Soil & Tillage Research 120 (2012) 1–7

Impact of ridge tillage on soil organic carbon and selected physical propertiesof a clay loam in southwestern Ontario

X.H. Shi a,b,c, X.M. Yang a,*, C.F. Drury a, W.D. Reynolds a, N.B. McLaughlin d, X.P. Zhang b

a Greenhouse and Processing Crops Research Centre, Agriculture & Agri-Food Canada, Harrow, ON N0R 1G0, Canadab Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, Chinac Graduate School of the Chinese Academy of Sciences, Beijing 100049, Chinad Eastern Cereal and Oilseed Research Centre, Agriculture & Agri-Food Canada, Ottawa, ON K1A 0C6, Canada

A R T I C L E I N F O

Article history:

Received 22 August 2011

Received in revised form 20 December 2011

Accepted 8 January 2012

Available online 3 February 2012

Keywords:

Ridge tillage

No-tillage

Mouldboard plow tillage

Soil organic carbon

Soil penetration resistance

Southwestern Ontario

A B S T R A C T

Ridge tillage (RT) creates a distinctly different soil environment relative to no-tillage (NT) and

mouldboard plow tillage (MP), which may in turn affect soil properties. In this study, the impacts of long-

term (29 years) RT on soil organic carbon (SOC), water content, bulk density and penetration resistance

were compared with NT and MP tillage on a clay loam soil under a corn (Zea mays L.)–soybean (Glycine

max L. Merr.) rotation. The ridges in RT were formed at 76-cm spacing and corn was planted in the center

of the ridges whereas soybean was planted in the shoulders of the ridges at 38-cm spacing. Soil samples

were collected from the ridge crest (i.e. corn row), from the two ridge shoulder positions and from the

interrow (furrow) positions of the ridges to evaluate both the spatial and profile distributions of the

selected soil properties under RT relative to NT and MP. Ridge tillage produced low SOC in the interrows,

high SOC in the crests and medium SOC in the shoulders relative to MP and NT. Soil water content was

higher in the interrows than in the crests of the ridges, while soil penetration resistance followed the

reverse trend. No-tillage resulted in a distinct SOC stratification with significantly higher SOC in surface

soil and slightly lower SOC in subsurface soil while a uniform distribution of SOC was observed in the

plow layer of MP soil. Hence, RT produced different SOC, water content, bulk density and penetration

resistance distributions than NT and MP. Twenty-nine years of RT management resulted in improved soil

physical conditions in the plow layer for crop root growth relative to NT and greater SOC stocks within

the plow layer compared to MP.

Crown Copyright � 2012 Published by Elsevier B.V. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Soil & Tillage Research

jou r nal h o mep age: w ww.els evier . co m/lo c ate /s t i l l

1. Introduction

Southwestern Ontario (�42–438N latitude by 79–838W longi-tude), is a major corn (Zea mays L.) and soybean (Glycine max L.Merr.) production region in Canada. Degradation of the predomi-nantly clay and clay-loam soils in this region is evidenced primarilyby a gradual deterioration of near surface soil structure resultingfrom mouldboard plow (MP) tillage and secondary disking andharrowing (Stone et al., 1990). Conservation tillage systems,particularly no-till (NT), are generally accepted as effectivetechnologies for maintaining both soil productivity and quality.NT has the advantages of reducing soil erosion and surface runoff(Nyakatawa et al., 2000; Zhang et al., 2009), decreasing the labor,time and fuel requirements for land preparation (Vyn et al., 1990;

Abbreviations: RT, ridge tillage; NT, no-tillage; MP, mouldboard plow tillage; SOC,

soil organic carbon.

* Corresponding author at: 2085 County Rd., 20, Harrow, ON N0R 1G0, Canada.

Tel.: +1 5197381292; fax: +1 5197382929.

E-mail address: Xueming.Yang@AGR.GC.CA (X.M. Yang).

0167-1987/$ – see front matter . Crown Copyright � 2012 Published by Elsevier B.V. A

doi:10.1016/j.still.2012.01.003

Raper and Bergtold, 2007; McLaughlin et al., 2008), slowing soilorganic matter loss (Koch and Stockfisch, 2006), and increasingsocial benefits (Uri et al., 1999). However, on the cool, humid, fine-textured and poorly drained soils of southwestern Ontario,continuous NT generally results in lower crop yields due to excesscrop residues, surplus soil water and lower soil temperatures in theearly spring (Dwyer et al., 2000; Drury et al., 2003), increased soilstrength (Reynolds et al., 2002; Drury et al., 2003), and reducedseedling emergence (Drury et al., 2003, 2006).

