-Black method involved adding concentrated sulfuric acid (20 mL)
188
to soil wetted with a dichromate (0.167 M) solution (10 mL). The reaction
189
mixture reaches a temperature of 110-120ºC, inducing oxidation (Rayment and
190
Higginson 1992). Organic C was determined by back-titrating ferrous sulfate
191
with the reactant mixture; the end-point was identified by a colour change from
192
orange-red to green (Rayment and Higginson 1992). Soils were ground with a T
193
100 N. V. Tema grinder for 5 s. Samples were analysed in triplicate and were
194
analysed in a random order to minimise systematic errors. The amount of soil
195
analysed was based on the OC determination from a commercial laboratory
196
(results not shown). For soils that contained less than 20 g C kg-1, 1 g of soil was
8
197
used and for soils that contained greater than 20 g C kg-1, 0.5 g of soil was used.
198
As the Kangaroo Island soil had OC content greater than 70 g kg-1 and the OC
199
content of Foul and Sturt Bay soils were not known, 0.2 g of soil was used.
200
201
Determination of OC using the Heanes method was carried out in an identical
202
manner, except that the reaction mixture was heated to keep it at 135ºC for 30
203
min (Rayment and Higginson 1992).
204
205
Statistical Analysis
206
Simple linear regression (SLR) and analysis of variance were conducted using
207
GenStat 13th edition software (VSN international). Assumptions of constant
208
variance and normality of data distribution were tested for each analysis. All the
209
data presented in tables and graphs are raw means.
210
211
Results and Discussion
212
213
Basic properties of the eighteen soils analysed in this study are shown in Table
214
1. The majority of the soil samples are from cropping systems, with the
215
remainder from pastoral and viticultural systems (Table 1). The soils vary in clay
216
content from 1% to 57% (Table 1). The soils are from different climatic zones,
217
as shown by the variation in mean temperature and annual rainfall (Table 1).
218
219
Dry Combustion
220
The total C contents of the eighteen soils, determined by dry combustion, ranged
221
from 7.8 to 91.9 g kg-1 (Table 2). In general, the calcarosols analysed had higher
222
total C contents than the non-calcareous soils, although the Kangaroo Island and
9
223
Koppio soils had total C contents as high as many of the calcarosols (Table 2).
224
The higher total C contents of the calcarosols are due to their high carbonate
225
contents, while the Kangaroo Island and Koppio soils are pastoral soils in which
226
high C inputs and low disturbance frequencies result in an accumulation of OC.
227
228
OC determination by correction for carbonate C
229
The calcium carbonate contents of the eighteen soils, determined using a
230
volumetric calcimeter, are shown in Table 2. For the non-calcareous soils, the
231
carbonate content was between 0 and 3%. These soils do not contain sufficient
232
carbonate to cause effervescence after the addition of a few drops of
233
hydrochloric acid (Isbell 2002), and are therefore are classed as non-calcareous.
234
For the calcarosols, the carbonate contents were in the range 13-85% (Table 2).
235
The OC contents calculated by correcting the total C content for the contribution
236
of carbonate-C to the total C content (0.12 times the calcium carbonate content)
237
are shown in Table 2. It is clear that this method was unsuccessful because
238
negative values were found for five of the calcarosols (Table 2).
239
240
This indicates that there was incomplete conversion of carbonate to CO2 during
241
the combustion process, leading to an underestimation of the total C content of
242
these soils. It has been shown that, in general, lower temperatures are required to
243
convert OC to CO2 than are required to thermally decompose carbonate to
244
release CO2 (Merry and Spouncer 1988; Matejovic 1997; Kerven et al. 2000;
245
Wright and Bailey 2001). In the current study, samples were heated to 1350ºC, a
246
temperature identified in the above studies as being sufficient to completely
247
convert calcium carbonate to CO2. However, it may be that all or some of the
248
carbonate in this set of soils, especially those with very high carbonate contents,
10
249
may be more resistant to decomposition, and may not be fully converted to CO2
250
under these conditions.
251
252
These finding contrast with those of Chichester and Chaison (1992) and Santi et
253
al. (2006), who reported no problems with this approach for measuring OC in
254
calcareous soil. However, they analysed soils with lower carbonate contents,
255
with a maximum values of 10% and 15%, respectively, compared to a maximum
256
value of 85% in this study. We only found negative values using this method for
257
soils containing more than 59% carbonate. It is also possible that the carbonate
258
in their soils included only less thermally stable forms.
259
260
Another problem with determining OC contents in highly calcareous soils by
261
subtracting carbonate C from total C is “catastrophic cancellation”, which occurs
262
when a relatively small value (in this case OC content) is calculated as the
263
difference between two much larger measured values (in this case total C and
264
carbonate-C) (Bisutti et al. 2004). This results in an uncertainty in the calculated
265
value that is much larger than for the measured values in relative terms. Thus,
266
even if the problem of incomplete conversion of carbonate to CO2 could be
267
addressed (e.g. by increasing either the reaction temperature or heating time or
268
both), this method would not be recommended for highly calcareous soils.
