Impact of Soil Texture on the Distributi

Published online January 6, 2006

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Impact of Soil Texture on the Distribution of Soil Organic Matter in
Physical and Chemical Fractions
Alain F. Plante,* Richard T. Conant, Catherine E. Stewart, Keith Paustian, and Johan Six
Six et al., 2002). Several studies have shown that soil texture influences aggregation (Kemper and Koch, 1966;
Chaney and Swift, 1984; Schlecht-Pietsch et al., 1994)
such that increased clay contents were associated with increased aggregation or aggregate stability. In increasing
soil aggregation, soil clay content indirectly affects soil
C storage by occluding organic materials, making them
inaccessible to degrading organisms and their enzymes.
Therefore, soil texture (particularly soil clay content) plays
direct and indirect roles in chemical and physical protection mechanisms.
Unprotected soil C is not intimately associated with
soil mineral particles and is not occluded within aggregates. Unprotected soil C can be defined operationally as
free particulate organic matter (POM), which includes
rapidly metabolized plant and associated microbial carbohydrates and more recalcitrant molecules derived
from resistant plant materials and microbial decomposition products (Golchin et al., 1994; Six et al., 2001).
Biochemically resistant C, defined operationally as organic C resistant to acid hydrolysis (Leavitt et al., 1996),

is an average of 1300 to 1500 yr older than whole-soil C
(Paul et al., 1997; Paul et al., 2001). Even in the presence
of cometabolites, specialized enzymes, and optimum environmental conditions, decomposition of this material
is slow, resulting in turnover times on the order of centuries to millennia. This pool of organic C is often associated with silt and clay minerals (Paul and Clark,
1989) but is protected from decomposition primarily
due to its complex chemical structure rather than by the
mineral association.
These observations suggest soil texture affects chemical and physical protection of soil C stocks, whereas
unprotected C and biochemically protected C should
vary largely independent of soil texture. The principle
of soil texture altering soil C levels and decomposition
kinetics has been integrated into several biogeochemical models (e.g., van Veen and Paul, 1981; Parton et al.,
1987) but has not been fully evaluated across a controlled soil textural sequence. In addition, the means by
which texture alters C dynamics in these models reflect
only the conceptual chemical protection and do not encompass physical protection mechanisms. There is an
increasing demand for new models that incorporate
measurable fractions rather than conceptual pools (e.g.,
Christensen, 1996; Arah and Gaunt, 2001), which has
been met with varying degrees of success (e.g., Sohi
et al., 2001; Skjemstad et al., 2004). The goal of this work

ABSTRACT
Previous research on the protection of soil organic C from decomposition suggests that soil texture affects soil C stocks. However, different pools of soil organic matter (SOM) might be differently related
to soil texture. Our objective was to examine how soil texture differentially alters the distribution of organic C within physically and chemically defined pools of unprotected and protected SOM. We collected
samples from two soil texture gradients where other variables influencing soil organic C content were held constant. One texture gradient
(16–60% clay) was located near Stewart Valley, Saskatchewan, Canada
and the other (25–50% clay) near Cygnet, OH. Soils were physically
fractionated into coarse- and fine-particulate organic matter (POM), siltand clay-sized particles within microaggregates, and easily dispersed siltand clay-sized particles outside of microaggregates. Whole-soil organic
C concentration was positively related to silt plus clay content at both
sites. We found no relationship between soil texture and unprotected
C (coarse- and fine-POM C). Biochemically protected C (nonhydrolyzable C) increased with increasing clay content in whole-soil samples,
but the proportion of nonhydrolyzable C within silt- and clay-sized
fractions was unchanged. As the amount of silt or clay increased, the
amount of C stabilized within easily dispersed and microaggregateassociated silt or clay fractions decreased. Our results suggest that for
a given level of C inputs, the relationship between mineral surface
area and soil organic matter varies with soil texture for physically and
biochemically protected C fractions. Because soil texture acts directly
and indirectly on various protection mechanisms, it may not be a universal predictor of whole-soil C content.

S

can be protected from decomposition and stabilized in soils by different mechanisms, including chemical protection by association with
mineral surfaces, physical protection by occlusion within
aggregates, and biochemical protection by recalcitrance
(Jastrow and Miller, 1997; Six et al., 2002; Krull et al.,
2003). Chemical stabilization of organic molecules through
mineral-organic matter binding is well established (Ladd
et al., 1985; Gonzalez and Laird, 2003). Even labile organic
material that would otherwise decompose quickly can be
protected from decomposition by close association with
silt and clay particles (Sørensen, 1972). Analyses synthesizing multiple studies suggest that stabilization capacity
is dictated by soil silt and clay content and the surface
area and reactivity of mineral soil particles (Hassink,
1997; Kiem et al., 2002; Kiem and Ko¨gel-Knabner, 2002;
OIL ORGANIC MATTER

A.F. Plante, R.T. Conant, C.E. Stewart, K. Paustian, and J. Six, Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO
80523; K. Paustin, Dep. of Soil and Crop Sciences, Colorado State
Univ., Fort Collins, CO 80523; J. Six, Dep. of Plant Sciences, Univ. of
California, Davis, CA 95616. Received 23 Nov. 2004. *Corresponding

author (alainfplante@hotmail.com).

