Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol52.Issue1-2.Sept1999:

Soil & Tillage Research 52 (1999) 1±9

Tillage effects on soil organic carbon distribution
and storage in a silt loam soil in Illinois
Xue-Ming Yang1,*, Michelle M. Wander
Turner Hall, S. Goodwin Ave., Department of NRES, University of Illinois, Urbana-Champaign, IL 61801, USA
Received 18 August 1998; received in revised form 27 January 1999; accepted 16 February 1999

Abstract
Interest in tillage impacts on sequestration of soil organic carbon (SOC) has increased greatly during recent years. The use of
reduced and no-tillage (NT) practices generally increases the SOC concentration in surface few centimeters when compared to
conventionally tilled soils. However, use of conservation tillage does not always result in increased SOC storage overall. The
effects of sample handling and data expressions on the assessment of tillage-induced SOC sequestration were investigated
using data collected from a tillage trial on a Thorp silt loam (US Taxonomy: ®ne-silty, mixed, mesic Argiaquic Argialboll;
FAO: Orthic Greyzems). The tillage experiment was established in 1986 in Illinois, USA. The NT treatment used no soil
disturbance except for planting. The disk tillage treatment included fall disking (7.5±10 cm deep) after corn (Zea mays L.) and
spring ®eld cultivation after soybean (Glycine max L.) production. The moldboard plowing treatment included fall moldboard
plowing (20±25 cm deep) after corn, followed by spring disking (7.5±10 cm deep) and ®eld cultivating; fall chisel plowing
(30±35 cm) was done after soybean, followed by spring disking and ®eld cultivation. Estimates of tillage impacts on SOC
sequestration varied with the soil depth considered, time of sampling, and sample handling technique. Results indicated that
tillage-induced changes in SOC occurred in the surface 30 cm. NT soil had greater C contents in the upper 30 cm when

assessed on a concentration and volumetric basis. Although the use of NT practices did increase C content stored as surface
residues and SOC concentrations in the top few cm of the soil, it did not increase SOC storage overall. # 1999 Elsevier
Science B.V. All rights reserved.
Keywords: Tillage; Soil organic carbon; Distribution; Storage; Equivalent mass

1. Introduction
Accurate assessments of tillage effects on soil
organic matter (SOC) sequestration are needed. A
small difference in SOC contents can substantially
*Corresponding author. Tel.: +1-519-824-4120
E-mail address: xyang@lrs.uoguelph.ca (X.-M. Yang)
1
Present address: Department of Land Resource Sciences,
University of Guelph, Guelph, Ont., Canada, N1G 2W1.

change soil C storage estimates because the terrestrial
carbon pool is so large. Changes in SOC contents have
environmental rami®cations, including contributions
to or amelioration of greenhouse gases (CO2 and CH4)
and ultimately global warming (Bouwman, 1989; Post

et al., 1990). Changes in SOC storage are also closely
related to soil quality and the long-term sustainability
of agriculture (Doran and Parkin, 1994).
Tillage strongly in¯uences SOC distribution and
storage by physically mixing soil and by distributing

0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 5 1 - 3

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X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

crop residues in the soil. Many studies have indicated
that the use of reduced or no-tillage (NT) practices
better protects the soil resource by increasing SOC as
compared to moldboard plow (MP) practices (e.g.,
Doran, 1987; Mahboubi et al., 1993). Kern and Johnson (1993) and Paustian et al. (1997) concluded that
the application of conservation tillage, in particular
NT, generally leads to higher SOC levels than do

conventional tillage. However, conservation tillage
practices have limited ability to increase SOC in
®ne-textured and poorly-drained soils and in sites
where cold climate constrains organic matter decay
(Carter and Rennie, 1982; Angers et al., 1995; Angers
et al., 1997; Paustian et al., 1997). In studies with three
®ne-textured poorly drained Illinois soils, Wander et
al. (1998) found NT practices had increased SOC in
surface soil (0±5 cm) at the expense of SOC stored at
depth (5±17.5 cm), as compared to conventional tillage. In a study that included many soil types and
climatic conditions in Illinois, Needelman et al. (Personal communication) con®rmed that NT and MP
practices have had different impacts on the vertical
distribution but not the quantity of SOC. In a typic
Argiudoll in Argentina, Alvarez et al. (1998) found
that the use of NT would not signi®cantly affect SOC
pools in a region where erosion rates were low.
In most organic matter studies, SOC refers to
organic matter within the soil. Residues accumulated
on the soil, and sometimes within the soil, are typically removed before C is quanti®ed. While in accord
with the de®nition of soil organic matter, which is the

