Open Journal of Soil Science, 2012, 2, 242-255
http://dx.doi.org/10.4236/ojss.2012.23029 Published Online September 2012 (http://www.SciRP.org/journal/ojss)
Tillage, Crop Rotation, and Cultural Practice Effects on
Dryland Soil Carbon Fractions
Upendra M. Sainju1*, Andrew W. Lenssen2, Thecan Caesar-TonThat1, Jalal D. Jabro1,
Robert T. Lartey1, Robert G. Evans1, Brett L. Allen1
1US Department of Agriculture, Agricultural Research Service, Northern Plains Agricultural Research Laboratory, Sidney, USA;
2Agronomy Department, Iowa State University, Ames, USA.
Received June 5th, 2012; revised July 9th, 2012; accepted July 22nd, 2012
ABSTRACT
Information is needed on novel management practices to increase dryland C sequestration and soil quality in the north-
ern Great Plains, USA. We evaluated the effects of tillage, crop rotation, and cultural practice on dryland crop biomass
(stems and leaves) yield, surface residue, and soil C fractions at the 0 - 20 cm depth from 2004 to 2008 in a Williams
loam in eastern Montana, USA. Treatments were two tillage systems (no-tillage [NT] and conventional tillage [CT]),
two crop rotations (continuous spring wheat [Triticum aestivum L.] [CW] and spring wheat-barley [Hordeum vulgaris
L.] hay-corn [Zea mays L.]-pea [Pisum sa tivum L.] [W-B-C-P]), and two cultural practices (regular [conventional seed
rates and plant spacing, conventional planting date, broadcast N fertilization, and reduced stubble height] and ecological
[variable seed rates and plant spacing, delayed planting, banded N fertilization, and increased stubble height]). Carbon
fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C miner-
alization (PCM). Crop biomass was 24% to 39% greater in W-B-C-P than in CW in 2004 and 2005. Surface residue C
was 36% greater in NT than in CT in the regular practice. At 5 - 20 cm, SOC was 14% greater in NT with W-B-C-P and
the regular practice than in CT with CW and the ecological practice. In 2007, POC and PCM at 0 - 20 cm were 23% to
54% greater in NT with CW or the regular practice than in CT with CW or the ecological practice. Similarly, MBC at
10 - 20 cm was 70% greater with the regular than with the ecological practice in NT with CW. Surface residue, PCM,
and MBC declined from autumn 2007 to spring 2008. No-tillage with the regular cultural practice increased surface
residue and soil C storage and microbial biomass and activity compared to conventional tillage with the ecological
practice. Mineralization reduced surface residue and soil labile C fractions from autumn to spring.
Keywords: Carbon Sequestration; Dryland Soil; Labile Carbon; Management Practices; Residue Carbon
1. Introduction
Sequestering atmospheric CO2 through plant photosyn-
thesis in the soil as organic matter is getting increased
attention, because reduction in CO2 concentration can
reduce global warming [1,2]. In croplands, C sequestra-
tion occurs when non-harvested crop residues, such as
stems, leaves, and roots, are returned to the soil, followed
by their placement at the soil surface using NT that re-
duces the mineralization of residue and SOC [3-5]. Car-
bon sequestration rates, however, depend on the balance
between the amounts of plant residue C input and the rate
of C mineralized in unmanured soil [6,7]. Other benefits
of increasing C storage include enhancement of soil
structure and soil water-nutrient-crop productivity rela-
tionships [8].
Incentives by governments and private companies for
the compensation of C credit in agricultural soils have
led to increased need for C sequestration and storage
using novel management practices. Some of these prac-
tices are NT management, diversified crop rotations, and
improved cultural practices. Studies have shown that NT
with continuous cropping can increase dryland soil C
storage compared to CT with crop-fallow [3-5]. In the
northern Great Plains, NT with continuous cropping sys-
tem can increase C sequestration by as much as 233 kg C
ha−1·yr−1 compared to a loss of 141 kg·C·ha−1·yr−1 in CT
with crop-fallow [9]. Sherrod et al. [4] found that in-
creased cropping intensity in NT increased dryland SOC
in the central Great Plains after 12 yr. Conversion from
CT to NT can sequester C at 570 ± 140 kg·C·ha−1·yr−1,
reaching equilibrium in 15 to 20 yr and enhanced crop
rotation can sequester at 200 ± 120 kg·C·ha−1·yr−1, reach-
ing equilibrium in 40 to 60 yr [10]. The NT management
can also increase annual crop yields due to efficient wa-
*Corresponding author.

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 243
ter use as a result of increased soil water conservation
from residue accumulation at the soil surface compared
to CT [11,12].
Continuous cropping of diversified crops in the rota-
tion can efficiently utilize water and nutrients and sustain
yields compared to monocropping in water-limited dry-
land farming systems [13,14]. One such crop is pea,
which uses less soil water than spring wheat and barley,
thereby resulting in more water available for succeeding
crops and increasing their yields [11,14]. Pea, being a
legume, also fixes N from the atmosphere and has higher
N concentration than spring wheat or barley [12,15]. Be-
cause of greater N supply, N fertilization rates to crops fol-
lowing pea can be reduced [12,14]. Other benefits of crop
diversification include effective control of weeds, diseases,
and pests [13,14], reductions in the risk of crop failure,
farm inputs, and duration of fallow, and improvement in
economic and environmental sustainability [16,17].