Abbreviations: CPOM, coarse particulate organic matter . 250 mm in
size; d-clay, easily dispersed clay-sized fraction; d-silt, easily dispersed
silt-sized fraction; fPOM, fine particulate organic matter 53–250 mm
in size; POM, particulate organic matter; magg-clay, microaggregatederived clay-sized fraction; magg-silt, microaggregate-derived silt-sized
fraction.

Published in Soil Sci. Soc. Am. J. 70:287–296 (2006).
Nutrient Management & Soil & Plant Analysis
doi:10.2136/sssaj2004.0363
ª Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

287

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288

SOIL SCI. SOC. AM. J., VOL. 70, JANUARY–FEBRUARY 2006

was to examine how soil texture alters the distribution of
organic C in physically and chemically defined pools of
soil organic matter for the long-term goal of parameterizing the impact of soil texture in a model of soil organic
C dynamics based on measurable fractions. While holding C inputs and other factors influencing soil C turnover (temperature, precipitation, litter quality, tillage,
etc.) reasonably constant, we evaluated the effects of
texture on unprotected and protected soil C stocks. The
fractionation scheme outlined by Six et al. (2002) aims to
isolate pools of organic C based on physical, chemical,
and biochemical mechanisms of protection of organic C
and was applied to samples from two in situ soil texture
gradients. Specifically, soil texture effects on unprotected
and physically protected C are evaluated by testing the
hypothesis that particulate organic matter content varies
largely independent of soil texture. Soil texture effects
on chemically protected organic C are evaluated by testing the hypothesis that silt- and clay-associated soil C
stocks are directly related to silt and clay content. The
hypothesis that the proportion of silt- and clay-associated
C that is nonhydrolyzable does not vary with soil texture

is tested to assess the impact of texture on biochemically
protected C.
MATERIALS AND METHODS
Sites and Sampling
Two sites were selected where in situ soil texture gradients
could be generated by sampling spatially while keeping all
other variables constant. The soils of the Saskatchewan site
developed under native grasslands, whereas the soils at the
Ohio site developed under native forest. Both sites have been
under long-term agricultural production, and each consists of a
small localized area with reasonably consistent parent materials, topography, and climate but with significant variability
in soil texture. Land management, including tillage type, tillage
frequency, tillage timing, and cropping histories, were similar
within each area. Analysis of crop yield and aboveground biomass data from the Saskatchewan gradient (McConkey and
Brandt, personal communication) showed only minor differences between textures and that interannual variability was
greater than the variability across textures. We therefore assumed that crop yields and soil organic C inputs were reasonably constant within each gradient. The result of the site
selection is that the dominant factor influencing soil C turnover is soil texture.
Saskatchewan
Soil samples were collected in April 2002 from six farm
fields located near Stewart Valley, Saskatchewan, Canada

(508179 N; 1078489 W). All sites were within 20 km of each
other and were conventionally tilled under long-term (901
years) cereal-pulse crop rotation including fallow periods
(usually wheat [Triticum aestivum], fallow, and fieldpea [Pisum
sativum]). Soils in the area are classified as Aridic Borolls
(Septre, Fox Valley, Birsay Haverville, and Birsay Hatton
series). Clay mineralogy in this area is dominated by montmorillonite, with some kaolinite and illite (Brierley et al.,
1996). With the assistance of staff from Agriculture & AgriFood Canada, we used soil survey maps and hand texturing
in the field to select six locations stratified across a soil texture
gradient that represented the treatment levels (i.e., textures).

Within each of the six treatment locations, three transects were
sampled and treated as replicates. Six surface soil (0–20 cm)
cores were collected for each replicate and separated into 0–5 cm
and 5–20 cm subsamples in the field. Due to soil compaction of
two clayey textures when using a Giddings soil probe, we handdug pits and sampled the soil horizontally at the two depths.
Only the surface (0–5 cm) subsamples were further analyzed
because these samples were expected to demonstrate the largest
differences in organic matter protection due to physical protection because of increased wet-dry and free-thaw cycles at the
soil surface.

Ohio
A second textural gradient was sampled in October 2002
from within a single 400-ha field near Cygnet, OH, located
approximately 15 km SW of the Ohio Agricultural Research
and Development Center’s Northwest Agricultural Research
Station near Hoytville, OH (41809 N; 84809 W). The soils of the
field consisted of Mollic and Aeric Epiaqualfs (Hoytville and
Napanee series), all managed under a corn (Zea mays), soybean (Glycine max), oat (Avena sativa) rotation with conventional tillage for the last 20 yr. Clay minerals in these soils
are dominated by illites (Collins et al., 2000). Similar to the
Saskatchewan site, soil survey maps and hand texturing were
used to identify four locations within a single farmer’s field,
which represented the treatment levels (i.e., soil textures).
Three transects within each of the treatment levels were sampled and treated as replicates. Seven surface soil (0–20 cm)
cores were collected from each replicate sample and separated
into 0- to 5- and 5- to 20-cm subsamples in the field. Only the
surface samples were further analyzed; the subsurface samples
were archived.