organic fraction of the total soil exclusive of undecayed plant and animal residues, removal of residues
may lead to incorrect and/or variable assessment of
soil C storage potential. The amounts of residue in soil
re¯ect disturbance and input patterns. Paustian et al.
(1997) noted that the contribution of the surface
residue to SOC buildup by NT was not fully accounted
for by comparisons of plowed and NT systems where
residues were removed. In such cases, the effect of NT
on total carbon storage would be underestimated.
Estimates of tillage effects on SOC storage differ
because of variations in sampling and data expression.
Ellert and Bettany (1995) questioned the validity of
SOC expressed in simple concentration units or as the
product of concentration and soil bulk density from
the same soil thickness. To resolve this, the equivalent
mass method was used to assess SOC sequestration

(Powlson and Jenkinson, 1981; Ellert and Bettany,
1995). Ellert and Bettany (1995) found that calculation of mass of C, N, P, and S on an equivalent mass
basis eliminated previously identi®ed tillage-based

differences in elemental contents.
Temporal changes in tillage effects on SOC storage
are poorly understood. Several studies suggest that
SOC contents increase rapidly during the ®rst 10 years
following conversion to NT practices and that this
period is followed by slower SOC increases (Staley
et al., 1988; Dick et al., 1991; Ismail et al., 1994).
Generally, these decadal scale trends ignore withinyear ¯uctuations in SOC contents and other properties
that in¯uence SOC storage estimates that could be
large enough to mask tillage effects. Estimates of SOC
sequestration may vary with time due to the dynamics
of C contained in residue and shifts in soil bulk density
(Markin et al., 1996). Both surface residues and bulk
density are known to vary within-season (Markin et
al., 1996). Failure to consider factors that vary in the
short-term may compromise estimates of long-term
trends.
Arbitrary and inconsistent approaches to estimating
of SOC sequestration can lead to variable results.
Several factors must be taken into account when

estimating SOC storage. Ignoring these factors can
result in incomplete estimates of SOC storage. The
objective of this study was to quantify tillage impacts
on SOC storage in a silt loam in Illinois, USA, while
considering the in¯uences of soil depth, volume or
mass considered, surface and soil incorporated residues, and date of sampling. Tillage effects were
summarized using both concentration and equivalent
mass-based assessments.

2. Materials and methods
2.1. Site description
Samples were collected from a tillage experiment
established on the Agricultural Engineering Research
Farm of the University of Illinois, at Urbana, IL. The
tillage treatments were initiated in Fall 1986. The soil
is a Thorp silty loam (US Taxonomy: ®ne-silty, mixed,
mesic Argiaquic Argialboll; FAO classi®cation:
Orthic Greyzems) with a pH of 6.6±6.8. The experimental plots were laid out in a completely randomized

X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

block with eight replicate plots. In this work, we
characterized three of the eight treatments included
in that tillage study. Samples were collected from NT,
disking tillage (DT), and MP treatments. The NT
treatment used no soil disturbance except for planting.
The DT treatment included fall disking (7.5±10 cm
deep) after corn (Zea mays L.) and spring ®eld cultivation after soybean (Glycine max L.) production. The
MP treatment included fall MP (20±25 cm deep) after
corn, followed by spring disking (7.5±10 cm deep) and
®eld cultivating; fall chisel plowing (30±35 cm) was
done after soybean, followed by spring disking and
®eld cultivation. Eight replicate plots for each main
tillage treatment were further split into south and north
®elds in which corn and soybeans were rotated
annually.
2.2. Soil sampling and handling
In 1994 and 1995, soil samples were collected after
fall ®eld operations. The samples were collected from
the top 30 cm with a 4.9 cm diameter splitable soil

core sampler (Forestry Supply, Jackson, MS) in midDecember. Three subsamples were collected from
each plot. Cores were divided into 0±5, 5±15, and
15±30 cm increments. Samples were collected in
similar fashion down to a 90 cm depth with the
hydraulic probe before planting in April 1997. These
samples were separated into eight intervals: 0±5, 5±10,
10±20, 20±30, 30±40, 40±50, 50±70, and 70±90 cm.
Samples collected on all dates were air dried and
weighed. A subsample, dried at 1058C, was used to
calculate water content; this value was used to calculate soil bulk density. Visibly identi®able crop residues
were removed manually from the 1994 and 1995
samples. No residues were removed from the 1997
samples.
Soil total carbon content was determined with a
LECO CNS-2000 (Leco, St. Joseph, MI). Total C was
assumed to equal SOC because this soil contained no
free carbonates.
We measured the percentage of the soil surface
covered with crop residue shortly after planting on
May 27, 1997. Amounts of crop residues were estimated using the following equation (Gilley et al.,