Increased C storage can also improve biological soil
quality as indicated by enhanced microbial biomass and
activity [5,18,19]. These can be estimated by measuring
labile C fractions, such as MBC and PCM [20,21]. The
MBC refers to C storage in the body of microorganisms
and also known as microbial biomass. Similarly, PCM
refers to soil C mineralization and is related to microbial
activity. Because of a large pool size and inherent spatial
variability, SOC changes slowly with management prac-
tices [18]. As a result, measurement of SOC alone does
not adequately reflect changes in soil quality and nutrient
status [18,19]. Active C fractions, such as MBC and
PCM that change rapidly with time due to management
practices, can better reflect changes in soil quality and
productivity that alter nutrient dynamics due to immobi-
lization-mineralization [20,21]. Similarly, POC that con-
tains coarse organic matter,has been considered as an
intermediate fraction of SOC between active and slow
fractions that changes rapidly due to management prac-
tices [22]. The POC also provides substrates for micro-
organisms and influences soil aggregation [23].
In the northern Great Plains, traditional farming sys-
tems, such as CT with wheat-fallow, for the last 50 to
100 yr have decreased SOC levels by 30% to 50% [7,24].
Both tillage and fallowing reduce SOC by increasing soil
organic matter mineralization [25,26] and reducing the
amount of plant residue returned to the soil [10,27]. As a
result, traditional farming systems have become unsus-
tainable and uneconomical [16]. Controlling weeds in
NT cropping systems is also a major concern in dryland
farming systems. Some of the cultural practices used for
weed control include high crop seeding rates, banded fer-
tilization, delayed planting and harvest, and increased
retention of crop residue [28,29]. Although information
is available about the use of NT with continuous crop-
ping in increasing soil C storage compared to CT with
crop-fallow, little is known about the combined effects of
tillage, diversified crop rotation, and cultural practice on
dryland surface residue and soil C fractions.
Because of the short growing season due to cold wea-
ther and lack of enough time for sampling before crop
planting in the spring, soil samples are normally taken
after harvest in the autumn for testing nutrient levels and
recommending fertilizer rates for succeeding crops in the
next year in the northern Great Plains [16,30]. As a result,
fertilizers are applied in the autumn instead of spring in
the succeeding year. This results in substantial loss of
nutrients, especially N and P, from fertilizers due to lea-
ching, surface runoff, volatilization, and denitrification.
To reduce this loss, it is recommended that soil tests be
done and fertilizers applied before planting in the spring
[30]. Since C mineralization can continue during the
winter, it could be possible that C levels vary in surface
residue and soil samples collected in the autumn and
spring. We hypothesized that NT with diversified crop
rotation and the ecological cultural practice would in-
crease surface residue and soil C fractions compared to
CT with monocropping and the regular cultural practice,
and that surface residue and labile C fractions would de-
cline more than SOC from the autumn to the succeeding
spring due to mineralization and lack of additional inputs,
regardless of management practices. Our objectives were
to: 1) determine the amount of dryland crop biomass
(stems and leaves) returned to the soil as influenced by
tillage, crop rotation, and cultural practice from 2004 to
2007 in eastern Montana, 2) quantify their effects on
surface residue and soil C fractions (SOC, POC, PCM,
and MBC) at 0 - 20 cm, and 3) examine changes in sur-
face residue and soil C fractions as influenced by man-
agement practices from autumn 2007 to spring 2008.
2. Materials and Methods
2.1. Experimental Site and Treatments
From 2004 to 2008, the experiment was carried out on a
dryland farm site, 8 km northwest of Sidney (47˚46′N,
104˚16′W; elevation 690 m), Montana, USA. The cli-
mate is semiarid, with mean monthly air temperature
ranging from −8˚C in January to 23˚C in July and August,
and mean annual precipitation (68-yr average) of 357
mm, 80% of which occurs during the crop growing sea-
son (April-November). The soil is a Williams loam (fine-
loamy, mixed, superactive, frigid, Typic Argiustolls)
with 350 g·kg−1 sand, 325 g·kg−1 silt, 325 g·kg−1 clay,
and 6.1 pH at the 0 - 20 cm depth. The SOC contents at 0
- 5 and 5 - 20 cm prior to the initiation of the experiment
in April 2004 were 8.8 and 21.7 Mg·C·ha−1, respectively.
Previous cropping system for the last 10 yr was CT with
spring wheat-fallow.
Treatments included two tillage practices (NT and CT),

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 244
two crop rotations (continuous spring wheat [CW] and
spring wheat-barley hay-corn-pea [W-B-C-P]), and two
cultural practices (regular and ecological). The cultural
practices used for crops in the rotation are described in
Table 1. The NT plots were left undisturbed, except for
applying fertilizer and planting crops in rows. The CT
plots were tilled one to two times a year with a field cul-
tivator to a depth of 7 - 8 cm for seedbed preparation and
weed control. In the four-year crop rotation in W-B-C-P,
each phase (spring wheat, hay barley, corn, and pea) of
the crop rotation was present in every year. Treatments
were arranged in a split-plot design in randomized com-
plete block with three replications. Tillage was consi-
dered as the main-plot and the factorial combination of
crop rotation and cultural practice as the split-plot treat-
ment. The split plot size was 12.2 m × 12.2 m.
2.2. Crop Management
At planting in April, 2004 to 2007, P fertilizer as mono-
ammonium phosphate (11% N, 23% P) at 56 kg·P·ha−1
and K fertilizer as muriate of potash (60% K) at 48
kg·K·ha−1 were banded to a depth of 5 cm to all crops.
Nitrogen fertilizer as urea (46% N) (either broadcast or
banded [Table 1]) and monoammonium phosphate
(banded) was applied at 101 kg·N·ha−1 for spring wheat,
67 kg·N·ha−1 for barley hay, and 78 kg·N·ha−1 for corn.
Pea received 6 kg·N·ha−1 from monoammonium phos-
phate. Nitrogen fertilization rate to each crop was ad-
justed for soil NO3-N content to a depth of 60 cm mea-
sured after harvest of previous crop in the autumn.
Spring wheat (cv. Reeder) was planted from early April
to mid-May and pea (cv. Majoret) and barley hay (cv.