269
270
OC determination by removal of carbonate C via acid pre-treatment
271
The other way to determine the OC content of calcarosols by dry combustion is
272
to use acid pre-treatment to remove carbonates (Nelson and Sommers 1996;
273
Skjemstad et al. 1998; Chatterjee et al. 2009). Sulfurous acid is most often used
274
for this purpose for soils (Nelson and Sommers 1996), whereas removal of
11
275
carbonates from other environmental materials (e.g. sediments and waters) prior
276
to OC determination often utilises other mineral acids (Bisutti et al. 2004). The
277
preference for sulfurous acid can be attributed to OC losses reported on
278
treatment with stronger mineral acids with oxidising potential (e.g. HCl, H2SO4
279
HNO3) (Gibbs 1977) and the potential for OC loss due to hydrolysis in strongly
280
acid conditions (Bisutti et al. 2004).
281
282
In this study, results from two different acid pre-treatments are compared. The
283
first involved a single treatment with an excess of sulfurous acid (pKa = 1.90).
284
The second acid pre-treatment involved multiple treatments with hydrofluoric
285
acid (pKa = 3.17). This second method was developed as a pre-treatment for
286
concentrating organic matter prior to solid-state 13C nuclear magnetic resonance
287
(13C NMR) analysis. Hydrofluoric acid is uniquely suited for this purpose
288
because, unlike, other weak acids, it dissolves most minerals present in soil
289
(Skjemstad et al. 1994).
290
291
The mass recoveries for the non-calcareous soils from sulfurous acid pre-
292
treatment ranged from 87% to 97% (Table 3). Recoveries below 100% were
293
expected due to the presence of low levels of carbonate-C in most soils (Table
294
2), along with small quantities of water-soluble or acid-soluble organic species
295
and small losses during manipulations. Total C contents following sulfurous acid
296
pre-treatment of the non-calcareous soils were 81-117% (average 94%) of the
297
values for untreated soils (Table 3). Again, these small losses can be attributed to
298
small losses during manipulations, small amounts of carbonate in these soils and
299
soluble or hydrolysable OC discarded in the supernatant.
300
12
301
The mass recoveries on sulfurous acid pre-treatment for the calcarosols ranged
302
from 53 to 79%. These recoveries were higher than initially expected, given the
303
high carbonate content of these soils and the expectation that sulfurous acid pre-
304
treatment would remove the carbonate in the sample. For example, the Foul Bay
305
soil contained 85% detectable carbonate, yet had a mass recovery of 71%
306
(Tables 2 and 3).
307
308
The high mass recoveries for the calcarosols were investigated by treating
309
finely-powered, pure calcium carbonate with sulfurous acid. Rather than
310
completely dissolving, a white precipitate was found to form, resulting in a mass
311
recovery of 77%. The white precipitate is most likely calcium sulfite, which is
312
formed when Ca2+ ions, released into solution as the calcium carbonate reacts,
313
combine with SO32- ions. Calcium sulfite has a low solubility, similar to that of
314
calcium carbonate. The formation of an insoluble product raises the possibility
315
that this product may prevent complete removal of carbonate due to the coating
316
of partially reacted carbonate particles with calcium sulfite. The residue from the
317
reaction of calcium carbonate with sulfurous acid was analysed by dry
318
combustion and only a small amount of C was detected (1.0 g kg-1). However,
319
for larger particles of carbonate that may be present in the calcarosols,
320
particularly those with very high carbonate contents, carbonate removal may
321
have been incomplete, due to calcium sulfite providing a protective coating
322
around unreacted carbonate. This has been reported previously for sulfurous acid
323
pre-treatment of calcareous sediments (Fernandes and Krull 2008).
324
325
The OC contents of the sulfurous acid treated calcarosols are lower than the total
326
soil C contents, confirming removal of some carbonate (Table 3). However, the
13
327
soil OC values still appear to be high for some of the calcarosols (Table 3). This
328
may, in part, be due to sulfite coating and protecting carbonate from acid attack,
329
as discussed above. Another possible explanation is that these soils contain a
330
form of carbonate that is more resistant to acid attack. Caughey et al. (1995)
331
reported that incomplete removal of small quantities of acid-resistant carbonate
332
on sulfurous acid treatment was a major source of error when analysing OC in
333
aquifer sediments containing much larger quantities of carbonate than OC.
334
Although calcite (CaCO3) is usually the dominant carbonate mineral in soil,
335
other carbonates that are more stable to acid treatment such as dolomite
336
(MgCa(CO3)2) and siderite (FeCO3) may also be present (Caughey et al. 1995;
337
Heron et al. 1997; Bisutti et al. 2004). A third possibility is that organic matter
338
may protect some carbonate from exposure to acid (Telek and Marshall 1974).
339
This is especially likely in shelly calcarosols, since the shell matrix contains
340
organic scaffolds that become apparent during acid etching of shells (Gordon
341
and Carriker 1980). It should be noted that the four calcarosols with the highest
342
C contents (>40 g kg-1) following sulfurous acid pre-treatment are all shelly
343
calcarosols (Foul Bay, Greenly, Streaky Bay and Sturt Bay).