1986):
Surface cover ˆ 100…1 ÿ eÿ0:114 weight †;

(1)

3

where surface cover is given as a percentage and
residue weight is measured in Mg haÿ1. The SOC
content of residues was calculated by multiplying
residue mass by 0.40. This value (0.401) was con®rmed by subsequent analysis of C in residues. Carbon
concentration and soil bulk density data were used to
compute SOC storage on a unit area basis to the depth
of plowing (30 cm).
Treatment impacts on the sequestration of C in soil
were based on values expressed in terms of equivalent
mass. Brie¯y, the C concentrations of samples from
each depth were multiplied by the corresponding bulk
density and depth dimension values. The maximum
average mass recovered from an individual plot

sampled to a 30 or 90 cm depth was used as the basis
for C sequestration comparisons. The values used
(4375, 4450, and 12 276 ton haÿ1) were based upon
NT samples collected from the 0±30 cm depth in Fall
1995 and Spring 1997, and from 0±90 cm in Spring
1997, respectively. The data collected in Fall 1994
were combined with the 1995 values because those
data were analyzed together. The total quantities of
SOC were then summed from the surface downward.
Variable amounts of soil (and the associated SOC)
were added to the less dense treatments from the
deeper depth to bring all samples to an equivalent
mass. The additional thickness of soil added to the DT
and MP plots for purposes of SOC comparison were
computed with the following equation:
Additional thickness ˆ …Equivalent mass† ÿ ……BD…l; i†
 TL…l; i† †=BD…bottom; i† †;

(2)

where BD is soil bulk density, TL the thickness of
depth l, l is index of depths, and i refers to either DT or
MP.
The bulk density and SOC concentration of the 15±
30 cm depths were used to estimate SOC additions to
the NT and MP treatments in Fall 1994 and 1995
samples. In Spring 1997, the density and C concentration of the 30±40 cm depths were used to compute
SOC added by the additional thickness in the 30 cm
equivalent mass. The density and SOC concentration
of the 70±90 cm depths were used to compute SOC
added by the additional thickness in the 90 cm equivalent mass. For more information about equivalent
mass- or depth-based analyses, the reader is referred
to Ellert and Bettany (1995).

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X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

2.3. Statistical analysis
Analysis of variance was performed using SAS
PROC MIXED (SAS Institute, 1996) to assess treatment effects on SOC concentration, soil bulk density,
and SOC storage. Since the sampling interval and
handling procedures of Fall 1994 and 1995 samples
differed from the 1997 samples, we analyzed those
data separately. Tillage and depth were considered
®xed effects. Block was considered a random effect.
Year was considered a ®xed effect for the analysis of
the 1994 and 1995 bulk density and SOC storage, and
as a random effect in all other analysis. Since depth
was a repeated measure we used an unstructured
covariance structure in the analysis. Least square
means difference was used to compare the effects
of treatment. Unless otherwise noted, signi®cance
was reported at the 5% probability level.

3. Results
3.1. Soil organic carbon concentration and storage
in Fall 1994 and 1995 samples
Among the main effects, depth had the greatest
impact on SOC concentration, soil bulk density, and