Haybet) from early to mid-April (Table 1) with a no-till
drill at a spacing of 20.3 cm. Corn (cv. 39T67-RR) was
planted in early May at a spacing of 76 cm in 2004 and
61 cm from 2005 to 2007. The narrow spacing for corn
from 2005 to 2007 compared to 2004 was adapted to use
soil water more efficiently and increase grain yield.
Growing season weeds were controlled with selective
post-emergence herbicides appropriate for each crop.
Contact herbicides were applied at preplanting and post-
harvest. In late June or early July, aboveground biomass
of barley was harvested for hay with a self-propelled
mower-conditioner and round baler after determining dry
yield from two 0.5 m2 plots at 60˚C for 3 d. In August,
total aboveground biomass (stems, leaves, and grains)
yield of spring wheat and pea was determined from two
0.5 m2 areas outside yield rows after determining dry
yields as above. At the same time, grain yield (oven-
dried basis at 60˚C) was determined by harvesting grains
from a swath of 1.5 m × 12.0 m using a combine har-
vester. In October, corn biomass and grain yields were
determined from areas as described above. Biomass
(stems and leaves) yield of spring wheat, pea, and corn
was determined by deducting grain yield from total
aboveground biomass. During harvest, grains were re-
moved but biomass of spring wheat, pea, and corn were
returned to the soil.
2.3. Surface Residue and Soil Sample Collection
and Analysis
In November 2007, a month after crop harvest, soil sur-
face residue samples were collected from five 30 cm ×
30 cm areas randomly in the central rows of the plot,
composited, washed with water to remove soil, and dried
in the oven at 60˚C for 7 d to obtain dry matter weight.
Samples were ground to pass a 1 mm screen to determine
C concentration. After removing the residue, soil samples
were collected with a hand probe (5 cm inside diameter)
from the 0 - 20 cm depth from five places in the central
rows of the plot. These were separated into 0 - 5, 5 - 10,
and 10 - 20 cm depth increments, composited within a
depth, air-dried, ground, and sieved to 2 mm for deter-
mining C fractions. The process was repeated again in
April 2008 by collecting surface residue and soil samples
and preparing exactly as above for C analysis. Because
of the difficulty in getting intact soil cores due to low
water content, only one additional core (5 cm inside di-
ameter) containing undisturbed sample was collected
from each depth in 2007 and 2008 to determine bulk
Table 1. Description of cultural practices (regular and ecological) used for crops in the rotation.
Crop Cultural practice Seeding rate (million seeds ha−1)N fertilization at plantingPlanting date Stubble height (cm)
Spring wheat Regular 2.23 Broadcast Early to mid April 20
Ecological 2.98 Banded Early to mid May 30
Pea Regular 0.60 Broadcast Early to mid April 5
Ecological 0.92 Banded Early to mid April 5
Barley hay Regular 2.23 Broadcast Early to mid April 5
Ecological 2.98 Banded Early to mid April 5
Corn Regular 0.04 Broadcast Early May 5
Ecological 0.05 Broadcast Early May 5

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 245
density by dividing the mass of the oven-dried (105˚C)
soil by the volume of the core. Soil bulk density was not
different among treatments and sampling dates averaged
1.34, 1.36, and 1.49 Mg·m−3 at 0 - 5, 5 - 10, and 10 - 20
cm, respectively.
Total C concentration (g·C·kg−1) in the surface residue
was determined by using a dry combustion C and N ana-
lyzer (LECO Corp., St Joseph, MI). Carbon content
(Mg·C·ha−1) in the residue was determined by multiply-
ing dry matter weight by C concentration. The SOC
concentration in soil samples was determined by using
the C and N analyzer as above after grinding the samples
to <0.5 mm and pretreating the soil with 5% H2SO3 to
remove inorganic C [31]. For determining POC, 10 g soil
sample was dispersed with 30 mL of 5 g·L−1 sodium
hexametaphosphate by shaking for 16 h and the solution
was poured through a 0.053 mm sieve [22]. The solution
and particles that passed through the sieve and contained
mineral associated and water soluble C were dried at
50˚C for 3 to 4 d and SOC concentration was determined
by using the analyzer as above after pretreating with 5%
H2SO3. The POC concentration was determined by the
difference between SOC in the whole-soil and that in the
particles that passed through the sieve after correcting for
the sand content.
The PCM in air-dried soils was determined by the
method modified by Haney et al. [32]. Ten grams of soil
sample was moistened with water at 50% field capacity
(0.25 m3·m−3 [33]) and placed in a 1 L jar containing
beakers with 2 mL of 0.5 M NaOH to trap evolved CO2
and 20 mL of water to maintain high humidity. Soils
were incubated in the jar at 21˚C for 10 d. At 10 d, the
beaker containing NaOH was removed from the jar and
PCM concentration was determined by measuring CO2
absorbed in NaOH, which was back-titrated with 1.5 M
BaCl2 and 0.1 M HCl. The moist soil used for determin-
ing PCM was subsequently used for determining MBC
by the modified fumigation-incubation method for
air-dried soils [34]. This container was incubated twice
because MBC determination required moist-soil. The
method also required mineralizable C to be flushed out
during the first incubation [34]. The moist soil was fu-
migated with ethanol-free chloroform for 24 h and placed
in a 1 L jar containing beakers with 2 mL of 0.5 M
NaOH and 20 mL water. As with PCM, fumigated moist
soil was incubated for 10 d and CO2 absorbed in NaOH
was back-titrated with BaCl2 and HCl. The MBC con-
centration was calculated by dividing the amount of
CO2-C absorbed in NaOH by a factor of 0.41 [35] with-
out subtracting the values from the non-fumigated con-
trol [34].