344
345
Low mass recoveries on HF pre-treatment were observed for both the non-
346
calcareous soils and calcarosols, ranging from 2.5% to 31.7% (Table 3). These
347
low mass recoveries were expected, as the HF dissolves most minerals in the
348
soil, concentrating the organic materials. This resulted in high total C contents in
349
the HF treated residues, ranging from 51 g kg-1 to 471 g kg-1.
350
351
The OC contents of the non-calcareous soils determined following HF pre-
352
treatment are lower (average 73%, range 60-88%) than their respective total C
14
353
contents (Table 3). It has previously been shown that HF-treatment results in the
354
loss of water-soluble OC species that are held in the soil through interactions
355
with soil minerals, especially clays and hydrous oxides (Skjemstad et al. 1994;
356
Schmidt et al. 1997). Since sulfurous acid does not remove these minerals,
357
losses of OC on sulfurous acid treatment are considerably lower than on HF-
358
treatment (Table 3).
359
360
The OC contents of the calcarosols following HF-treatment are much lower
361
(average 21%, range 12-37%) than their total C contents (Table 3). Most of this
362
difference is due to the removal of carbonate-C in the calcarosols, but again
363
there will also be losses of soluble OC sorbed to the mineral matrix. In the case
364
of the calcarosols it is not possible to distinguish between these losses from this
365
data alone.
366
367
Comparison of C contents determined following sulfurous acid and hydrofluoric
368
acid pre-treatment
369
There was a strong linear relationship (R2 = 0.99) between soil OC contents
370
following sulfurous acid and HF pre-treatments for non-calcareous soils (Figure
371
1). As discussed above, the OC contents determined with sulfurous acid pre-
372
treatment were higher, due to greater losses associated with HF pre-treatment.
373
For the calcarosols, OC contents determined by the two methods did not exhibit
374
a linear relationship and HF and sulfurous acid pre-treatments gave very
375
different apparent OC contents (Figure 1). Following HF pre-treatment of
376
calcarosols, the maximum soil OC content was 23.6 g kg-1, compared to 54.8 g
377
kg-1 following sulfurous acid pre-treatment (Table 2). This 30 g kg-1 difference
15
378
between the two pre-treatments is probably mainly due to the incomplete
379
carbonate removal on sulfurous acid pre-treatment.
380
381
Although the calcarosols did not exhibit an overall linear relationship, three out
382
of the nine soils do conform to the linear relationship shown by the non-
383
calcareous soils (Figure 1). All three of these soils contain lower (<35%)
384
calcium carbonate contents (Cungena, Kadina and Maitland). We suggest that
385
this indicates that these three soils contain little or no carbonate following
386
sulfurous acid treatment. On the other hand, for the other six calcarosols, only
387
HF pre-treatment appears to have successfully removed all of the carbonate and
388
thus enabled an OC content to be determined free from any interference from
389
carbonate-C. Based on these findings, it can be concluded that HF pre-treatment
390
is the only one of our three dry combustion methods for which it can be
391
confidently assumed that the presence of carbonate does not directly
392
compromise the determination of OC contents in highly calcareous soils.
393
394
Wet Oxidation
395
It is well known that the Walkley-Black method does not completely oxidise all
396
soil organic matter, as the exothermic reaction does not result in temperatures
397
high enough for complete reaction (Nelson and Sommers 1996; Skjemstad et al.
398
1998; Chatterjee et al. 2009). This has been shown in comparisons of Walkley-
399
Black OC values with values determined by dry combustion for non-calcareous
400
soils (Nelson and Sommers 1996; Skjemstad et al. 1998; Chatterjee et al. 2009).
401
These underestimations have led to the modification of the Walkley-Black
402
method to improve the extent of oxidation. The Heanes method is one such
16
403
variation. It is very similar to the Walkley-Black method, except it involves
404
applying external heat to increase the rate of OC oxidation (Heanes 1984).
405
406
In this study, OC content determined using the Heanes method ranged from 7.3
407
to 71.2 g kg-1, which was higher than values determined using the Walkley-
408
Black method, which ranged from 5.8 to 63.4 g kg-1 (Table 4). These differences
409
were statistically significant for every soil (P < 0.001, LSD 1.2). A strong linear
410
relationship was found between OC values determined by the Walkley-Black
411
and Heanes methods, which was very similar for calcareous and non-calcareous
412
soils. On average, values for the Heanes method were 25% higher (Table 4).
413
Kerven et al. (2000) reported similar findings, with higher OC values determined
414
using the Heanes method.