SOC storage (Table 1). Comparisons of SOC concentration and bulk density averaged to 30 cm, and of
SOC storage, considered on an aerial basis, are shown
in Table 2. In general, the use of NT practices
increased SOC concentrations in top few centimeters
and soil bulk density in the top 30 cm as compared to
the DT and MP treatments. When these data were
combined to estimate SOC storage, the results suggest
SOC storage was 25% greater in the NT than in the MP
treatment. Differences among depths in SOC storage
were due to differences in the soil volume compared,
which increased from the top to the bottom depth. The
sum of the C contained in the top and middle depths,
which when combined had a volume equal to the
bottom depth, exceeded the amount of C in the bottom
depth. Based upon Table 2, NT and DT soils stored
25% and 15%, respectively, more SOC in the top
30 cm than did MP soils. The NT and DT soils were
17% and 13% denser than the MP soils. Therefore,
most of the increase in SOC observed for NT and DT
soils was associated with an increase in soil mass.
There was an interaction between tillage and depth.
Tillage effects on SOC contents varied among soil
depths (Tables 1 and 3). Only in the surface (0±5 cm)
were the SOC contents of the NT soil greater than in
the DT and MP soils. The data based upon the same
equivalent mass showed that the NT soils contained

Table 1
Variance analyses of F-statistic (F) and probability (P) for effects on SOC concentration, soil bulk density, and SOC storagea
SOC content (g C kgÿ1 soil)

Bulk density (Mg Mÿ3)

SOC storage (Mg C haÿ1)

F

F

P

F

7.94
39.93
D
54.58
3.42
D
D

0.0051
0.0001

NS
21.68
D
279.07
2.00
D
D

Fall-collected samples (1994 and 1995)
Year
NSb
Tillage
7.49
Year  tillage
NS
Depth
113.56
Year  depth
3.27
Tillage  depth
11.22
Tillage  depth  year
Dc
Spring-collected samples (1997)
Tillage
Depth
Tillage  depth

2.22
276.63
2.95

P

0.0008
0.0001
0.0412
0.0001

0.1119
0.0001
0.0005

NS
7.97
2.23

0.0001
0.0358

0.0001
0.0086

NS
85.10
116

P

0.0001
0.0001
0.1400

0.0001
0.0679

a
The data from Fall 1994 and 1995 are presented separately from spring 1997 data due to different depth intervals and methods used for
preparing soil samples for organic carbon determination. Depth for 1994 and 1995 was 0±30 cm and for 1997 was 0±90 cm.
b
Effect was not significant at P ˆ 10% level.
c
Terms dropped (D) from the model due to lack of statistical significance at P ˆ 0.25.

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X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9
Table 2
SOC concentration, soil bulk density, and SOC storage (volumetric
basis) under different tillage management and in different depths of
fall samples (1994 and 1995)a
SOC
(g C kgÿ1 soil)

Bulk density
(Mg Mÿ3)

SOC storedb
(Mg C haÿ1)

Tillage
No tillage
Disk tillage
Moldboard plow

15.1a
14.4b
13.7b

1.38a
1.30b
1.16c

58.5a
53.8ab
46.6b

Depth
0±5 cm
5±15 cm
15±30 cm

17.1a
14.5b
11.6c

1.13b
1.34a
1.37a

9.8c
19.4b
23.8a

Factor

a

Values followed by different letters in the same column are
significantly different at P < 0.05.
b
Soil organic carbon storage (Mg C haÿ1) ˆ SOC (g kgÿ1)/
10  10 000(m 2 )  thickness of depth (m)  bulk density
(Mg mÿ3). Values are the sum of all three depths.

2.5 (4.7%) and 4.7 (8.6%) Mg SOC haÿ1 more SOC
than did the DT and MP soils, respectively (Table 3).
The DT soils contained 2.2 (3.5%) Mg SOC haÿ1
more than the MP soils. Although the NT soils still
contained more SOC than did the MP soils when
storage was assessed on an equivalent mass basis,
the difference was not statistically signi®cant
(P > 0.10). The volume-based comparison of NT
and CT soils (Mg C haÿ1), obtained by summing
SOC concentrations to a 30 cm depth, underestimated
C storage in the MP treatment by 8.1 Mg haÿ1 (15%).
Table 3
SOC contents and storage expressed on equivalent mass basis:
spring samples (1994 and 1995)a
Factors
Depth (g C kgÿ1)
0±5 cm
5±15 cm
15±30 cm

No
tillage

Disk
tillage

Moldboard
plow

19.6a
14.0c
11.8d

17.1b
14.4c
11.6d

14.8c
15.0c
11.4d

56.9a

54.7a

Equivalent massb (Mg C haÿ1)
59.4a
a

Values followed by different letters in depth and tillage
interaction effect are significantly different at P < 0.05. Values in
the same row followed by different letters are significantly different
at P < 0.05.
b
The equivalent mass is 4373 Mg haÿ1.