The SOC, POC, PCM, and MBC contents (Mg or
kg·C·ha−1) at 0 - 5, 5 - 10, and 10 - 20 cm depths were
calculated by multiplying their concentrations (g·C·kg−1)
by bulk density at each depth and thickness of the soil
layer. Total content at 0 - 20 cm was determined by
summing the contents at 0 - 5, 5 - 10, and 10 - 20 cm.
2.4. Data Analysis
Data for crop biomass, surface residue, and soil C frac-
tions at each depth were analyzed using the MIXED
procedure of SAS, with year considered as the fixed ef-
fect [36]. Tillage, crop rotation, and cultural practice
were considered as fixed effects and replication and till-
age × replication as random effects. Since each phase of
the crop rotation was present in every year, data for
phases were averaged within a rotation and used for a
crop rotation for the analysis. Means were separated by
using the least square means test when treatments and
interactions were significant [36]. Statistical significance
was evaluated at P ≤ 0.05, unless otherwise stated.
3. Results and Discussion
3.1. Crop Biomass Yield
Biomass (stems and leaves) yield varied between crop
rotations from 2004 to 2007 (Table 2), as year and crop
rotation × year interaction were significant (P ≤ 0.05) but
tillage, cultural practice, and their interactions with crop
rotation and year were not (data not shown). Biomass,
averaged across tillage and cultural practices, was greater
in W-B-C-P than in CW in 2004 and 2005. Biomass was
greater in 2004 and 2006 than in 2005 in CW and greater
in 2004 than in other years in W-B-C-P. Averaged across
tillage, crop rotations, and cultural practices, biomass
was greater in 2004 than in other years.
The greater biomass in W-B-C-P than in CW in 2004
and 2005 was due to higher corn yield. During these
years, biomass in corn was two to three times greater
than that in spring wheat, barley hay, and pea. Biomass
was lower in 2005 than in other years, probably due to
less uniform distribution and/or below-average precipita-
tion. This was especially true for spring wheat in CW.
Precipitation amounting to 115 mm fell in June but only
2 mm in September in 2005 compared to 68-yr average
(72 mm in June and 34 mm in September) and other
years. The growing season (April-October) precipitation
was 11 mm greater in 2006 but 10 mm lower in 2005
than the normal precipitation (311 mm). Since dryland
crop yields depend on the amount and distribution of
precipitation due to the absence of irrigation, large varia-
tions in crop yields can occur from year to year due to
variations in growing season precipitation [3,5,12]. Till-
age and cultural practice did not influence biomass yield.
Several researchers [3,5,12] also found that tillage did
not influence dryland crop biomass yield in the northern
Great Plains. Since biomass, being a source of C input,
was returned to the soil during grain harvest, the amount

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 246
Table 2. Effect of crop rotation on crop biomass (stems and leaves) returned to the soil from 2004 to 2007.
Crop rotationa
Year CW (Mg·ha−1) W-B-C-P (Mg·ha−1) Mean (Mg·ha−1)
2004 5.40abBc 6.71aA 6.06a
2005 3.83bB 5.32bA 4.58c
2006 5.61aA 4.77bA 5.19b
2007 5.10abA 5.31bA 5.21b
aCrop rotations are CW, continuous spring wheat; and W-B-C-P, spring wheat-barley hay-corn-pea; bNumbers followed by different lowercase letters within a
column (among years) are significantly different at P ≤ 0.05 by the least square means test; cNumbers followed by different uppercase letters within a row
(between crop rotations) are significantly different at P ≤ 0.05 by the least square means test.
of biomass as influenced by crop rotation can influence
surface residue and soil C fractions, as discussed below.
3.2. Soil Surface Residue Carbon
Residue incorporation into the soil and mineralization
due to tillage, cultural practice, and time resulted in sig-
nificant tillage, year, and tillage × cultural practice inter-
action in surface residue amount and C content (Table 3).
Surface residue amount, averaged across crop rotations
and years, was greater in NT with the regular and eco-
logical practices than in CT with the regular practice.
Surface residue C was greater in NT than in CT in the
regular practice. Averaged across crop rotations, cultural
practices, and years, surface residue amount and C con-
tent were 27% to 30% greater in NT than in CT. Aver-
aged across treatments, surface residue amount and C
content declined from 51% to 53% from autumn 2007 to
spring 2008. Surface residue between crop rotations did
not vary, possibly because mean biomass across rotations
was not different (Table 3).
The increased surface residue amount and C content in
NT with the regular practice could be a result of reduced
mineralization due to surface placement of the residue. In
contrast, incorporation of the residue into the soil to a
greater depth could have increased mineralization,
thereby reducing surface residue amount and C content
in CT with the regular practice [37]. Although crop bio-
mass yield was not influenced by tillage and cultural
practice, biomass was somewhat lower in the regular
than in the ecological practice in CT, which could have
resulted in lower surface residue amount and C content in
this treatment. Surface residue amount and C content
declined from autumn 2007 to spring 2008 probably due
to increased mineralization and/or loss of residue due to
the actions of wind and water. During this period, more
than half of surface residue amount and C were lost, in-
dicating that substantial losses of residue C can occur
even during the cold weather in the winter. Increased
surface residue amount can reduce soil erosion, increase
C sequestration, and sustain dryland crop yields due to
increased soil water conservation [37,38]. Since C stored
in surface residue form an important part of C cycle,
management practices that increase surface residue C can
also influence soil C fractions [37].
3.3. Soil Organic Carbon
Differences in crop residue quality and quantity and C
mineralization rates among treatments also resulted in
significant tillage × crop rotation × cultural practice in-
teraction on SOC at 5 - 10, 10 - 20, and 0 - 20 cm (Table
4). At 10 - 20 and 0 - 20 cm, cultural practice, year, and
tillage × crop rotation and tillage × cultural practice in-
teractions were significant.