415
416
Comparison between dry combustion and wet oxidation techniques
417
A key aim of this study was to compare dry combustion and wet oxidation
418
techniques for measuring the OC content of calcarosols, especially those with
419
very high carbonate contents. Two of the three dry combustion techniques were
420
found unreliable for highly calcareous soils: subtracting a separately determined
421
carbonate C content from total C determined by dry combustion gave negative
422
values for OC, presumably due to incomplete detection of carbonate C by dry
423
combustion, and sulfurous acid pre-treatment appeared to overestimate OC in
424
the majority of calcarosols due to incomplete removal of carbonate. Therefore it
425
is only valid to compare the third dry combustion technique, i.e. following HF-
426
treatment, with wet oxidation values. We have chosen to compare against the
427
Heanes values, based on the more complete OC recovery for this method,
428
though given the close correlation between Heanes and Walkley-Black values,
17
429
similar conclusions could be drawn from comparisons against Walkley-Black
430
values as well.
431
432
Regression analysis of the OC content determined by the Heanes method and
433
dry combustion following HF pre-treatment showed that non-calcareous and
434
calcareous soils are described by distinctly different relationships (P = 0.01).
435
Strong linear correlations were observed for both the non-calcareous soils (R2 =
436
0.99) and calcarosols (R2 = 0.76) (Figure 3). The strong linear correlation for the
437
calcarosols confirms that the HF pre-treatment has successfully removed all the
438
carbonate, overcoming the problem encountered with sulfurous acid pre-
439
treatment, illustrated in Figure 1.
440
441
The OC contents of the non-calcareous soils determined by the HF-dry
442
combustion method were slightly lower than those determined by the Heanes
443
method (71-92% of Heanes values, average 81%; Figure 3), while for the
444
calcarosols, the OC contents determined by the HF-dry combustion method were
445
much lower than those determined by the Heanes method (39-70% of Heanes
446
value, average 57%; Figure 3).
447
448
Since HF-treatment demonstrably removes all of the carbonate from the
449
calcarosols, the different relationships for the calcareous and non-calcareous
450
soils in Figure 3 cannot be caused by carbonate-C directly affecting
451
measurement of OC content by dry combustion. This implies that the carbonate
452
in the calcarosols is having an indirect effect on OC determination by the two
453
methods. In particular, it is clear that the ratio of OC content determined by the
454
HF-dry combustion method to that determined by the Heanes method is
18
455
substantially lower for the calcarosols. The simplest explanation for this is that
456
the calcarosols contain a larger proportion of soluble OC associated with or
457
sorbed to soil minerals than the non-calcareous soils. This soluble OC would be
458
lost during pre-treatment with HF (and hence would not be detected by the HF-
459
dry combustion method), but would be detected by the Heanes method. Two
460
possible forms of such soluble OC in calcarosols are OC incorporated within the
461
matrix of shell material and OC sorbed to the carbonate minerals.
462
463
Shells contain around 1-5% organic material, mostly β-chitin, acidic proteins
464
and glycoproteins (Jacob et al. 2008). These organic materials provide a scaffold
465
for shell formation, as carbonate is formed on top and around this organic
466
material. These soluble OC compounds incorporated in the shell matrix would
467
be released when sulfuric acid dissolves the carbonate during Heanes OC
468
determination. Fernandes and Krull (2008) reported large losses of OC on acid
469
treatment of carbonate-rich sediments, some of which they attributed to loss of
470
soluble organic species released from shelly material.
471
472
There is also a possibility that a substantial quantity of OC is sorbed to the
473
carbonate minerals, as there were two calcic calcarosols (which do not contain
474
shelly material) that had considerably higher OC contents when measured using
475
the Heanes method (Figure 3). It has long been recognised that carbonate
476
particles in seawater are strong sorbents of soluble organic species (Chave 1965;
477
Suess 1970). In fact, this process has important implications for carbonate
478
formation and dissolution in seawater, with the sorbed organics inhibiting both
479
of these processes (Morse et al. 2007). More recently, sorption of soluble
480
organic species to carbonate minerals has been reported to be a potentially
19
481
important mechanism of organic matter stabilisation in calcareous soils
482
(Grünewald et al. 2006; Grünewald et al. 2008).
483
484
Finally, it should be noted that OC incorporated into carbonate minerals is likely
485
to have much less influence on soil fertility than other forms of organic matter.
486
As it is in a stabilised form, it does not have the reactive surface area available to
487
provide the binding role important for aggregate stability and it is likely to have
488
low availability to soil microbes, and thus provide little in the way of nutrient
489
release. This has been demonstrated for carbonate-rich sediments, in which the
490
~50% of OC occluded in carbonate minerals was shown to be virtually inert
491
(Ingalls et al. 2004). Therefore the OC content determined by dry combustion
492
following HF-treatment (which doesn’t include soluble OC occluded in
493
carbonate minerals) may be the more relevant value for assessing the fertility
494
status of calcarosols. The low values of this measure of OC for the calcarosols
495
(average 13.0 g OC / kg soil, range 5.4-23.6 g / kg) are consistent with the
496
generally low fertility of these soils.
497
498
Conclusions
499
500
Large inconsistencies were found between different methods used for OC
501
determination in highly calcareous soils. Some of these inconsistencies can be
502
attributed to error caused by incomplete decomposition of carbonate during dry
503
combustion and incomplete removal of carbonate during sulfurous acid pre-
504
treatment. However, there also appears to be an indirect effect of carbonate that
505
results from association of substantial quantities of soluble OC with carbonates.