Ridge tillage (RT) has been attracting a growing interest sincethe early 1980s. The RT system, proposed as a compromisebetween MP and NT, is ‘‘a tillage system in which ridges arereformed atop the planted row by cultivation, and the ensuing rowcrop is planted into ridges formed in the previous growing season’’(Gregorich et al., 2001). It is not as common as MP and NT, but it isestablished throughout the world (Muller et al., 2009) and widelyemployed in mid-western USA for corn, sorghum (Sorghum bicolor

L. Moench.) and soybean production (Klein et al., 1996).The well-recognized advantages of RT relative to NT or MP

include improved soil fertility, water and pest management

ll rights reserved.

Fig. 1. Soil sampling plan for (a) ridge tillage (RT); and (b) for no-tillage (NT) and

mouldboard plow tillage (MP). The black circles indicate the locations of core

sampling. In the soybean phase of the rotation, soybean rows were planted at

positions P2, P4, P6 and P8.

X.H. Shi et al. / Soil & Tillage Research 120 (2012) 1–72

(Bezdicek et al., 2003; Jiang and Xie, 2009), reduced soil erosion(Kardous et al., 2005; Liu et al., 2006; Jiang and Xie, 2009), deceasedgreenhouse gas emission (Patino-Zuniga et al., 2009), increasedsoil organic carbon (SOC) (Gao et al., 2008; Doumbia et al., 2009;Shao et al., 2009), increased active soil depth (Lal, 1990; Gaynorand Findlay, 1995; Pikul et al., 2001), and improved soiltemperature and moisture environment for seed germination inearly spring (Stone et al., 1990; Doumbia et al., 2009; He et al.,2010). Ridge tillage also can produce corn and soybean yields equalto and even higher than those of MP tillage (Vyn et al., 1990; Archeret al., 2002; Odjugo, 2008; Gursoy et al., 2010; He et al., 2010).However, some studies indicate that RT could increase P loss(Gaynor and Findlay, 1995) and soil bulk density (Pikul et al.,2001), and decrease SOC content in the top 20-cm compared withMP (Pikul et al., 2001). Although RT may have some disadvantages,it is more versatile than other minimum-tillage systems (Lal, 1990)and it is often an economically viable alternative to MP (Archeret al., 2002).

Ridge tillage is characterized by a permanent row-interrowconfiguration where the rows are in the same location every year.It requires use of matched-width equipment including planters,fertilizer side-dressing equipment, sprayers, and harvest machin-ery. Wheel traffic is often confined to the same interrow for eachpass of the machinery for certain field operations, which results indistinctly different traffic intensity among the interrows. Studiesshowed that some soil properties varied substantially at differentpositions across the ridge (Cassel and Nelson, 1985; Unger, 1995;Muller et al., 2009). The ridges and interrows created by the RTsystem add additional micro-sites to existing spatial heterogeneityfor soil characteristics such as SOC concentration (Burton et al.,2006). Thus, RT fields should be conceptualized and managed asthree distinct soil zones (i.e. ridge centers, ridge shoulders andinterrows), and not as a single unit (Liebig et al., 1993). Theperformance of RT in cool, humid fine-textured soils is not welldocumented with respect to overall SOC sequestration and soilphysical properties. In particular, the spatial distributions of SOCand soil physical properties among the interrows, shoulders andcrests of the ridges are still largely unknown. Hence, the purpose ofthis study was to compare the lateral and profile variations of SOCand some selected soil physical properties on a Brookston clayloam soil under RT, NT and MP in southwestern Ontario, Canada.We believe that this information will be useful in assessing RT as atillage system option, and in developing appropriate samplingregimes to assess lateral and profile changes of soil properties infuture RT studies.

2. Materials and methods

2.1. Experimental site and treatments

This study was established in the fall of 1982 on a Brookstonclay loam, a poorly drained lacustrine soil (Orthic Humic Gleysol,Canadian Soil Classification; mixed, mesic Typic Argiaquoll, USDASoil Taxonomy), located at the Hon. Eugene F. Whelan ExperimentFarm, Agriculture and Agri-Food Canada, Woodslee, Ontario,Canada (428130N; 828450W). The average soil texture in top 15-cm was 28% sand, 35% silt and 37% clay by weight and soil pHranged from 6.1 to 6.5. The mean annual air temperature andprecipitation were 8.7 8C and 827 mm, respectively (30 yearaverage). Surface slopes were <0.5% at the field site, and thus soilerosion and surface runoff were negligible.