Soil Textural Analysis
Once returned to the laboratory, individual soil core samples were weighed and subsampled for moisture content, and

bulk density was determined using the core volume. The six to
seven individual cores for each replicate were composited to
form the replicate sample. The field-moist soil samples were
passed through an 8-mm sieve by gently breaking apart the
soil. The samples were air-dried, sieved to 2 mm, and stored at
room temperature.
Soil texture was determined using a modified version of the
standard hydrometer method without removal of carbonates
or organic matter (Gee and Bauder, 1986). Briefly, 30 g of airdry, 2-mm sieved soil were shaken for 16 h in a 250-mL
Nalgene bottle with 100 mL of 5 mg L21 sodium hexametaphosphate and 10 glass beads (10 mm in diameter). Soil clay
contents were determined using hydrometer readings taken at
1.5 and 24 h and appropriate interpolation calculations. Coarse
and fine sand contents were determined by pouring and washing the suspension over 250-mm and 53-mm sieves after the
sedimentation was completed. Materials retained on the sieves
were oven dried at 608C and weighed. Soil silt contents were
determined by difference.

Soil Physical and Chemical Fractionations
Microaggregate Isolation
Microaggregates were isolated using a method described by

Six et al. (2000a). Briefly, 50 g of air-dried whole soil were
submerged in deionized water for 30 min to promote slaking
of macroaggregates and poured onto a 250-mm mesh screen
inside a cylinder and reciprocally shaken (120 rev min21) with
50 glass beads (10 mm in diameter) until the complete dis-

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PLANTE ET AL.: IMPACT OF SOIL TEXTURE ON THE DISTRIBUTION OF SOIL/ORGANIC MATTER

ruption of all macroaggregates was achieved. Disruption of
microaggregates was prevented by a continuous flow of water
that immediately flushed the ,250-mm material out of the
shaker and onto a 53-mm sieve (Six et al., 2000a). The fraction
retained on the 250-mm mesh consisted of coarse (POM and
250–2000 mm sand) and comprised the coarse POM fraction
(CPOM .250 mm). The materials retained on the 53-mm
sieve were wet sieved by hand for 2 min at approximately
50 cycles per minute, and finer materials were gently washed
off to isolate stable microaggregates (53–250 mm). The

suspension passing the 53-mm sieve was centrifuged to isolate
the easily dispersed silt-sized fraction (d-silt, 2–53 mm). The
supernatant was flocculated using 0.25 M MgCl2 and CaCl2
and centrifuged to isolate the easily dispersed clay-sized fraction (d-clay, ,2 mm). Fraction suspensions were oven dried at
608C and weighed. Mass and organic C balances were used to
determine the completeness of recovery after the microaggregate isolation procedure.
Dispersion of Microaggregates and POM Isolation
Five to six grams of microaggregate (53–250 mm) samples,
isolated in the previous procedure, were dispersed by shaking
for 18 h with 25 mL of 0.5 g mL21 sodium hexametaphosphate
and 12 glass beads (4 mm in diameter) in 50-mL centrifuge
tubes to isolate fine POM (Cambardella and Elliott, 1992)
and microaggregate-derived mineral fractions. After shaking,
the suspension was poured over a 53-mm sieve and washed
thoroughly to isolate the fine POM fraction (fPOM, 53–250 mm),
which includes fine sand and fine POM that was originally outside the microaggregates and the occluded POM that is released
on dispersion of the microaggregates. The suspension that
passed the 53-mm sieve was centrifuged as described previously
to isolate the microaggregate-derived silt- and clay-sized fractions (magg-silt and magg-clay). Fractions were subsequently
oven dried at 608C and weighed. Mass and organic C balances
of the POM isolation procedure were used to determine the
completeness of recovery.
The microaggregate-derived fractions were corrected for
sand content to determine C concentrations on a true microaggregate basis. Fine sand contents (53–250 mm) of the samples determined during particle-size analysis were subtracted
from the mass of the microaggregate fractions determined
during the first isolation procedure. The organic C concentrations of the fine POM and microaggregate-derived silt- and
clay-sized fractions could then be expressed on a sand-free
microaggregate basis:

g fPOM C
g fPOM C
5
kg sand free magg
g fPOM
g magg
g fPOM
kg soil
3
3
g magg g fine sand
kg magg
2
kg soil
kg soil