Table 4
SOC concentration by depths: Spring 1997 samplesa
Depth (cm)

No tillage
ÿ1

Concentration (g C kg
0±5
19.3a
5±10
13.2b
10±20
13.3a
20±30
9.1b
30±40
6.0a
40±50
4.7a
50±70
3.5a
70±90
2.8a

Disk tillage

soil in depth)
19.3a
15.0a
14.6a
10.1ab
6.1a
4.9a
3.9a
2.7a

C Storage (Mg C haÿ1 in depth)
0±5
13.1a
12.9a
0±10
23.8a
24.4a
0±20
43.8a
45.3a
0±30
57.2a
59.8a
0±40
65.6a
68.1a
0±50
72.0a
74.8a
0±70
81.4a
85.6a
0±90
89.3a
93.5a

Moldboard plow
15.6b
14.0ab
13.9a
11.6a
5.4a
4.5a
3.7a
3.1a

10.5b
20.8b
39.3b
56.3a
64.4a
70.5a
81.0a
89.8a

a
Values followed by the different letters in the same row are
significantly different at P < 0.05.

3.2. Soil organic carbon concentration and storage
in Spring 1997 samples
To fully credit SOC sequestration by treatments,
residues were not removed from the 1997 samples.
Tillage alone had no impacts on SOC storage, bulk
density, and SOC storage (Table 1). Tables 4 and 5
show that SOC concentration, soil bulk density, and
SOC storage once again varied with depth. Soils in the
0±5 and 5±10 cm depths had the lowest, and highest,
bulk density, respectively, in the entire 90 cm pro®le
for NT and DT soils. The MP soil was denser in 30±
40 cm layer too. The NT and DT soils had signi®cantly
higher SOC concentrations than the MP soils in the 0±
5 cm depth (Table 4). The SOC contents sharply
declined with depth in NT soils, and below 5 cm,
NT SOC contents were generally equal to the values
obtained for the DT and MP soils. The SOC contents
of the 20±30 cm depth were greater in the MP than NT
soils.
As was true for fall-collected samples, soil bulk
density varied among tillage treatments in Spring
1997 (Table 5). The NT soils were denser than MP
soils in the top 5±20 cm. Contrarily, MP soils were
denser between 30±40 cm than were the NT and DT

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X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

Table 5
Soil bulk density and soil mass by depth: Spring, 1997a
Thickness (cm)

0±5
5±1 0
10±20
20±30
30±40
40±50
50±70
70±90
a

Soil bulk density (Mg Mÿ3)

Soil mass (Mg haÿ1)

No tillage

Disk tillage

Moldboard plow

No tillage

Disk tillage

Moldboard plow

1.34a
1.62a
1.52a
1.45a
1.39b
1.36a
1.36a
1.43a

1.34a
1.54ab
1.44ab
1.45a
1.36b
1.39a
1.37a
1.45a

1.35a
1.45b
1.35b
1.46a
1.51a
1.42a
1.38a
1.46a

670
810
1516
1454
1385
1361
2710
2860

670
768
1438
1454
1363
1391
2732
2902

675
725
1350
1459
1509
1420
2750
2910

Values followed by different letters in the same row are significantly different at P < 0.05.

Table 6
Impact of buried and surface residues on estimates of SOC storage expressed on an equivalent mass basis: Spring 1997 samples, where no
buried residues were removeda
Sample handling

No tillage

Disk tillage

Moldboard

SOC storage on equivalent mass Mg C haÿ1 (4450 Mg top soil  30 cm)
Excluding SOC in surface residue
57.2a
Including SOC in surface residueb
59.8a

60.5a
62.6a

57.6a
57.8a

SOC storage on equivalent mass Mg C haÿ1 (12 776 Mg top soil  90 cm)
Excluding SOC in surface residue
89.3a
Including SOC in surface residue
92.0a