The three tillage treatments (RT, NT, MP) were arranged in arandomized complete block design with two replicates and12 � 35 m plots. Monoculture corn was grown from 1983 to1996. In 1997, each plot was split lengthwise into two sub-plots(6 � 35 m) and a corn-soybean rotation grown with both crops

present in each main plot in each year from 1997 until present. Forall three tillage treatments, the plant rows were parallel tosubsurface tile drains. The typical sequence of operations in the RTsystem for the corn phase was: (1) ridges rebuilt to 10-cm heightand 76-cm spacing after soybean harvest at the end of Septemberusing a Buffalo ridge cultivator (Fleischer Manufacturing Inc.,Columbus, NE, USA); (2) application of pre- and post-emergenceherbicides as required in spring; (3) corn planting on ridge crestswith a no-till planter and application of starter fertilizer in Maywith dates dependent upon the moisture content of soil; (4) side-dressing nitrogen fertilizer at corn 6 leaf stage in mid- to late June;(5) corn harvest with all residues retained in field in Autumn. Thesequence of operations for the soybean phase of ridge tillageincluded: (1) application of pre- and post-emergence herbicides asrequired in spring; (2) soybean planting at 38-cm row spacing witha no-till planter in Spring (late May to early June); (3) soybeanharvest normally at the end of September; and (4) ridges rebuiltafter soybean harvest using. The ridges were in the same locationeach year, and were maintained at 10-cm above the mean soilsurface (Fig. 1). In the corn year, the top of the ridge was skimmedoff just prior to planting, and the corn row was planted into thecenter of the truncated ridges. In the following soybean year,soybean was planted into the shoulders (38-cm spacing). Therewas no soil disturbance in the NT treatment, except for plantingand side-dress application of nitrogen fertilizer in corn. The MPtreatment included mouldboard plowing to 15–17-cm depth eachfall, and secondary cultivation (Kongskilde triple K cultivator) ordisking and harrowing in the following spring just before planting.No field cultivation was applied to the RT plots during the soybeanphase.

2.2. Sample collection

On 20th September 2010, soil cores (3.4-cm in diameter, 0–60-cm depth) were collected from each corn plot using a hydraulicallypowered Concord soil core sampler (Fargo, ND, USA). Due to thedry summer and fall in the study year, the soil was very dry at thetime of sample collection. Only soil cores without visiblecompaction were used in this study. The sampling design, includedtwo transects perpendicular to the corn rows in each plot (one inthe North end and one in the South end), each consisting of ninepositions (P1–P9) with an interval of 19-cm lateral distancebetween adjacent sampling points (Fig. 1). The locations of thesampling positions from east to west are as follows: P1, in a light-traffic interrow; P2, in the shoulder adjacent to light-trafficinterrow; P3, in the ridge crest or crop row; P4, in the shoulderadjacent to heavy-traffic interrow; P5, in heavy-traffic interrow;

X.H. Shi et al. / Soil & Tillage Research 120 (2012) 1–7 3

P6, in the shoulder adjacent to heavy-traffic interrow; P7, in croprow; P8, in the shoulder adjacent to light-traffic interrow; and P9,in light-traffic interrow. The same sampling design with the samespacing relative to the corn rows and light and heavy wheel trafficwas also applied to NT and MP. The heavy-traffic interrow wassubjected to tractor and implement wheel traffic for planting andharvesting every year, and for side-dressing N application andridge re-building in the corn years. The light-traffic interrow wassubjected to wheel traffic only for herbicide spraying. As NT andMP do not have ridges, the so called ‘‘shoulder’’ under NT and MP inthis study was located mid way between the crop row andinterrow (Fig. 1b). The soil cores were transported back to thelaboratory, and cut into six depth-segments (0–5, 5–10, 10–20, 20–30, 30–45 and 45–60-cm). For RT, the core depth was measuredfrom the respective soil surfaces of the crests, shoulders andinterrows; for NT and MP, the depth reference was at the soilsurface.

2.3. Soil physical measurements

Soil penetration resistance (cone penetration index) wasmeasured in situ at each of the nine sampling points (Fig. 1) to45-cm depth at intervals of 1.5-cm on the same day of soilsampling, using a RIMIK CP-40 cone penetrometer (cone basalarea = 1.2 cm2, cone angle = 308) (Agridry Rimik Pty Ltd., Too-woomba, Australia). The bulk density of each soil core segment wasestimated using the inner diameter of the soil core sampler, thesegment length, and the oven-dry weight of the core segment. Allsoil core samples were air-dried for several days until weight wasstable and then the water content in air-dry soil was determined byoven-drying eight randomly selected sub-samples and applyingthe average of air-dry soil moisture content to all samples.