½ ð

Þ


The sand correction was not applied on a whole-soil basis because we were concerned only with the composition of the
microaggregates and the distribution of organic C within them
as a function of changing soil clay content.
Acid Hydrolysis
Easily dispersed and microaggregate-derived silt- and claysized fractions were subjected to acid hydrolysis to isolate a
resistant pool of organic C using a modification of the method
described in Paul et al. (1997) without the pretreatment for

289

removal of carbonates. Briefly, 0.5 g of sample was refluxed
at 958C for 16 h in 25 mL of 6 M HCl. When insufficient
material was recovered during previous fractionation steps,
less material (down to 0.3 g) was used or individual replicates were combined. After refluxing, the suspension was
filtered and washed with deionized water over a glass fiber
filter. The residue was then washed from the filter into a
specimen cup, oven dried at 608C, and weighed. The proportion of nonhydrolyzable C was determined using the following equation:

%NHC ¼




gC
massafter
3
massbefore
kg sample after


gC
kg sample before

which accounts for mass loss during acid hydrolysis and recovery of residues. Mass loss was found to be minor.

Carbon and Nitrogen Analyses
Total C and N analyses were done on the whole soil and
each isolated fraction using a CHN analyzer (model LECO
CHN-1000; Leco Corp., St. Joseph, MI). Results of soil
carbonate determination by the pressure transducer method
(Sherrod et al., 2002) indicated that carbonates were not
present (data not shown), and thus total C concentrations can
be equated to organic C concentrations.

Statistical Analyses
Linear relationships between the sample mass content of
various fractions and the organic C contents within these fractions were tested using ordinary least squares linear regression.
Linear regression was used in spite of not having an explicit
independent variable because we sought only the presence or
absence of a relationship and because the error around the Xaxis variable was found to be lower than that on the Y-axis
variable. The linear relationships were in the form Y 5 a 1
bX, where X was whole-soil clay or silt content and Y was
the organic C content within various fractions. Data from the
Saskatchewan texture gradient in Stewart Valley (SK) and the
gradient in Hoytville (OH) were generally analyzed separately.
Overall comparisons between the sites or between fractions
within a site (all textures combined) were done using Student
t tests assuming equal variances and were considered statistically significant at P , 0.05. Differences in the response of
organic C contents in various fractions to soil texture were
tested using standard ANOVA techniques.

RESULTS
Whole-Soil Texture and Organic
Carbon Concentration
The range of soil textures at the Saskatchewan site was
wider than at the Ohio site (Table 1). Soil clay contents in
the Saskatchewan texture gradient ranged from 159 to
603 g clay kg21 soil and ranged from 249 to 492 g clay kg21
soil at the Ohio site. Whole-soil organic C ranged from
7.0 to 18.8 g C kg21 soil at Saskatchewan and from 16.5 to
26.5 g C kg21 soil at Ohio (Table 1). The relationship
between soil texture, as represented by whole-soil silt
plus clay content, and the total amount of organic C
stored within the soils (Fig. 1) was statistically significant

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SOIL SCI. SOC. AM. J., VOL. 70, JANUARY–FEBRUARY 2006

Table 1. Bulk soil characteristics of 0–5 cm surface soil samples (mean 6 SD, n 5 3).

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Particle-size distribution
Site

Sample†

Whole-soil
organic C
21

Stewart Valley, SK

Hoytville, OH

1
2
3
4
5
6
1
2
3
4

g kg
7.0 6 1.8
10.3 6 1.1
18.8 6 2.3
15.1 6 0.8
17.4 6 0.8
13.9 6 0.9
16.5 6 1.2
24.2 6 1.7
24.5 6 3.3
26.5 6 0.6

Whole-soil
total N

C/N

Bulk density

21

g kg
0.7 6 0.2
1.2 6 0.2
2.0 6 0.4
1.5 6 0.1
2.0 6 0.1
1.6 6 0.3
1.5 6 0.1
2.1 6 0.2
1.9 6 0.3
2.7 6 0.1

23

10.5
8.6
9.9
10.1
8.7
8.9
11.2
11.3
12.8
9.7

6
6
6
6
6
6
6
6
6
6

2.7
0.4
2.4
0.1
0.3
1.1
0.1
1.0
0.9
0.2

Mg m
1.59 6 0.12
1.35 6 0.11
1.27 6 0.14
1.20 6 0.13
0.83 6 0.11
0.75 6 0.16
1.47 6 0.17
1.34 6 0.18
1.24 6 0.17
1.29 6 0.14

Coarse sand
(.250 mm)
21

g kg
148 6 23
20 6 32
14 6 44
665
863
261
200 6 41
179 6 15
61 6 10
42 6 8

Fine sand

Silt

Clay

53–250 mm
639 6 31
430 6 35
367 6 32
96 6 51
70 6 19
48 6 4
350 6 62
340 6 4
281 6 20
122 6 17

2–53 mm
54 6 13
253 6 54
366 6 53
490 6 17
452 6 21
346.4 6 24
201 6 16
220 6 2
315 6 10
344 6 16

,2 mm
159 6 9
296 6 7
253 6 18
408 6 38
470 6 35
603 6 26
249 6 11
260 6 19
348 6 5
492 6 9

† Samples are ranked in order within sites from highest to lowest whole-soil sand content (lowest to highest silt 1 clay content).

in the Ohio (P 5 0.012, r 2 5 0.48) and Saskatchewan
(P 5 0.0028, r 2 5 0.46) texture gradients.