At 5 - 10 cm, SOC, averaged across years, was greater
in NT with CW and the ecological practice or in NT with
W-B-C-P and the regular practice than in CT with CW
and the ecological practice (Table 4). At 10 - 20 cm,
SOC was greater in NT with W-B-C-P and the regular
practice than in NT with W-B-C-P and the ecological
practice or in CT with CW and the regular and ecological
practices. At 0 - 20 cm, SOC was greater in NT with CW
and the ecological practice or in NT with W-B-C-P and
the regular practice than in NT with W-B-C-P and the
ecological practice or in CT with CW and the ecological
practice. Averaged across tillage, crop rotations, and
years, SOC at 10 - 20 and 0 - 20 cm was 2.4% to 4.3%
greater with the regular than with the ecological practice.
Averaged across treatments, SOC at 10 - 20 and 0 - 20
cm reduced by 3.7% to 4.8%, respectively from autumn
2007 to spring 2008.
Changes in SOC due to treatments occurred mostly at
the subsurface than in the surface soil. This was kind of a
surprise given the residue accumulation at the surface
due to NT and incorporation into the soil due to CT oc-
curred to a depth <10 cm. While the coefficient of varia-
tion for SOC ranged from 8.6% in 2007 to 9.1% in 2008
at 0 - 5 cm, it ranged from 8.5% in 2007 to 10.0% in
2008 at 5 - 10 and 10 - 20 cm. Since the variability in
SOC at the surface and subsurface layers were similar, it
could be possible that differences in the amount of resi-
due incorporation into the soil due to CT and/or root
growth among treatments resulted in SOC changes more
in the subsurface than in the surface soil.
The greater SOC at 5 - 10, 10 - 20, and 0 - 20 cm in

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 247
Table 3. Effects of tillage and cultural practice on soil surface residue amount and C content in 2007 and 2008.
Soil surface residue
Tillagea Cultural practiceb Year
Amount (Mg·ha−1) C content (Mg·C·ha−1)
NT Regular 4.04 1.50
Ecological 3.71 1.36
CT Regular 2.72 1.03
Ecological 3.21 1.22
LSD (0.05) 0.99 0.40
Means
NT 3.87ac 1.43a
CT 2.97b 1.13b
2007 4.64a 1.72a
2008 2.26b 0.81b
Significanced
Tillage (T) * *
Crop rotation (C) NS NS
T × C NS NS
Cultural practice (P) NS NS
T × P * *
C × P NS NS
T × C × P NS NS
Year (Y) *** ***
T × Y NS NS
C × Y NS NS
T × C × Y NS NS
P × Y NS NS
T × P × Y NS NS
C × P × Y NS NS
T × C × P ×Y NS NS
aTillage practices are CT, conventional tillage; and NT, no-tillage; bSee Table 2 for the description of cultural practices; cNumbers followed by different letters
within a column in either tillage practice or year are significantly different at P ≤ 0.05 by the least square means test; dSignificance levels: *P ≤ 0.05; **P ≤ 0.01;
***P ≤ 0.001; NS, Not significant.
NT with CW and the ecological practice or in NT with
W-B-C-P and the regular practice than in CT with CW
and the regular and ecological practices could be the re-
sults of increased crop residue returned to the soil, fol-
lowed by decreased mineralization of residue and soil
organic matter due to reduced soil disturbance. Crop
biomass returned to the soil was greater in W-B-C-P than
in CW in 2004 and 2005 (Tab le 2). While CT can reduce
dryland soil C storage compared to NT [5,9,25], in-
creased residue returned to the soil due to diversified
crop rotation and increased cropping intensity can in-
crease SOC levels [3-5]. The lower SOC at 10 - 20 and 0
- 20 cm in the ecological than in the regular practice was
probably due to increased organic matter mineralization
as a result of longer fallow period from delayed planting.
The lower bulk density in the ecological practice also
may have partly contributed to lower SOC in this treat-
ment. During the fallow period in the winter, SOC did
not change at 0 - 5 and 5 - 10 cm but declined at 10 - 20
cm, possibly a result of absence of crop residue input
and/or increased mineralization of SOC and residue in
the subsurface compared to the surface layers. Because
of differences in SOC levels from autumn to spring at the
subsurface soil layer, it may be a good idea to collect soil
samples at the same time of the year for SOC analysis if
deeper samples >10 cm are collected.
The SOC level at 0 - 20 cm at the initiation of the ex-
periment in April 2004 was 30.5 Mg·ha−1. The SOC le-
vels at 0 - 20 cm between 2007 and 2008 ranged from
28.0 Mg·C·ha−1 in CT with CW and the ecological prac-
tice to 31.2 Mg·C·ha−1 in NT with CW and the ecological
practice (Table 4). This suggests that SOC decreased by

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 248
Table 4. Effects of tillage, crop rotation, and cultural practice on soil organic C (SOC) content at the 0 - 20 cm depth in 2007
and 2008.
SOC content (Mg·C·ha−1)
Tillagea Crop rotationb Cultural practicec Year 0 - 5 cm 5 - 10 cm 10 - 20 cm 0 - 20 cm
NT CW Regular 8.1 7.2 14.8 30.1
Ecological 8.7 7.9 14.6 31.2
W-B-C-P Regular 8.2 7.7 15.1 31.0
Ecological 8.2 7.2 13.2 28.6
CT CW Regular 8.4 7.1 14.0 29.5
Ecological 8.0 6.8 13.3 28.1
W-B-C-P Regular 8.1 7.4 14.3 29.8
Ecological 8.0 7.3 14.6 29.9
LSD (0.05) NS 0.7 1.0 2.1
Mean
Regular 8.2ad 7.3a 14.5a 30.0a
Ecological 8.2a 7.3a 13.9b 29.4b
2007 8.3a 7.4a 14.6a 30.3a
2008 8.1a 7.1a 13.9b 29.1b
Significancee
Tillage (T) NSb NS NS NS
Crop rotation (C) NS NS NS NS
T × C NS NS ** *
Cultural practice (P) NS NS *** **
T × P NS NS *** *
C × P NS NS NS NS
T × C × P NS * ** ***
Year (Y) NS NS *** **
T × Y NS NS NS NS
C × Y NS NS NS NS
T × C × Y NS NS NS NS
P × Y NS NS NS NS
T × P × Y NS NS NS NS
C × P × Y NS NS NS NS
T × C × P ×Y NS NS NS NS
aTillage practices are CT, conventional tillage; and NT, no-tillage; bCrop rotations are CW, continuous spring wheat; and W-B-C-P, spring wheat-barley
hay-corn-pea.; cSee Table 2 for the description of cultural practices; dNumbers followed by different letters within a column in either cultural practice or year
are significantly different at P ≤ 0.05 by the least square means test; eSignificance levels: *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; NS, Not significant.