2.4. Soil organic carbon analysis

Visibly identifiable crop residues were manually removed andthe soil was then ground to pass through a 0.25-mm sieve. Totalsoil C content was determined using a Leco CN 2000 analyzer (LecoCorp., St. Joseph, MI, USA). Since the soils were virtually free of

Table 1Soil properties (relative values) calculated using the average of 8 sample positions (P1–P8

the average of the samples located in the shoulders (S) (P2, P4, P6, and P8), interrows

Depth (cm) Avg. Soil organic C Soil bulk density

S I C S I

No-tillage

0–5 1.00 1.01 0.96 1.02 0.99 1.03

5–10 1.00 1.05 0.96 0.94 0.99 0.98

10–20 1.00 0.99 0.99 1.03 0.99 1.00

20–30 1.00 1.00 0.85 1.15 1.00 0.99

30–45 1.00 1.02 1.00 0.97 1.00 0.99

P valuea 0.24 0.13 0.60 0.18 0.94

Mouldboard plow tillage

0–5 1.00 1.00 1.00 1.00 1.02 1.00

5–10 1.00 0.99 1.00 1.02 1.02 0.95

10–20 1.00 1.02 0.96 1.01 0.99 1.02

20–30 1.00 0.99 0.96 1.07 1.00 0.98

30–45 1.00 0.98 0.91 1.13 1.01 0.98

P value 0.43 0.10 0.14 0.31 0.29

Ridge tillage

0–5 1.00 1.02 0.90 1.07 1.03 0.96

5–10 1.00 0.96 0.93 1.16 1.01 1.03

10–20 1.00 1.03 0.92 1.02 1.01 1.02

20–30 1.00 0.98 0.65 1.39 1.01 1.00

30–45 1.00 0.93 0.91 1.24 1.01 0.98

P value 0.44 0.06 0.05 0.02 0.88

a Probability associated with a Student’s paired t-test, with a two-tailed distribution be

carbonates, SOC was assumed to equal total C. The quantity of SOC,on an equivalent-soil-mass basis, was calculated as outlined inEllert and Bettany (1995).

2.5. Statistical analysis

We hypothesized that under RT the average value of a given soilproperty would be located on ridge shoulders at P2, P4, P6 and P8(Fig. 1a). To test this, we averaged the soil properties (SOC, soil bulkdensity, oven-dry soil water content and soil penetration resis-tance) for: (1) the overall average, P1–P8; (2) the shoulder location,P2, P4, P6 and P8; (3) the furrow location, P1 and P5; and (4) thecrest location, P3 and P7 (Fig. 1a and Table 1). Each of the averageswas normalized by dividing by the average across the two ridges(P1–P8) and then compared statistically using SAS (SAS Institute,2000) (Table 1). The same data treatment was also applied to NTand MP (Fig. 1b and Table 1).

The statistical comparison assumed a randomized completeblock design with sub-sampling. Tillage type main effects on soilproperties were tested using one-way analysis of variance(ANOVA), and means of soil properties among tillage type werecompared using the least significant difference (LSD) test. Thestatistical significance level was set at p = 0.05.

3. Results and discussion

3.1. Spatial distribution of SOC and selected soil physical properties

Ridge tillage produced an indistinct ‘‘sine-wave’’ pattern of SOCcontent across sampling positions in comparison to the moreuniform values under NT and MP (Fig. 2). The relatively uniformlateral distributions of SOC under NT and MP was mainly due to thehomogenizing effects of the two tillage treatments, i.e. nodisturbance for NT and thorough mechanical mixing under MP.In addition, the position of plant rows in NT and MP likely shiftedover the years because there were no permanent markers for therows. Consequently, crop root effects on the lateral distribution ofSOC would probably be negligible. On the other hand, existingridges in RT provide a guide for ridging and planting in subsequent

, across two ridges) (standardized avg. = 1.00) versus soil properties calculated using

(I) (P1, P5 and P9), and crests (C) (P3 and P7) under three tillage systems.