506
The results from this study suggest that this soluble OC may be either sorbed to
20
507
carbonate minerals (in calcic calcarosols) or incorporated within the carbonate
508
matrix (in shelly calcarosols). Further research is needed to confirm these
509
mechanisms of stabilisation of OC by the carbonate minerals.
510
511
The large inconsistencies in OC determination can have important consequences
512
for land management decisions, including nutrient cycling and C storage. Dry
513
combustion following HF-treatment and the Heanes wet oxidation technique
514
both provide uncompromised measurements of OC in calcarosols, in that there
515
are no direct biases caused by carbonate, but they give different measures
516
because the former excludes soluble OC associated with carbonate, whereas the
517
latter includes this form of OC. Hydrofluoric acid pre-treatment successfully
518
removes all the carbonate prior to combustion with 100% efficiency but only
519
detects the insoluble OC. The Heanes wet oxidation technique provides a near
520
complete oxidation of organic materials, including soluble OC associated with
521
carbonaceous materials. Sulfurous acid pre-treatment followed by dry
522
combustion and correcting for carbonate-C by subtracting carbonate-C from
523
total C were unsuccessful in determining OC in the calcarosols.
524
525
This study shows that careful consideration is needed when analysing soil OC in
526
calcarosols. The end-use of the OC measurement will affect the choice of
527
method. The Heanes method would be the best method for determining OC for
528
the purpose of C sequestration, as it is important to measure all OC. HF pre-
529
treatment followed by dry combustion may be the best method for determining
530
OC for the purpose of evaluating the chemical, physical and biological
531
properties controlling the fertility of the soil, as it excludes OC strongly
532
associated with carbonate minerals. However, HF pre-treatment is not a
21
533
commercially practical method because HF is very toxic and requires special
534
equipment to be safely handled. Further research is required to confirm how
535
much OC in calcarosols is occluded in carbonate minerals, but the results from
536
this study suggest that it is at least 5-10 g OC / kg soil for highly calcareous
537
soils.
538
539
Acknowledgements
540
541
This research was funded by the Australian Grains Research and Development
542
Corporation (GRDC). We also gratefully acknowledge scholarship support for
543
AS through The Commonwealth Hill Scholarship, The Ransom Mortlock Trust
544
Scholarship and The Phil Watters Memorial Scholarship.
545
546
22
547
References
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
Allison LE, Moodie CD (1965) Carbonate: volumetric calcimeter method. In
'Methods of soil analysis. Part 2. Chemical and microbiological methods.' Ed.
CA Black) pp. 1389. (American Society of Agronomy: Madison, WI)
Battle-Aguilar J, Brovelli A, Porporato A, Barry DA (2011) Modelling soil
carbon and nitrogen cycles during land use change. A review. Agronomy for
Sustainable Development 31, 251-274.
Bertrand I, Holloway RE, Armstrong RD, McLaughlin MJ (2003) Chemical
characteristics of phosphorus in alkaline soils from southern Australia.
Australian Journal of Soil Research 41, 61-76.
Bisutti I, Hilke I, Raessler M (2004) Determination of total organic carbon - an
ovrview of current methods. Trends in Analytical Chemistry 23, 716-726.
Brye KR, Slaton NA (2003) Carbon and nitrogen storage in a Typic Albaqualf
as affected by assessment method. Communications in Soil Science and Plant
Analysis 34, 1637-1655.
Caughey ME, Barcelona MJ, Powell RM, Cahill RA, Grøn C, Lawrenz D,
Meschi PL (1995) Interlaboratory study of a method for determining nonvolatile
organic carbon in aquifer materials. Environmental Geology 26, 211-219.
Chatterjee A, Lal R, Wielopolski L, Martin MZ, Ebinger MH (2009) Evaluation
of different soil carbon determination methods. Critical Reviews in Plant
Sciences 28, 164-178.
Chave KE (1965) Carbonates: Association with organic matter in surface
seawater. Science 148, 1723-1724.
Chichester FW, Chaison J, R.F. (1992) Analysis of carbon in calcareous soils
using a two temperature dry combustion infrared instrumental procedure. Soil
Science 153, 237-241.
Fernandes M, Krull E (2008) How does acid treatment to remove carbonates
affect the isotopic and elemental composition of soils and sediments?
Environmental Chemistry 5, 33-39.
Gibbs RJ (1977) Effect of combustion temperature and time, and of the
oxidation agent used in organic carbon and nitrogen analyses of sediments and
dissolved organic material. Journal of Sedimentary Petrology 47, 547-550.
Gordon J, Carriker MR (1980) Sclerotized protein in the shell matrix of a
bivalve mollusc. Marine Biology 57, 251-260.
Grünewald G, Kaiser K, Jahn R (2008) Hydrotalcite - A potentially significant
sorbent of organic matter in calcareous alkaline soils. Geoderma 147, 141-150.