93.5a
95.5a

89.8a
90.1a

a

Values followed by different letters in the same row are significantly different at P < 0.05.
No tillage contained 2.64 Mg haÿ1 OC in surface residues, disk tillage 2.07 Mg haÿ1, and moldboard plow 0.22 Mg haÿ1 of C,
respectively.
b

soils (Table 5). Changes in bulk density greatly in¯uenced SOC storage within speci®c depth ranges when
calculated on a volume basis (Table 6). Low SOC
storage in the 0±5 cm of MP soil was associated with
low SOC concentration in that depth (Table 4).
Although soil bulk density was similar at the 20±
30 cm depth in all plots, the higher SOC concentrations of the MP soil resulted in estimates of SOC
storage that were 27% and 17% greater than estimates
of SOC stored at that depth in the NT and DT soils.
There were no signi®cant differences in SOC storage
among the three treatments in soil below the 30 cm
depth.
Inclusion of residues formerly removed from 1994
and 1995 samples revealed that the NT soil did not
contain signi®cantly more SOC than the MP soil
(Table 6). When SOC sequestration was assessed on
an equivalent mass basis using either the bulk density
and SOC contents above the added thickness or the

data associated with the actual thickness added, produced different estimates of SOC storage. Use of data
obtained from above the added thickness resulted in
over-estimation of SOC contained in an equivalent
mass of the tilled plots (Table 7) because of higher
SOC concentration than beneath soil. Even though the
carbon contained in surface residues varied with tillage practices (Table 8); tillage impacts on overall
SOC storage remained insigni®cant when all residues
were accounted for (Table 6).

4. Discussion
Use of NT practices has not resulted in signi®cant
accumulation of SOC during the ®rst decade after NT
adoption at this site. This may be due, at least in part,
to the fact that erosion is not as signi®cant a factor in
central as it is in southern Illinois, where use of NT

7

X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

Table 7
A comparison of two methods (A and B) used to compute the length and SOC of the additional thickness of soil used to determine SOC
sequestration with the equivalent mass based methoda
No tillageb

Additional thickness (cm)
Associated SOC (Mg C haÿ1)

Disk tillage

Moldboard plow

Ac

Bd

A

B

A

B

0
0

0
0

0.80b
1.20a

0.88a
0.73b

1.65a
2.80a

1.59a
1.30b

a

The values followed by different letters in the same tillage are significantly different at P < 0.05.
NT values are used as the basis for comparison, equal 100%.
c
Values were computed based on bulk density and SOC content data collected to a 30 cm depth.
d
Values were computed based on bulk density and SOC content data collected to a 40 cm depth.
b

practices has been shown to increase SOC conservation (Hussian, 1997), and to the relative short period of
time.
The most apparent difference among the three
treatments was the vertical distribution of SOC. The
SOC was concentrated at the surface of the NT and DT
soils and was more evenly distributed within the MP
soil. The SOC concentrations declined rapidly in both
DT and NT soil with increasing depth. Wander et al.
(1998) and Needelman et al. (Personal communication) have reported similar results for Illinois soils.
Our data are consistent with other work in the Midwest
(Dick et al., 1991; Karlen and Cambardella, 1996),
that indicates that the use of NT practices alter the
vertical distribution of SOC in these non-erosion
prone soils.
The inclusion of surface residues in the 1997 samples affected sample ranking in SOC storage
(DT  MP  NT with buried residue only, and
DT  NT  MP with surface and buried residue)
for the surface 30 cm soils. Samples containing no
residues (1994
and
1995)
were
ranked
(NT > DT > MP). This suggests that the DT soils
contained more residues than the other two treatments

and that the MP soil contains more residues than the
NT soils. In general, MP incorporate the most, and NT
practices the least, amount of residues into the soil
(Carter and Rennie, 1982; Doran, 1987; Collins et al.,
1992; Tindall and Crabtree, 1980; Stott, 1991).
Accordingly, the nearly identical SOC content of all
three treatments of 1994 and 1995 subsurface soils,
which residues were removed, suggests that SOC
decay rates were fastest in the MP, intermediate in
the DT, and slowest in the NT soil.
Estimates of tillage impacts on SOC sequestration
depended upon several factors. Results based on a
shallow sampling depth and volume-based comparisons positively credit the ability of NT practices to
sequestrate SOC (Fleige and Baeumer, 1974; Granatstein et al., 1987; Campbell et al., 1989). Our fallcollected data suggested that NT soil had higher SOC
concentrations than DT soil, which in turn, had higher
SOC concentrations than MP soil. These concentration-based results did not re¯ect differences in the
density of the soil volume compared. Volume-based
estimates of SOC indicated NT soils contained more
SOC than MP soils. Such volume-based comparisons
have been rightly challenged because they consider