Soil water content Soil penetration resistance

C S I C S I C

0.98 0.97 0.99 1.07 1.04 1.01 0.91

1.04 0.99 0.95 1.06 1.08 0.63 0.85

1.01 0.98 0.96 1.08 1.11 0.90 0.88

1.01 0.99 0.99 1.02 1.10 0.85 0.94

1.01 1.01 0.99 0.99 1.10 0.79 1.00

0.43 0.16 0.05 0.05 0.00 0.06 0.03

0.97 0.95 1.00 1.10 0.96 1.09 0.99

1.02 0.97 1.01 1.05 0.92 0.65 1.13

1.00 0.99 1.01 1.02 0.98 0.77 1.27

1.02 0.98 1.07 0.97 0.99 0.77 1.25

1.00 0.97 1.10 0.96 1.03 0.75 1.19

0.86 0.01 0.15 0.52 0.23 0.06 0.03

0.98 1.00 1.14 0.87 1.27 0.62 0.83

0.95 1.01 1.10 0.87 1.15 0.67 1.03

0.97 1.00 1.07 0.92 1.08 0.65 1.19

0.98 0.98 1.08 0.95 1.12 0.64 1.12

1.00 0.98 1.09 0.96 1.09 0.65 1.17

0.04 0.37 0.00 0.01 0.01 0.00 0.36

tween the average (Avg.) and a give position of shoulder (S), interrow (I), or row (R).

5

10

15

20

25

30

35

40

NT

So

il o

rgan

ic c

arb

on

(g

kg

-1)

5

10

15

20

25

30

35

40MP

Sample locati on

P1 P2 P3 P4 P5 P6 P7 P8 P9

0

5

10

15

20

25

30

35

40

RT

0-5 cm 5-10 cm 10-20 cm

20-30 cm 30 -45 cm 45-60 cm

Fig. 2. Variation of soil organic C (SOC) content with sampling positions and depths

under no-tillage (NT), mouldboard plow tillage (MP) and ridge tillage (RT). The

sampling positions are as indicated in Fig. 1. The vertical bars indicate standard

error (n = 4).

Sample location

P1 P2 P3 P4 P5 P6 P7 P8 P9

So

il w

ater

co

nte

nt

(%)

9.0

10.5

12.0

13.5

15.0

16.5

18.0

19.5

21.0 0-5 cm 5-10 cm 10-20 cm

20-30 cm 30-45 cm 45-60 cm

Fig. 3. Variation of soil water content with sampling positions and depths under

ridge tillage (RT). The sampling positions are as indicated in Fig. 1a. The vertical bars

indicate standard error (n = 4).

X.H. Shi et al. / Soil & Tillage Research 120 (2012) 1–74

years and plant residues are preferentially incorporated into thecrest and shoulder positions during ridge reformation. In addition,crop roots were most abundant in the crest and shoulder areas, sodead roots and above-ground residues relocated to the crests andshoulders by ridging each year served as additional substrate forsoil microorganisms (Kramer and Gleixner, 2006). Thus, it was notsurprising that SOC level tended to be greatest in the crests, lowestin the interrows, and intermediate in the shoulders (Fig. 2). Cornroots that penetrated deep into soil at the same position year afteryear would result in an accumulation of dead roots underneath theplant row, which could contribute to higher SOC content in deepsoil under the row on the crest of the ridges relative to theinterrow. Liebig et al. (1995) found a negative impact of wheeltraffic on soil respiration and we speculate that this could havecontributed to the higher SOC content observed in the near-surface(0–5-cm) of the heavy-traffic interrow (P5) relative to light-trafficinterrow (P1, P9) (Fig. 2).

The similar SOC content across the crest, shoulder, and interrowof RT soil at 10–20-cm depth (Fig. 2) was likely related to thedistribution of fibrous corn roots. Kovar et al. (1992) found thatcorn root density was higher in the row (ridge crest) at surface (0–15-cm) and in the interrow at subsurface (15–30-cm) under RT,whereas the root density was highest in the row and decreasedwith distance from the row under MP and NT. Accordingly, we canpresume that greater proliferation of lateral roots at 10–20-cm inthe row and interrow under RT may be responsible for therelatively uniform SOC at that depth.