23
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
Grünewald G, Kaiser K, Jahn R, Guggenberger G (2006) Organic matter
stabilization in young calcareous soils as revealed by density fractionation and
analysis of lignin-derived constituents. Organic Geochemistry 37, 1573-1589.
Heanes DL (1984) Determination of total organic-C in soils by an improved
chromic acid digestion and spectrophotometric procedure. Communications in
Soil Science and Plant Analysis 15, 1191-1213.
Heron G, Barcelona MJ, Andersen ML, Christensen TH (1997) Determination of
nonvolatile organic carbon in aquifer solids after carbonate removal by sulfurous
acid. Ground Water 35, 6-11.
Hockaday WC, Masiello CA, Randerson JT, Smernik RJ, Baldock JA,
Chadwick OA, Harden JW (2009) Measurement of soil carbon oxidation state
and oxidative ratio by 13C
nuclear magnetic resonance. Journal of Geophysical Research 114, GO2014.
Holloway RE, Bertrand I, Frischke AJ, Brace DM, McLaughlin MJ, Shepperd
W (2001) Improving fertiliser efficiency on calcareous and alkaline soils with
fluid sources of P, N and Zn. Plant and Soil 236, 209-219.
Ingalls AE, Aller RC, Lee C, Wakeham SG (2004) Organic matter diagenesis in
shallow water carbonate sediments. Geochimica and Cosmochimica Acta 68,
4363-4379.
Isbell RF (2002) 'The Australian Soil Classification.' (CSIRO Publishing:
Collingwood, Vic)
Jacob DE, Soldati AL, Wirth R, Huth J, Wehrmeister U, Hofmeister W (2008)
Nanostructure, composition and mechanisms of bivalve shell growth.
Geochimica and Cosmochimica Acta 72, 5401-5415.
Kerven GL, Menzies NW, Geyer MD (2000) Soil carbon determination by high
temperature combustion - a comparison with dichromate oxidation procedures
and the influence of charcoal and carbonate carbon on the measured value.
Communications in Soil Science and Plant Analysis 31, 1935-1939.
Kowalenko CG (2001) Assessment of Leco CNS-2000 analyzer for
simultaneously measuring total carbon, nitrogen and sulphur in soil.
Communications in Soil Science and Plant Analysis 32, 2065-2078.
Lettens S, De Vos B, Quataert P, van Wesemael B, Muys B, van Orshoven J
(2007) Variable carbon recovry of Walkley-Black analysis and implications for
national soil organic carbon accounting. European Journal of Soil Science.
Manlay RJ, Feller C, Swift MJ (2007) Historical evolution of soil organic matter
concepts and their relationships with the fertility and sustainability of cropping
systems. Agriculture, Ecosystems and Environment 119, 217-233.
24
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
Matejovic I (1997) Determination of carbon and nitrogen in samples of various
soils by the dry combustion. Communications in Soil Science and Plant Analysis
28, 1499-1511.
McBeath TM, Armstrong RD, Lombi E, McLaughlin MJ, Holloway RE (2005)
Responsiveness of wheat (Triticuum Aestivum) to liquid and granular
phosphorus fertilisers in southern Australian soils
Australian Journal of Soil Research 43, 203-212.
Merry RH, Spouncer LR (1988) The measurement of carbon in soils using a
microprocessor-controlled resistance furnace. Communications in Soil Science
and Plant Analysis 19, 707-720.
Mikhailova EA, Noble RRP, Post CJ (2003) Comparison of soil organic carbon
recovery by Walkley-Black and dry combustion methods in the Russian
Chernozem. Communications in Soil Science and Plant Analysis 34, 1853-1860.
Morse JW, Arvidson RS, Lüttge A (2007) Calcium carbonate formation and
dissolution. Chemical Reviews 107, 342-381.
Nelson DW, Sommers LE (1996) Total carbon, organic carbon, and organic
matter. In 'Methods of soil analysis. Part 3. Chemical methods.' Ed. DL Sparks)
pp. 961-1010. (Soil Society of America and American Society of Agronomy:
Madison, WI)
Rayment GE, Higginson FR (1992) 'Australian laboratory handbook of soil and
water chemical methods.' (Inkata Press: Melbourne)
Santi C, Certini G, D'Acqui LP (2006) Direct determination of organic carbon
by dry combustion in soils with carbonates. Communications in Soil Science and
Plant Analysis 37, 155-162.
Schmidt MWI, Knicker H, Hatcher PG, Kogel-Knabner I (1997) Improvement
of 13C and 15N CPMAS NMR spectra of bulk soils, particle size fractions and
organic material by treatment with 10% hydrofluoric acid. Eur. J. Soil Sci. 48,
319-328.
Skjemstad JO, Clarke P, Taylor JA, Oades JM, Newman RH (1994) The
removal of magnetic materials from surface soils. A solid state 13C CP/MAS
n.m.r. study. Aust. J. Soil Res. 32, 1215-1229.