Organic Carbon Concentrations of Physically
Isolated Fractions
Recovery of mass (as CPOM, microaggregates, d-silt,
and d-clay) after the microaggregate isolation procedure
was 97.9 6 1.1% (mean 6 standard deviation), and total
organic C recovery was 96.9 6 9.8%. Mass recovery (as
fPOM, magg-silt, and magg-clay) after the POM isolation
from microaggregates procedure was 98.8 6 1.5%, and
organic C recovery was 98.5 6 14.5%. The high variance
of C recovery in the POM isolation procedure was due
primarily to the high variability in the amount of fPOM
obtained, and its organic C concentration was due to
interfering sand contents. The two-step physical fractionation scheme was successful in isolating noncomposite pools of organic C (Smith et al., 2002) because C
recovery was high and because there was no redundancy

in organic C allocation between the fractions. However,
their usefulness as modelable, functional pools can be
assessed only when their dynamic behaviors become
properly described.
The mass distributions of samples after physical fractionation reflect their textural composition (Fig. 2).
In general, the proportion of mass associated with the
coarse (.250 mm) and fine (53–250 mm) sand plus
POM fractions decreases with increasing clay content,
whereas the mass of the microaggregate-derived silt
increased with increasing clay content in the Saskatchewan soils and the mass of the easily dispersed siltsized fraction increased with increasing clay content in
the Ohio soils.
For all textures combined, approximately 76% of the
organic C in the fractions was associated with mineral
fractions (silt and clay-sized fractions); this was nearly
the same at both sites (P 5 0.99) (Fig. 3). Across all
textures within the Saskatchewan texture gradient, organic C was greater in the microaggregate-derived silt-

Fig. 1. Relationships between whole-soil organic C concentration and soil fine fraction (silt 1 clay) content for the Saskatchewan and Ohio texture
gradients.

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PLANTE ET AL.: IMPACT OF SOIL TEXTURE ON THE DISTRIBUTION OF SOIL/ORGANIC MATTER

291

Fig. 2. Soil mass distribution of physical fractions isolated from soils in the Saskatchewan and Ohio texture gradients. Samples are ranked in order
from highest to lowest whole-soil sand content (lowest to highest silt 1 clay content) within sites.

and clay-sized fractions than in the easily dispersed
fractions (49% of the organic C stock versus 27%, P ,
0.001), whereas the reverse was true in the Ohio gradient (30% versus 47%, P , 0.001). Within the Ohio
texture gradient, the proportion of the organic C associated with microaggregate-derived or easily dispersed
mineral fractions did not seem to differ with increasing
soil clay content (P 5 0.39 and r 2 5 0.075 for microaggregate-derived and P 5 0.068 and r 2 5 0.29 for easily
dispersed mineral fractions), but significant trends were
observed in the Saskatchewan gradient (P , 0.001 and
r 2 5 0.79 for microaggregate-derived and P , 0.001 and
r 2 5 0.74 for easily dispersed mineral fractions). As the
soil clay content increased, the proportion of organic
C associated with the microaggregate-derived fractions
increased from approximately 38 to 65%, with a concomitant decrease in the easily dispersed mineral fractions from 34 to 18%.
The organic C concentrations of individual isolated
fractions showed varying responses to soil texture in the
two soils (Fig. 3). In the Saskatchewan texture gradient,
statistically significant relationships were found for the
fPOM, magg-silt, and magg-clay fractions (three out of
six fractions). The magg-silt fraction was the only one
with a positive slope, whereas the organic C stocks in the
fPOM and magg-clay fractions decreased with increasing whole-soil silt 1 clay content. In the Ohio texture
gradient, organic C concentrations decreased in the maggclay fraction, increased in the d-silt fraction, and showed

no significant trends with increasing soil silt 1 clay content
in the remaining fractions.
After correction for sand particles, the mass of microaggregates increased with increasing whole-soil clay content in both texture gradients (P , 0.001, r 2 5 0.93 for
Saskatchewan and P , 0.001, r 2 5 0.76 for Ohio; data
not shown). Total sand-free microaggregate-associated
C decreased with increasing soil clay content in the
Saskatchewan soils, whereas no trend was observed in
the Ohio soils (Fig. 4). The trend in the Saskatchewan
soils is attributable to decreases in fine POM and in the
magg-clay associated C.
When the organic C concentrations of individual fractions are expressed on a per mass of fraction basis rather
than on a per mass of soil basis, microaggregate-derived
and easily dispersed silt- and clay-sized C concentrations
significantly decreased with increasing content of the
fraction in the soil (Fig. 5 and 6), although the relationship in the magg-silt fraction from the Ohio gradient
was weaker (P 5 0.085, r 2 5 0.27). When compared with
each other, these trends did not differ between the easily
dispersed versus the microaggregate-derived fractions.