Table 8
Residue percent coverage and C contents after planting 1997a
Crop (Summer 1996)

Soybean
Corn
Mean

No tillage

Disk tillage

Moldboard plow

Cover (%)

C (Mg haÿ1)

Cover (%)

C (Mg haÿ1)

Cover (%)

C (Mg haÿ1)

40
18
26

4.48b
1.74
2.64

35
7
21

3.78
0.64
2.07

3
2
2.5

0.27
0.18
0.22

a

Percentage of surface covered with residues was determined using the string intercept method.
SOC contained in residues was computed with following equation: Surface cover ˆ 100  (1s ÿ eÿ0.114
above ground residues contained 40% carbon.
b

weight

) (Gilley et al., 1986). The

8

X.-M. Yang, M.M. Wander / Soil & Tillage Research 52 (1999) 1±9

unequal soil masses (Ellert and Bettany, 1995). When
our data were assessed using the equivalent mass
method, tillage effects on SOC storage were not found
signi®cant. The form in which data was expressed
determined the conclusions drawn about tillage
impacts on SOC sequestration.
We found that SOC concentration and bulk density
varied within the plow layer. We suggested that a
reasonable comparison of SOC storage should
include, at minimum, the entire rooting zone. Differences in bulk density and SOC concentration observed
below this depth suggest that inclusion of deeper
depths may be warranted. The MP soils had a denser
plow pan at 30±40 cm, which coincided with an area
of relatively lower SOC concentration. According to
Mann (1985), inclusion of deeper soil depths reduces
the variability of estimates of SOC sequestration.
There were signi®cant differences in the bulk density of samples collected on different dates even
though there were no changes in the corresponding
SOC concentrations. Markin et al. (1996) also
reported that soil bulk density varied with time.
Clearly, bulk density can in¯uence estimates of
SOC sequestration expressed on a volumetric or
equivalent mass basis. Even though changing bulk
density did alter our results in absolute terms, they did
not affect assessment of general treatment effects on
SOC sequestration. We found that within-season
variability in bulk density had a minimal impact on
our results when equivalent masses of soil were
compared.
There is controversy over how to deal with residues
when estimating SOC storage. Removal of surface
residues may discount the effect of NT practices on
SOC sequestration (Paustian et al., 1997) because NT
practices frequently lead to the accumulation of surface residues (Tindall and Crabtree, 1980; Stott,
1991). In our study, the inclusion of C in surface
residue did not signi®cantly alter conclusions about
differences between MP and NT soils; it did, however,
change estimates of absolute C contents. In addition,
inclusion of residues buried within soils, which are
also removed by sieving, in¯uenced our results. By
including buried residues, we noted signi®cantly
higher SOC concentration in the 20±30 cm depths
of the MP soils than in the NT soils. When residues
were removed, there were no differences in the SOC
concentrations of the CT and NT soils at this depth.

When buried residues were included in estimates of
SOC storage within the top 30 cm, the three treatments
were found to be nearly identical. These ®ndings show
both surface and buried residues contribute to SOC
storage, and both types of residues should be assessed
in studies of SOC sequestration.
We found the equivalent mass-based method provides a more accurate way to assess tillage impacts on
SOC (or other elements) storage than do concentration
and volume-based measures. However, assumptions
and data handling affect the results regardless of the
means of data expression selected. When, in keeping
with the recommendations of Ellert and Bettany
(1995), the bulk density and SOC concentration of
the 15±30 depth were used to estimate C stored in the
thickness of soil added, results differed from the
estimates obtained using values collected from the
deeper depth. Detailed sampling at small depth increments beyond the depth of tillage will further diminish
errors.

5. Conclusions
Except for the accumulation of surface residues and
SOC in the top few centimeters, the use of NT
practices did not increase C storage at this site.
Tillage-induced changes in SOC concentrations and
soil bulk density were concentrated in the top 30 cm.
Estimates of tillage impacts on SOC sequestration
varied with the soil depth considered, sampling time,
sample handling, and the methods used to quantify
SOC storage. Use of concentration- or volume-based
comparisons produced erroneous and misleading
results. The equivalent mass-based method provided
a means to assess C sequestration that can be adapted
to accurately compare the effects of different tillage
systems on C storage. In order to obtain accurate
estimates of C sequestration and compare tillage
practices, the C contents of surface and subsurface
residues and of subsoil should be considered.

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