Skjemstad JO, Janik LJ, Taylor JA (1998) Non-living soil organic matter: what
do we know about it? Australian Journal of Experimental Agriculture 38, 667680.
Soil Survey Staff (1999) 'Soil Taxonomy.' 2nd edn. (United States Department
of Agriculture, Natural Resources Conservation Service: Washington DC)
Soon YK, Aboud S (1991) A comparison of some methods for soil organic
carbon determination. Communications in Soil Science and Plant Analysis 22,
943-954.
25
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
Suess E (1970) Interaction of organic compounds with calcium carbonate - I.
Association phenomena and geochemical implications. Geochimica and
Cosmochimica Acta 34, 157-168.
Telek G, Marshall N (1974) Using a CHN analyzer to reduce carbonate
interference in particulate organic carbon analyses. Marine Biology 24, 219-221.
Walkley A, Black IA (1934) An examination of the Degtjareff method for
determining soil organic matter, and a proposed modification of the chromic
acid titration method. Soil Science 37, 29-38.
Wright AF, Bailey JS (2001) Organic carbon, total carbon, and total nitrogen
determinations in soils of variable calcium carbonate contents using a Leco CN2000 dry combustion analyzer Communications in Soil Science and Plant
Analysis 32, 3243-3258.
26
716
Figure captions
717
718
Figure 1: Plot of total carbon (TC) content of soils following hydrofluoric acid
719
pre-treatment versus TC following sulfurous acid pre-treatment. The best fit
720
linear relationship for non-calcareous soils (solid line) is shown. A 1:1 line
721
(dashed line) is also shown to aid comparison.
722
Figure 2: Plot of Heanes organic carbon (OC) versus Walkley-Black OC. The
723
best fit linear relationship for all soils (solid line) is shown. A 1:1 line (dashed
724
line) is also shown to aid comparison.
725
Figure 3: Plot of total carbon content of soils following hydrofluoric acid pre-
726
treatment versus Heanes organic carbon. The best fit linear relationships for
727
calcarosols and non-calcareous soils (solid line) are shown. A 1:1 line (dashed
728
line) is also shown to aid comparison.
729
27
730
731
Table 1: Sample locations, climatic data and basic properties of the soils used in
this study.
Sample Location
732
733
Clay
pH
content
(H2O)
(%)
Landuse
Annual
Rainfall
(mm)*
Mean
Temperature
(º C)*
Min Max
Non-calcareous Soils
Balranald, NSW
Dooen, Vic
Drillham, Qld
Kangaroo Island, SA
Koppio, SA
Newdegate, WA
Pinnaroo, SA
Tanunda, SA
Walpeup, Vic
8.9
8.5
8.3
6.8
6.2
5.4
8.2
7.6
8.1
21
57
38
10
18
27
1
42
12
Cropping
Cropping
Cropping
Pastoral
Pastoral
Cropping
Cropping
Viticulture
Cropping
320
413
649
484
485
367
326
467
333
10
7.9
12.2
11.6
9.7
8.7
12.1
9.1
9.8
24.3
21.4
27.1
19.1
20.9
23.2
29.6
21.5
23.1
Calcarosols
Cungena, SA
Foul Bay, SA
Greenly, SA
Kadina, SA
Maitland, SA
Pt Kenny, SA
Streaky Bay, SA
Sturt Bay, SA
Warramboo, SA
8.3
8.3
8.4
8.6
8.4
8.6
8.2
7.9
8.3
6
15
22
33
35
24
11
18
9
Cropping
Cropping
Cropping
Cropping
Cropping
Cropping
Cropping
Cropping
Cropping
327
438
490
388
504
378
378
438
344
11.0
12.9
11.6
10.5
11.2
12.2
12.2
12.9
10.3
24.0
20.3
20.7
23.0
21.6
23.0
23.0
20.3
23.6
* Data from the Bureau of Meteorology http://www.bom.gov.au/
28
734
735
736
Table 2: Total carbon (measured by dry combustion), carbonate (measured by
volumetric titration) and organic carbon calculated by subtracting carbonate
carbon from total carbon.
Sample Location
Non-calcareous Soils
Balranald
Dooen
Drillham
Kangaroo Island
Koppio
Newdegate
Pinnaroo
Tanunda
Walpeup
737
738
739
740
741
Total Carbon
(g C/kg soil)
21.1
13.3
10.9
75.1
42.7
18.7
15.2
20.4
7.8
Carbonate
(% CaCO3/g
soil)
2
1
1
1
0
0
0
3
1
Organic
Carbon* (g
C/kg soil)
18.7
12.0
10.0
74.4
42.7
18.7
14.9
16.5
7.7
Calcarosols
Cungena
53.1
32
15.3
Foul Bay
87.3
85
-15.1†
Greenly
91.9
68
10.8
Kadina
36.8
13
21.4
Maitland
45.0
14
28.3
Pt Kenny
57.3
59
-12.9†
Streaky Bay
72.9
83
-27.0†
Sturt Bay
84.9
76
-6.8†
Warramboo
46.5
69
-36.0†
*Organic carbon was calculated as total C – (0.12 x CaCO3), based on the stoichiometric mass of
C in CaCO3
†
Negative values indicate that total carbon was greater than carbonate C. This can be incomplete
conversion of carbonate to CO2 during dry combustion analysis – see text.