For the RT treatment, soil water content was highest in theinterrows, lowest in the crests, and intermediate in the shouldersof the ridges (Fig. 3). This was likely caused by redistribution ofprecipitation and soil water (Muller et al., 2009), because rainfalling on the ridge crests tends to rapidly run off and infiltrate intothe lower interrow spaces. Also, the corn roots are concentrated in

the ridges and consequently extract more soil water there thanfrom the interrows. Ridging had a distinct effect on soil penetrationresistance relative to NT and MP (Fig. 4). Penetration resistancewas higher in the crest and shoulder than in the interrow of theridges, and was negatively correlated with soil water content(Cassel and Nelson, 1985; Materechera and Mloza-Banda, 1997).As expected, penetration resistance in the frequently traffickedinterrow (P5) of the RT treatment was greater than in theinfrequently trafficked interrows (P1 and P9) (Fig. 4), as was foundby Liebig et al. (1993). Ridging had no appreciable spatial effect onsoil bulk density compared with NT and MP (data not shown),which was in agreement with the study of Vinther and Dahlmann-Hansen (2005) on a poorly drained soil. This provided furtherevidence that soil strength as manifested in soil cone penetrationresistance is more sensitive than soil bulk density to ridging andcompaction (Liebig et al., 1993). The differences in the effects ofridging on soil water content, soil penetration resistance and SOCsuggest that SOC was less sensitive to ridging than soil physicalproperties at this field site.

3.2. The optimal location for soil sampling under three tillage systems

For SOC and soil bulk density under NT and MP, the averagesfrom the samples collected at the ‘‘crests (row)’’ (P3 and P7), at the‘‘shoulders’’ (P2, P4, P6 and P8), and at the ‘‘interrows’’ (P1, P5, P9)(Fig. 1b) were not significantly different from the overall averagesof P1–P8, which indicates uniform spatial distribution of these soilproperties under NT and MP (Table 1). However, the soil watercontent was under-estimated (p = 0.05) using the samples from theinterrow (P1, P5, and P9) and over-estimated (p = 0.05) using thesamples from the crests (P3 and P7) for NT soils. This means thatthe soil water contents measured at the shoulders could representthe average of water content for NT. On the other hand, the soilwater contents at the interrow and crest positions under MP weresimilar to the averages of water content across the shoulders. Forpenetration resistance in NT, the measurements made at theshoulders led to over-estimation, and the measurements made atthe crests and interrows resulted in under-estimations. For MP, themeasurements conducted at the crests over-estimated and themeasurements at the interrows under-estimated the penetrationresistance. Although the differences between the overall averageand the various sampling locations were often statisticallysignificant under NT and MP, the magnitudes of the differencesvaried and were often less than 5%, such as for water content.

For RT, samples collected from the shoulders seem to providegood estimation of the mean SOC, bulk density and soil water

Sample location

P9P8P7P6P5P4P3P2P10

1000

2000

3000

4000

5000

1000

2000

3000

4000

5000

Soil

penetr

ati

on r

esi

stance (

kP

a)

1000

2000

3000

4000

5000

RT

NT

MP

0-5 cm 5-10 cm 10-20 cm

20-30 cm 30-45 cm

Fig. 4. Variation of soil penetration resistance with sampling positions and depths

under no-tillage (NT), mouldboard plow tillage (MP) and ridge tillage (RT). The

sampling positions are as indicated in Fig. 1. The vertical bars indicate standard

error (n = 4).

Soil organic carbon (g k g-1)

0 5 10 15 20 25 30 35

Soil

dep

th (

cm)

0

5

10

15

20

25

30

35

40

45

50

55

NT

MP

RT

Fig. 5. Effects of tillage practices on the depth profiles of soil organic C content (the

average of P1–P8 for all three soils). (NT, no-tillage; MP, mouldboard plow tillage;

RT, ridge tillage). The horizontal bars indicate standard error (n = 4).

Table 2Soil organic C (SOC) stocks on an equivalent-soil-mass basis among tillage systems.

Equivalent-soil-mass (Mg ha�1)

Tillage system 1554 3072 4735

SOC stock (Mg C ha�1)

No-tillage 38.5 a* 69.2 a 90.0 a

Mouldboard plow 29.6 c 59.0 c 80.3 c

Ridge tillage 35.5 b 64.1 b 85.4 b

* Values within a column followed by the same letter are not significantly

different at p < 0.05.

X.H. Shi et al. / Soil & Tillage Research 120 (2012) 1–7 5

content averaged across two ridges, with relative differenceswithin �2–7% for SOC, 0–3% for bulk density, and �0–2% for watercontent (Table 1). The samples collected from the interrows over-estimated soil water content and under-estimated SOC while thesamples collected from the crests showed opposite trends. Soil bulkdensity was less variable, however, with the crest and interrow valuesdiffering by �5% relative to the overall average. For soil penetrationresistance, none of the three sampling positions provided goodrepresentation of the overall average (Table 1). Our study supportsLiebig et al.’s (1993) argument that RT fields should be conceptualizedand managed as three distinct soil zones, not as a single unit.