500 kg·C·ha−1·yr−1 in CT with CW and the ecological
practice but increased by 140 kg·C·ha−1·yr−1 in NT with
CW and the ecological practice. The loss in SOC from
2004 to 2008 in CT with CW and the ecological practice
is larger than the reported value 141 kg·C·ha−1·yr−1 in CT
with crop-fallow but the gain in NT with CW and the
ecological practice is smaller than 233 kg·C·ha−1·yr−1 in
NT with continuous cropping under dryland farming
system in the northern Great Plains [9]. Possibly, the
experiment needs to be continued for more than five
years to obtain a stable C sequestration rate, since several
researchers [3,37,39] have concluded the need for long-
term experiment on C sequestration to observe the effects
of management practices on soil C storage under dryland
cropping systems in the northern Great Plains. This is
because lower crop biomass returned to the soil due to
limited precipitation compared with humid regions and
cold weather reduces turnover rate of plant C into SOC
[3,5].
3.4. Soil Particulate Organic Carbon
Similar to SOC, POC content varied between cultural
practices at 5 - 10 and 10 - 20 cm and between years at

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 249
all depths (Table 5). Interactions were significant for
tillage × year at 5 - 10 cm, crop rotation × year at 10 - 20
cm, and tillage × crop rotation × year at 0 - 5, 10 - 20,
and 0 - 20 cm.
The POC at 0 - 5, 10 - 20, and 0 - 20 cm, averaged
across cultural practices, was greater in NT with CW
than in other tillage and crop rotations in 2007 (Table 5).
In 2008, POC at 10 - 20 cm was greater in W-B-C-P than
in CW in CT but at other depths, it was not different be-
tween tillage and crop rotations. From autumn 2007 to
spring 2008, POC at 0 - 5 and 0 - 20 cm declined rapidly
in NT with CW and CT with W-B-C-P compared to
other treatments. Similarly, during this period, POC at 10
- 20 cm declined more in NT with CW or W-B-C-P than
in other treatments. Averaged POC across treatments,
however, declined from 16% to 18% at all depths from
autumn 2007 to spring 2008. Averaged across tillage,
crop rotations, and years, POC at 5 - 10 and 10 - 20 cm
was greater in the regular than in ecological practice.
As with SOC, greater POC at 0 - 5, 10 - 20, and 0 - 20
cm in NT with CW than in other tillage and crop rota-
tions in 2007 could be the results of an undisturbed soil
Table 5. Effects of tillage, crop rotation, and cultural practice on soil particulate organic C (POC) content at the 0 - 20 cm
depth in 2007 and 2008.
POC content (Mg C·ha−1)
Year Tillagea Crop rotationb Cultural practicec
0 - 5 cm 5 - 10 cm 10 - 20 cm 0 - 20 cm
2007 NT CW 2.49 1.77 3.56 7.82
W-B-C-P 2.04 1.57 2.96 6.57
CT CW 2.00 1.58 2.90 6.48
W-B-C-P 2.14 1.69 2.91 6.74
2008 NT CW 1.84 1.50 2.52 5.86
W-B-C-P 1.86 1.52 2.55 5.93
CT CW 1.86 1.30 2.28 5.44
W-B-C-P 1.64 1.26 2.76 5.66
LSD (0.05) 0.28 NS 0.48 0.82
Mean
2007 2.16ad 1.65a 3.08a 6.89a
2008 1.80b 1.39b 2.53b 5.72b
Regular 1.99a 1.55a 2.88a 6.42a
Ecological 1.98a 1.49b 2.73b 6.20a
Significancee
Tillage (T) NS NS NS NS
Crop rotation (C) NS NS NS NS
T × C NS NS NS NS
Cultural practice (P) NS * * NS
T × P NS NS NS NS
C × P NS NS NS NS
T × C × P NS NS NS NS
Year (Y) *** *** *** ***
T × Y NS * NS NS
C × Y NS NS * NS
T × C × Y ** NS * *
P × Y NS NS NS NS
T × P × Y NS NS NS NS
C × P × Y NS NS NS NS
T × C × P ×Y NS NS NS NS
aTillage practices are CT, conventional tillage; and NT, no-tillage; bCrop rotations are CW, continuous spring wheat; and W-B-C-P, spring wheat-barley
hay-corn-pea; cSee Tabl e 2 for the description of cultural practices; dNumbers followed by different letters within a column in either year or cultural practice
are significantly different at P ≤ 0.05 by the least square means test; eSignificance levels: *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; NS, Not significant.