.
29
Table 3: Soil organic carbon (OC) content (g C/kg soil) determined by dry combustion in soils pre-treated with sulfurous acid (H2SO3) and
hydrofluoric acid (HF). OC was calculated by multiplying mass recovery by the total carbon content of the acid treated residue (g C/kg soil). TC
is total carbon (g C/kg soil).
Sample Location
Non-calcareous Soil
Balranald
Dooen
Drillham
Kangaroo Island
Koppio
Newdegate
Pinnaroo
Tanunda
Walpeup
Calcarosols
Cungena
Foul Bay
Greenly
Kadina
Maitland
Pt Kenny
Streaky Bay
Sturt Bay
Warramboo
TC – no pretreatment
(g C/kg soil)
Mass recovery from
H2SO3 pretreatment (g/g)*
TC of H2SO3
treated
residue
(g C/kg soil)
OC H2SO3
treated
(g C/kg soil)
Mass recovery
from HF pretreatment (g/g)*
21.1
13.3
10.9
75.1
42.7
18.7
15.2
20.4
7.8
0.92
0.87
0.91
0.94
0.97
0.97
0.96
0.88
0.97
17.1
11.7
12.6
87.8
39.8
18.8
12.8
16.5
7.0
15.8
10.2
11.4
82.5
38.7
18.4
12.2
14.6
6.8
0.055
0.034
0.037
0.152
0.082
0.029
0.038
0.060
0.025
243
237
211
434
404
471
324
198
263
13.3
8.0
7.7
65.8
33.1
13.4
12.3
11.8
6.7
17.2
76.6
83.7
20.2
29.0
46.2
64.7
59.7
32.6
10.6
54.8
44.3
15.8
22.4
25.4
41.4
43.8
18.8
0.120
0.317
0.130
0.033
0.044
0.071
0.085
0.129
0.045
69
51
181
402
378
126
100
153
119
8.2
12.7
23.6
13.3
16.5
8.9
8.5
19.8
5.4
53.1
0.61
87.3
0.71
91.9
0.53
36.8
0.79
45.0
0.77
57.3
0.55
72.9
0.64
84.9
0.73
46.5
0.58
*gram of acid treated residue per gram of whole soil
TC of HF treated
residue
(g C/kg soil)
OC HF
treated
(g C/kg soil)
30
Table 4: Organic carbon determination by the Walkley-Black and Heanes wet oxidation
techniques. OC is organic carbon (g C/kg soil).
Location
Walkley-Black
OC (g C/kg soil)
Heanes OC
(g C/kg soil)
Non-calcareous Soils
Balranald
Dooen
Drillham
Kangaroo Island
Koppio
Newdegate
Pinnaroo
Tanunda
Walpeup
15.1
8.7
7.9
63.4
34.1
15.7
10.8
11.9
5.8
17.3
11.3
9.8
71.3
42.0
19.0
13.3
15.1
7.3
Calcarosols
Cungena
Foul Bay
Greenly
Kadina
Maitland
Pt Kenny
Streaky Bay
Sturt Bay
Warramboo
10.6
28.1
28.8
13.4
21.3
16.9
10.1
28.1
7.5
14.6
32.2
33.9
19.2
27.1
21.2
14.7
31.9
9.3
31
90
Non-calcareous soils
80
TC following HF treatment (g C/kg soil)
Calcarosols
70
60
50
40
y = 0.75x + 0.33
R² = 0.99
30
20
10
0
0
10
20
30
40
50
60
70
80
90
TC following sulfurous acid treatment (g/kg)
Figure 1: Plot of total carbon (TC) content of soils following hydrofluoric acid pretreatment versus TC following sulfurous acid pre-treatment. The best fit linear
relationship for non-calcareous soils (solid line) is shown. A 1:1 line (dashed line) is also
shown to aid comparison.
32
80
Non-calcareous soils
70
y = 1.11x + 2.0
R² = 0.99
Calcarosols
Heanes OC (g/kg)
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
Walkley-Black OC (g/kg)
Figure 2: Plot of Heanes organic carbon (OC) versus Walkley-Black OC. The best fit
linear relationship for all soils (solid line) is shown. A 1:1 line (dashed line) is also shown
to aid comparison.
33
80
Non-calcareous soils
TC following HF treatment (g C/kg soil)
70
Calcarosols
60
y = 0.92x - 1.9
R² = 0.99
50
40
30
20
y = 0.59x - 0.3
R² = 0.76
10
0
0
10
20
30
40
50
60
70
80
Heanes OC (g/kg)
Figure 3: Plot of total carbon content of soils following hydrofluoric acid pre-treatment
versus Heanes organic carbon. The best fit linear relationships for calcarosols and noncalcareous soils (solid line) are shown. A 1:1 line (dashed line) is also shown to aid
comparison.
34