3.3. Tillage effects on SOC and selected soil physical properties

No-tillage resulted in a distinct SOC stratification withsignificantly higher SOC in surface soil (0–5-cm, 29.1 g kg�1)and slightly lower SOC in subsurface soil (17.5 g kg�1 at 10–20-cmand 8.8 g kg�1 at 20–30-cm) relative to MP (18.6 kg�1 at 0–5 cm,19.0 g kg�1 at 10–20-cm, and 10.4 g kg�1 at 20–30-cm) (Fig. 5). Thestratification effect of NT on SOC has been reported previously onthe same soil (Shi et al., 2011; Yang et al., 2008) as well as in otherstudies (Angers et al., 1995; Franzluebbers, 2002; Olson et al.,2005; Dolan et al., 2006; Liang et al., 2007). No-tillage hadsignificantly greater SOC storage than MP on equivalent-massbasis (1554, 3072 and 4735 Mg ha�1 soil equivalent-mass,corresponding to 0–10-cm and 0–20-cm soil depths for MP, and0–30-cm depth for RT, respectively) (p < 0.05) (Table 2). Thehomogenizing effect of annual fall mouldboard plowing andsecondary spring cultivating, disking and harrowing was undoubt-edly the reason for the uniform SOC distribution in the top 20-cmof MP. Retention of crop residues on the surface and a concomitantlack of carbon input at greater depths probably resulted in thedistinct enrichment of SOC at the surface NT soil (Shi et al., 2011).

Although the stratification phenomenon was the most strikingfeature of NT, there was a significantly higher total SOC in NT thanMP on an equivalent mass basis (Table 2). This result was differentfrom the previous study where no significant difference in SOCstock was found between two tillage practices based on thesamples collected before 2004 (Yang et al., 2008). This may meanthat the impacts of NT on SOC stock vary with time; hence, we willcontinue to monitor this effect.

The SOC levels at 0–5-cm (25.5 g kg�1) and 10–20-cm(16.4 g kg�1) in RT were 12.4% and 5.9% lower than those of NT,but slightly higher than NT at 20–30-cm depth (9.8 g kg�1) (Fig. 5).The SOC profiles below the plow layer for RT, NT and MP exhibitedsimilar monotonic curves, and decreased from about 16–18 g kg�1

at 15-cm depth to about 3 g kg�1 at 40–60-cm depth (Fig. 5). TheSOC content was similar for RT and MP from 25 to 60-cm, but RTwas significantly greater than MP in the 0–5-cm depth (p < 0.05)(Fig. 5). Ridge tillage resulted in significantly lower (p < 0.05) SOCstorage than NT in the 0–30-cm depth but higher (p < 0.05) SOCstorage than MP on a 4735 Mg ha�1 soil equivalent-mass basis(Table 2). These results are in agreement with other studies(Agbede and Ologunagba, 2009; Varvel and Wilhelm, 2010). TheSOC levels in the 20–60-cm depth were similar in RT, NT and MPtreatments (Fig. 5). Surface residue incorporation in the nearsurface soil accompanying ridge building but with only limitedresidue incorporation or soil mixing at deeper soil depths likelycontributed to carbon accumulation at the surface soil under RT.The fact that SOC levels among three tillage treatments weresimilar over the 20–60-cm depth in cool, clay loam soils suggeststhat the effect of tillage on SOC was limited primarily in the plowlayer.

Soil bulk density varied from 1.39 to 1.61 Mg m�3 within thetop 10-cm and increased to 1.63–1.70 Mg m�3 for the depth-increments below 20-cm (Table 3). For this fine-texture soil, bulk

Table 3Soil bulk density for the various tillage systems and depths.

Tillage system Soil bulk density (Mg m�3)

0–5-cm 5–10-cm 10–20-cm 20–30-cm 30–45-cm 45–60-cm

No-tillage 1.45 Ab* 1.48 Bb 1.53 Bb 1.63 Ba 1.68 Aa 1.68 Aa

Mouldboard plow 1.51 Ac 1.61 Aab 1.52 Bc 1.65 Aab 1.68 Aa 1.68 Aa

Ridge tillage 1.39 Ac 1.53 Ab 1.59 Ab 1.67 Aa 1.69 Aa 1.70 Aa

Crest 1.37 1.45 1.54 1.64 1.68 1.71

Shoulder 1.43 1.55 1.61 1.69 1.71 1.70

Interrow 1.35 1.57 1.62 1.67 1.66 1.68

* Values within a column followed by the same uppercase letter are not significantly different at p < 0.05. Values within a row followed by the same lowercase letter are not

significantly different at p < 0.05.