Acid Hydrolysis of Physically Isolated Fractions
The proportion of organic C remaining after acid hydrolysis treatment of whole-soil samples was slightly
higher (P 5 0.084) in the Ohio soils than in the Saskatchewan soil (Table 2). Nonhydrolyzable C increased

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292

SOIL SCI. SOC. AM. J., VOL. 70, JANUARY–FEBRUARY 2006

Fig. 3. Organic C concentrations of (a) coarse sand and POM, (b) fine sand and POM, (c) microaggregate-derived silt-sized, (d) microaggregatederived clay-sized, (e) easily dispersed silt-sized, and (f) easily dispersed clay-sized fractions isolated from soils in the Saskatchewan and
Ohio texture gradients. Samples are ranked in order from highest to lowest whole-soil sand content (lowest to highest silt 1 clay content)
within sites.

with increasing whole-soil clay content in both soils (P 5
0.007 in the Saskatchewan texture gradient and P 5
0.006 in the Ohio gradient). No relationships between
whole-soil clay content and the nonhydrolyzable C of the
silt- and clay-sized fractions from either soil were
observed, regardless whether the fractions were microaggregate derived or easily dispersed. No differences in
the proportion of nonhydrolyzable C between easily
dispersed and microaggregate-derived fractions were
observed for the silt-sized (P 5 0.12) or clay-sized (P 5
0.46) fractions of both soils. Overall, the silt-sized
fractions had higher proportions of nonhydrolyzable C
than the clay-sized fractions (P , 0.001).

DISCUSSION
Soil Texture Effects on Whole-Soil
Carbon Concentration
Although the degree of association of SOM with soil
mineral surfaces (particularly soil clays) has long been
recognized as a mechanism for the stabilization of or-

ganic C, whole-soil clay content is not always a good
predictor of whole-soil organic C concentration. The
direct evidence for the long-term effect of soil texture
on organic C storage is derived primarily from correlations in soil databases and is inconsistent. Nichols (1984)
found a strong correlation (r 5 0.86) between soil clay
content and organic C concentration in the Southern
Great Plains. Percival et al. (2000), however, found that
soil clay content explained little of the variation in
organic C accumulation (r 2 , 0.05) in New Zealand. The
current study showed a significant relationship between
soil texture and soil organic C concentration at Ohio
and Saskatchewan. The slope of the relation from the
Saskatchewan soils was slightly lower than in Ohio, although the difference was not statistically significant
(P 5 0.27). Lower organic C concentrations and the
slightly lower slope in the Saskatchewan gradient are
likely due to the lower C input levels in this gradient.
Wheat straw plus grain yields in the general vicinity of
the Saskatchewan texture gradient average between
1.8 and 5.3 Mg ha21 (Campbell and Zentner, 1993),

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PLANTE ET AL.: IMPACT OF SOIL TEXTURE ON THE DISTRIBUTION OF SOIL/ORGANIC MATTER

293

Fig. 5. Relationships between organic C concentrations (g C kg21
fraction) in the easily dispersed and microaggregate-derived siltsized fractions and whole-soil silt contents for the (a) Saskatchewan
and (b) Ohio texture gradients.

Fig. 4. Organic C concentrations of (a) fine sand and POM, (b)
microaggregate-derived silt-sized, and (c) microaggregate-derived
clay-sized fractions on a sand-free microaggregate mass basis.
Samples are ranked in order from highest to lowest whole-soil sand
content (lowest to highest silt 1 clay content) within sites.

whereas corn grain yields in the area near the Ohio
texture gradient average between 6 and 10 Mg ha21
(Dick et al., 1997). These differing crop types and yields
result in contrasting organic C inputs to the two gradients. These were estimated to be 90 to 140 g C m22 yr21
in Saskatchewan and 310 to 420 g C m22 yr21 in Ohio,
based on crop yield data and formulas relating yields
to above- and belowground biomass similar to those

reported by Kong et al. (2005). In the wheat production system in Saskatchewan, C inputs are relatively low
due to low mean annual precipitation and the inclusion of fallow periods in the crop rotations. In Ohio,
greater mean annual precipitation and continuous corn
or corn-soybean rotations result in more C being returned to the soil. Therefore, even if there might be an
increased capacity for stabilization due to greater soil
clay content, the relationship between whole-soil C concentration and clay content was not fully expressed in
the SK gradient because there is little C input available
to be stabilized.