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 250
and surface placement of residue that reduced the miner-
alization of residue and coarse soil organic matter. The
greater POC level with this treatment was, however,
short lived because greater rate of decline in its level
occurred in NT with CW than in other treatments from
2007 to 2008, thereby resulting in non-significant diffe-
rence in POC between treatments at all depths, except at
10 - 20 cm, in 2008. This indicates that coarse fraction of
soil organic matter can be enriched by using no-tillage
with continuous spring wheat compared to conventional
tillage with monocropping or crop rotation but the level
was reduced from autumn to the following spring, pro-
bably by increased mineralization. The proportion of
SOC in POC at 0 - 20 cm was also greater in NT with
CW (226 g·POC·kg−1 SOC) than in other tillage and crop
rotations (207 to 208 g·POC·kg−1 SOC) in 2007 but was
not significantly different among treatments in 2008.
This suggests that coarse organic matter was mineralized
at a rapid rate compared to whole soil organic matter, a
case similar to that observed by other researchers [5,18,
22,40].
The lower POC in the ecological than in the regular
practice at the subsurface layers (5 - 10 and 10 - 20 cm)
could be the results of increased mineralization of coarse
organic matter due to longer fallow period and/or re-
duced residue input due to delayed planting, a case simi-
lar to that observed for SOC. As with SOC, changes in
POC occurred more at the subsurface than in surface
layers. Probably differences in crop root growth and/or
various levels of residue incorporation to a greater depth
due to tillage (e.g. greater spring wheat stubble height
[Table 2] reduced the amount of residue incorporation
into the soil in the ecological than in the regular practice)
influenced POC levels between the two cultural practices
more in the subsurface than in the surface soil.
3.5. Soil Potential Carbon Mineralization
The PCM content varied between years at all depths,
except at 10 - 20 cm. (Table 6). Interactions were sig-
nificant for tillage × year at all depths and tillage × cul-
tural practice × year at 5 - 10, 10 - 20, and 0 - 20 cm.
Crop rotation and its interaction with other treatments
were not significant.
The PCM at 5 - 10 cm, averaged across crop rotations,
was greater in NT with the regular practice than in CT
with the ecological practice in 2007 (Table 6). At 10 - 20
and 0 - 20 cm, PCM was greater in NT with the regular
practice than in CT with the regular and ecological prac-
tices. In 2008, PCM at 0 - 20 cm was greater in NT with
the regular practice than in CT with the regular practice.
From autumn 2007 to spring 2008, PCM at 5 - 10 and 10
- 20 cm declined rapidly in NT with the regular practice
compared to other treatments, thereby resulting in non-
significant difference among tillage and cultural practices
in 2008. Averaged across crop rotations and cultural
practices, PCM was greater in NT than in CT at all
depths in 2007 and at 0 - 20 cm in 2008. In contrast to
SOC and POC, PCM at all depths, averaged across treat-
ments, declined from 17% to 34% from autumn 2007 to
spring 2008.
The greater PCM in NT with the regular practice than
in CT with the regular and ecological practices was pro-
bably a result of greater substrate availability, since SOC
and POC were greater in this treatment (Tables 4 and 5).
This is consistent with those reported by several re-
searchers [18,21,40] who found that greater substrate
availability increased C mineralization and microbial
activity in NT than in CT. Increased C mineralization
due to residue incorporation into the soil, followed by
longer duration of fallow probably reduced PCM in CT
with the regular and ecological practices. As with SOC
and POC, greater level of PCM in NT with the regular
practice was short lived, as substrate availability declined
at a rapid rate with this treatment from autumn 2007 to
spring 2008. Since PCM declined both at the surface and
subsurface soil layers at higher rates during this period in
contrast to slower rate of decline for SOC and POC at the
subsurface layers, PCM could be a sensitive indicator of
changes in soil organic matter with management practice
and time, a case similar to those observed by various
researchers [20,21]. Although variability was higher in
PCM (CV 28% - 55%) than in SOC and POC (CV 7% -
25%), small changes in SOC and POC levels due to
management practices and mineralization with time re-
sulted in large changes in PCM level.
3.6. Soil Microbial Biomass Carbon
Like SOC and POC, changes in MBC occurred at the
subsurface layers, with significant interactions for tillage
× crop rotation × year at 0 - 20 cm, cultural practice ×
year at 5 - 10 cm, and tillage × crop rotation × cultural
practice × year at 10 - 20 and 0 - 20 cm (Table 7). The
MBC, however, varied significantly with crop rotation
and years at all depths. The variability was lower for
MBC (CV 15% - 40%) than for PCM but greater than for
SOC and POC.
In 2007, MBC at 10 - 20 cm was greater in NT with
CW and the regular practice than in NT with CW and the
ecological practice, NT with W-B-C-P and the ecological
practice, or in CT with CW and the regular practice (Ta-
ble 7). At 0 - 20 cm, MBC was greater in NT with CW
and the regular practice than in NT with CW and the
ecological practice or in CT with CW and the regular
practice. From 2007 to 2008, MBC at 10 - 20 and 0 - 20
cm declined rapidly in NT with CW and the regular prac-
tice and CT with CW or W-B-C-P with the ecological
practice compared to other treatments, thereby resulting
in non-significant difference between treatments in 2008.

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 251
Table 6. Effects of tillage and cultural practice on soil potential C mineralization (PCM) content at the 0 - 20 cm depth in
2007 and 2008.
PCM content (kg·C·ha−1)
Year Tillagea Cultural practiceb 0 - 5 cm 5 - 10 cm 10 - 20 cm 0 - 20 cm
2007 NT Regular 39 35 78 152
Ecological 42 32 60 134
CT Regular 33 25 47 105
Ecological 28 23 50 101
2008 NT Regular 24 18 43 85
Ecological 24 19 52 95
CT Regular 31 20 52 103
Ecological 30 19 50 99
LSD (0.05) NS 12 28 48
Mean
2007 36ac 29a 59a 124a
2008 27b 19b 49a 95b
Significanced
Tillage (T) NS NS NS NS
Crop rotation (C) NS NS NS NS
T × C NS NS NS NS
Cultural practice (P) NS NS NS NS
T × P NS NS NS NS
C × P NS NS NS NS
T × C × P NS NS NS NS
Year (Y) * * NS *
T × Y * * * **
C × Y NS NS NS NS
T × C × Y NS NS NS NS
P × Y NS NS NS NS
T × P × Y NS * * *
C × P × Y NS NS NS NS
T × C × P ×Y NS NS NS NS
aTillage practices are CT, conventional tillage; and NT, no-tillage; bSee Table 2 for the description of cultural practices; cNumbers followed by different letters
within a column in a year are significantly different at P ≤ 0.05 by the least square means test; dSignificance levels: *P ≤ 0.05; **P ≤ 0.01;***P ≤ 0.001; NS, Not
significant.