X.H. Shi et al. / Soil & Tillage Research 120 (2012) 1–76

densities at 0–20-cm depth were above the upper limit foradequate root-zone aeration (1.25–1.30 Mg m�3) and fell withinthe range where root elongation becomes severely restricted (1.4–1.6 Mg m�3) (Reynolds et al., 2007). The high bulk densities in thisstudy are attributed to high shrinkage of this shrinking-swellingclay loam soil (shrinkage limit � 18%). No rainfall occurred formore than one and half month prior to sampling which led to soilmoisture contents (0–60-cm) generally close or below thepermanent wilting points. In a multiple year study on the sameplots, Yang et al. (2008) found the BDs (0–20-cm) of NT and MPwere between 1.22 and 1.43 based on the soils collected beforeplanting or after harvest when the soils were wetter. Soil bulkdensity did not differ in the 30–60-cm depth among the threetillage treatments (Table 3), indicating homogeneity of thisphysical property in that depth range. Soil cone penetrationresistance increased monotonically from 0 to 45-cm depth underMP treatment, from 0 to 9-cm depth under NT and from 0 to 12-cmdepth under RT (Fig. 6). Soil penetration resistance was lowerunder MP than under NT from 0 to 21-cm depth, which is likely aresult of plowing (Fig. 6). However, soil penetration resistance wasconsistently higher under MP than under NT from 22.5 to 45-cmdepth (Fig. 6). Ridge tillage produced a similar overall penetrationresistance profile to NT, although penetration resistance washigher over the 12–45-cm depth, and lower over the 0–12-cmdepth relative to that under NT (Fig. 6). Hence, seedlinggermination and early growth would experience somewhat morefavorable soil strength conditions at the 0–12-cm depth under RTthan NT. The greater soil strength (penetration resistance) atdepths greater than 12-cm under RT relative to NT may reflect soilcompaction over time by the tractor and ridge cultivator. The zoneof relatively unimpeded root growth (i.e. penetration resistance

Soil penet rati on resista nce (kPa)

0 500 100 0 150 0 200 0 250 0 300 0 3500

Soil

dep

th (

cm)

0

6

12

18

24

30

36

42

48

NT MPRTUpper limit

Fig. 6. Effects of tillage practices on soil penetration resistance (NT, no-tillage; MP,

mouldboard plow tillage; RT, ridge tillage). The vertical line distinguishes the soil

penetration resistance value (2000 kPa) above which root growth becomes

significantly impeded in this humid, fine-textured soil. The horizontal bars

indicate LSD values at p = 0.05 (n = 4).

<2000 kPa, Reynolds et al., 2002) was very shallow (�4.5-cm)under NT, slightly deeper (�6.0-cm) under RT, and much deeper(�10.5-cm) under MP (Fig. 6). The result of higher soil penetrationresistance under NT than under MP within 0–21-cm depth isapproximately consistent with findings by Drury et al. (2003) andShi et al. (2011) for this Brookston clay loam soil in different fieldsites.

4. Conclusions

Ridge tillage on Brookston clay loam soil resulted in stronglateral variation in soil water content, soil penetration resistanceand SOC. Ridge tillage produced low and high SOC in the furrows(interrows) and crests (rows) of the ridges, respectively, withintermediate SOC on the shoulders. Soil penetration resistance inthe ridge crest and shoulder was higher than in the interrow, whilesoil water content showed the reverse trend. While the samplescollected from the shoulders of the RT ridge can represent theaverage SOC, bulk density and soil water content across the ridge,measurements at more locations are needed to obtain the overallaverage soil penetration resistance. These results are likely causedby a combination of plant residue redistribution by ridgerebuilding, and root growth. After 29 years, RT improved thequality of the Brookston clay loam soil in southwestern Ontario, asevidenced by decreased soil penetration resistance and bulkdensity in surface soil relative to NT, and greater in SOC stockswithin 0–30-cm depth relative to MP (p < 0.05).

Acknowledgments

Financial support for this research was provided by theAgriculture & Agri-Food Canada (AAFC). We appreciate financialassistance from the PhD Scholarship Program of the Ministry ofEducation, People’s Republic of China and AAFC. Our gratitude isalso extended to Mary-Anne Reeb for field assistance, to JohnGoerzen for lab assistance, and to the AAFC farm staff foragronomic operation and plot maintenance. The constructivecomments of two anonymous reviewers are gratefully acknowl-edged.

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