Soil Texture Effects on Unprotected and
Physically Protected Carbon
Few researchers have reported on the effect of soil
texture on the amount of POM. Although they found
no relationship between whole-soil organic C and soil
texture in a series of cultivated Cambisols, Ko¨lbl and
Ko¨gel-Knabner (2004) found that the amount of organic
C present as POM occluded in aggregates increased
with increasing soil clay content. In contrast, they found
that the amount of organic C present as free POM

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

294

SOIL SCI. SOC. AM. J., VOL. 70, JANUARY–FEBRUARY 2006

Fig. 6. Relationships between organic C concentrations (g C kg21
fraction) in the easily dispersed and microaggregate-derived claysized fractions and whole-soil clay contents for the (a) Saskatchewan
and (b) Ohio texture gradients.

was not related to clay content. We did not distinguish
between free and occluded POM in the current study
but separated the total POM based on size. In terms of
mass, the coarse (.250 mm) and fine (53–250 mm) POM
fractions decreased with increasing clay content in the
Saskatchewan and Ohio soils. This is more likely attributable to changes in sand content of the fraction
rather than the POM itself. Organic C concentrations in
the POM fractions showed no relationship with texture,
except the fPOM fraction in the Saskatchewan gradient,

which showed a slight decrease. These results seem to
contradict those previously reported in the literature,
which generally showed increases in total POM with
increasing soil clay content (e.g., Needelman et al., 1999).
Our results are consistent with those reported by Franzluebbers and Arshad (1997), who showed no response
in total POM with soil texture. It is likely that a direct
causal relationship between soil clay content and POMassociated C does not exist but that the relationship is
indirect through the effects of soil clay on aggregation. In
the Saskatchewan texture gradient, the combined results
of increased microaggregate mass and decreased organic
C with increased soil clay content suggest a trend similar
to that observed in the silt- and clay-associated organic
C. The increased microaggregation diluted the associated C because the silt- and clay-sized materials comprising the microaggregates had decreasing organic C
concentrations. In addition, our results suggest that with
reasonably similar C inputs, increasing microaggregation
due to greater soil clay contents across the gradient diluted the amount of fine POM rather than promoting its
increased retention.
Examination of the total clay recovery from the microaggregate and POM isolation procedures reveals lower
clay contents than those determined in the particle-size
analysis. This suggests that dispersion was incomplete
during the physical fractionation and that clays are likely
to be found in silt-sized microaggregates. Complete
dispersion of the soil was not the objective of the physical
fractionation but does limit the inferences that can be
made concerning organic matter associated with the siltsized fraction. The microaggregate-derived, silt-sized
fraction increased dramatically in weight and C associated with it across the Saskatchewan gradient, whereas in
the Ohio gradient the easily dispersed silt-sized fraction
increased (Fig. 2). Thus, the structural unit in which C is
accumulated seems to differ between the two texture
gradients. We suggest that this is related to the overall
difference in the range of textures between the two gradients; the soil textures at the Ohio site were sandier than
at the Saskatchewan sites. In the less sandy soils of
Saskatchewan, there was a higher capacity for the formation of stable microaggregates within macroaggregates
(Oades, 1984; Six et al., 2000a) than in the coarser soils
of OH. Therefore, organic C was preferentially protected
in silt-sized aggregates occluded within the microaggregates, whereas silt-sized aggregates, with the associated

Table 2. The proportion of nonhydrolyzable carbon (%) in various mineral fractions (mean particle-size distribution [g kg21] SD, n 5 3).
Site
Stewart Valley, SK

Hoytville, OH

Sample†

Whole-soil

Easily
dispersed silt

1
2
3
4
5
6
1
2
3
4

31.7 6 2.9
46.1 6 1.9
40.5 6 4.3
48.9 6 0.4
50.2 6 6.0
46.1 6 4.9
41.4 6 2.0
43.4 6 5.0
54.9 6 1.9
54.8 6 4.7

53.5 6 3.6
56.4 6 5.8
40.5 6 5.2
46.9 6 3.4
64.4 6 3.4
58.5 6 6.6
62.6 6 10.1
52.3 6 1.6
60.9 6 0.4
60.0 6 8.7

Microaggregatederived silt
56.2‡
59.0 6
43.2 6
60.0 6
65.1 6
51.7 6
62.2 6
60.9 6
68.4 6
67.1 6

4.1
2.5
0.6
5.4
9.4
2.6
2.0
7.8
6.5

Easily
dispersed clay
51.5 6 0.8
58.7 6 10.2
46.8 6 11.2
45.8 6 10.7
50.3 6 4.3
44.6 6 7.0
42.7 6 2.4
46.8 6 2.5
51.1 6 1.0
45.6 6 14.7

† Samples are ranked in order within sites from highest to lowest whole-soil sand content (lowest to highest silt 1 clay content).
‡ n 5 1. Replicates composited because of lack of material recovered during microaggregate POM analysis.

Microaggregatederived clay
44.4‡
48.1 6
40.4 6
51.3 6
44.2 6
50.1 6
64.4 6
50.8 6
40.4 6
54.0 6

2.7
9.4
2.8
2.1
1.7
21.0
1.7
14.2
3.6

PLANTE ET AL.: IMPACT OF SOIL TEXTURE ON THE DISTRIBUTION OF SOIL/ORGANIC MATTER

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

occlusion of C, would form directly outside of microaggregates at the Ohio site.