Averaged across tillage, cultural practices, and years,
MBC at all depths was 11% to 14% greater in W-B-C-P
than in CW. Similar to PCM, MBC at all depths, aver-
aged across treatments, declined from 22% to 24% from
autumn 2007 to spring 2008.
Greater substrate availability due to increased SOC
and POC probably increased MBC in NT with CW and
the regular practice at the subsurface soil in 2007. As the
substrate availability decreased, MBC also declined from
2007 to 2008. Similar to PCM, MBC decline occurred at
all depths, with a greater rate of decline than in SOC and
POC. This verifies that MBC is also an active fraction of
soil organic matter [20,21]. Diversified crop rotation
clearly increased MBC in W-B-C-P than in CW, which
was especially noted in CT in 2007. Increased amount of
crop residue returned to the soil (Table 2) probably in-
creased MBC in W-B-C-P compared to CW. Several re-
searchers [4,26,40] also reported that MBC increased
with diversified crop rotation compared to monocropping
due to increased crop residue returned to the soil.
4. Conclusion
Differences in tillage, crop rotations, and cultural prac-
tices among treatments influenced crop biomass (stems
and leaves) returned to the soil, surface residue, and soil
C fractions. In contrast to our hypothesis, no-tillage with
monocropping and the regular cultural practice increased

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 252
Table 7. Effects of tillage, crop rotation, and cultural practice on soil microbial biomass C (MBC) content at the 0 - 20 cm
depth in 2007 and 2008.
MBC content (kg·C·ha–1)
Year Tillagea Cultural practiceb Crop rotationc
0 - 5 cm 5 - 10 cm 10 - 20 cm 0 - 20 cm
2007 NT Regular CW 99 79 196 374
W-B-C-P 94 73 167 334
Ecological CW 101 72 115 288
W-B-C-P 106 79 153 338
CT Regular CW 82 54 120 256
W-B-C-P 101 79 180 360
Ecological CW 84 61 165 310
W-B-C-P 111 85 184 380
2008 NT Regular CW 59 58 114 231
W-B-C-P 85 57 135 277
Ecological CW 93 52 108 253
W-B-C-P 79 60 114 253
CT Regular CW 67 61 129 257
W-B-C-P 80 65 121 266
Ecological CW 67 52 122 241
W-B-C-P 78 56 128 262
LSD (0.05) NS NS 38 59
Mean
2007 97ad 73a 160a 330a
2008 76b 57b 122b 255b
CW 81b 61b 134b 276b
W-B-C-P 92a 68a 148a 308a
Significancee
Tillage (T) NS NS NS NS
Crop rotation (C) * * * **
T × C NS NS NS NS
Cultural practice (P) NS NS NS NS
T × P NS NS ** NS
C × P NS NS NS NS
T × C × P NS NS NS NS
Year (Y) *** *** *** ***
T × Y NS NS NS NS
C × Y NS NS NS NS
T × C × Y NS NS NS *
P × Y NS * NS NS
T × P × Y NS NS NS NS
C × P × Y NS NS NS NS
T × C × P ×Y NS NS * *
aTillage practices are CT, conventional tillage; and NT, no-tillage; bSee Table 2 for the description of cultural practices; cCrop rotations are CW, continuous
spring wheat; and W-B-C-P, spring wheat-barley hay-corn-pea.; dNumbers followed by different letters within a column in either year or crop rotation are sig-
nificantly different at P ≤ 0.05 by the least square means test; dSignificance levels: *P ≤ 0.05; ** P ≤ 0.01; ***P ≤ 0.001; NS, Not significant.

Tillage, Crop Rotation, and Cultural Practice Effects on Dryland Soil Carbon Fractions 253
surface residue and soil C fractions but diversified crop
rotation increased microbial biomass C. The increases
occurred at subsurface soil layers for non-labile C frac-
tions but occurred at all layers for labile C fractions.
However, the increases were short lived as C fractions
declined rapidly compared to other treatments from au-
tumn 2007 to spring 2008, probably a result of increased
mineralization during the fallow period in the winter.
This resulted in similar levels of soil C fractions among
treatments in 2008. The ecological cultural practice did
not increase surface residue and soil C fractions com-
pared to the regular practice probably due to longer fal-
low period and continued C and N mineralization. For
increasing dryland C storage and soil quality, no-tillage
with diversified crop rotation and/or the regular cultural
practice can be used compared to conventional tillage
with monocropping and/or the ecological practice in the
northern Great Plains, USA. For obtaining stable C levels
and sequestration rates, however, long-term experiment
than the present four years of study may be needed be-
cause of declining levels of surface residue and C frac-
tions from autumn to spring. As a result, surface residue
and soil samples would be preferable to collect at the
same time (e.g. either after crop harvest in the autumn or
before planting in the spring) of the year for determina-
tions of C fractions under dryland cropping systems.
5. Acknowledgements
We sincerely acknowledge the help provided by Joy
Barsotti, Chris Russell, and Johnny Rieger for collection
and analysis of surface residue and soil samples in the
field and laboratory and Michael Johnson and Mark Gaf-
fri for management of field plots for tilling, planting,
herbicide and pesticide application, and harvest.
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