Soil Texture Effects on Chemically
Protected Carbon
The most surprising results of the study were those of
the silt- and clay-associated soil organic C. Soil texture is
often used as a surrogate for surface area and reactivity,
particularly when the mineralogy of the clays are similar.
The limited surface reactivity of a soil is provided as evidence for the potential existence of a limited stabilization
capacity (Hassink and Whitmore, 1997; Six et al., 2002).
The second hypothesis of this study was that silt- and
clay-associated soil C contents would be directly related
to whole-soil silt and clay content, such that any increases
in total soil C could be attributed to increases in silt
and clay contents, which themselves would have similar
organic C concentrations. What we found instead were
decreasing concentrations of organic C within the siltand clay-associated fractions and therefore a mixed response in terms of soil C concentrations. In other words,
given reasonably consistent organic C inputs within each
site, the silt- and clay-associated organic matter became
diluted with greater amounts of silt and clay across the
gradients. A soil that has reached its stabilization capacity would have a constant organic C concentration
in the silt- and clay-associated fractions, and thus any
changes in texture would be reflected in the whole-soil
saturation capacity. The soils in the current study seem to
be far removed from such a stabilization capacity.

Soil Texture Effects on Biochemically
Protected Carbon
Acid hydrolysis is often proposed as a chemical means
of isolating a fraction of biochemically or microbially resistant organic matter. We originally hypothesized that
the proportion of silt- and clay-associated C that is nonhydrolyzable would not vary with soil texture. Our results
supported this hypothesis because no significant differences in proportions of nonhydrolyzable C were found
across textures or between the sources (microaggregatederived versus easily dispersed) of the silt- and clay-sized
fractions. Acid hydrolysis is incapable of accounting for
physical and chemical protection mechanisms because
organic matter sorbed to mineral surfaces from easily
dispersed fractions was equally susceptible to acid hydrolysis compared with those from microaggregate-derived
reactions. However, our results showing that organic
matter associated with silt-sized fractions was more resistant to acid hydrolysis than clay-associated organic
matter support reports in the literature (Tiessen and
Stewart, 1983; Anderson and Paul, 1984; Christensen and
Sorensen, 1985; Six et al., 2000b) that concluded that siltassociated organic matter is the most stable fraction.
Therefore, the relative contribution of silt and clay to the
soil texture might alter the total amount of nonhydrolyzable C, and thus the size of the biochemically protected
pool does indeed vary with texture. This supports the
practice of some models of soil organic matter dynamics to
use soil texture to modify the proportion of organic matter

295

in the stable pool. However, this outcome may be confounded by the relative amounts of C associated with the
silt- and clay-sized fractions.

CONCLUSIONS
We found no significant relationship between soil texture and unprotected (coarse- and fine-POM) organic
C. The role of texture in the physical protection of soil C
seemed to differ between the Saskatchewan and Ohio
texture gradients. In Saskatchewan, a higher capacity to
form stable microaggregates resulted in increased organic C in the microaggregate-derived silt-sized fraction
with increasing soil silt and clay content. In the Ohio
gradient, the easily dispersed silt-sized fraction showed
the greater response. Mineral-associated, or chemically
protected, organic C was strongly affected by the soil silt
and clay contents, as would be expected through the role
of surface properties. As the amount of silt or clay increased, the concentration of organic C associated with
easily dispersed and microaggregate-associated silt or
clay fractions decreased. Biochemically protected (nonhydrolyzable) C increased with increasing clay content
in whole-soil samples, but the proportion of nonhydrolyzable C within silt- and clay-sized fractions was
unchanged. Our results suggest that soil texture, as represented by soil clay or silt 1 clay content, may not
always be a good predictor of whole-soil organic C content. This is likely because soil texture affects organic C
storage through direct and indirect mechanisms. For an
assumed constant level of C inputs within each of the
texture gradients observed, the relationship between
mineral surface area, as expressed by soil silt and clay
content, and soil organic matter seem to vary according
to the mechanisms by which the organic matter is stabilized in the soil, whether by predominantly physical,
chemical, or biochemical protection.
ACKNOWLEDGMENTS
The authors thank Brian McConkey and Kelsey Brandt
from Agriculture & Agri-Food Canada for site assistance in
Saskatchewan; Matthew Davis, Frank Thayer, and Nathan
David from the Ohio Agricultural Research and Development
Center for site assistance in Ohio; and Shane Cochran, Joyce
Dickens, Mike Katz, Sarah Moculeski, and Jodi Stevens for
laboratory assistance during the soil fractionations. This project was supported by the Office of Research (BER), U.S. Department of Energy Grant no. DE-FG03-00ER62997, and
Grant no. DE-FG02-04ER63912.

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