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A wide view of no-tillage practices and soil organiccarbon sequestrationXueming Yanga, Craig F. Drurya & Michelle M. Wanderb

a Greenhouse & Processing Crops Research Centre, Agriculture & Agri-Food Canada,Harrow, ON, Canadab Department of Natural Resources & Environmental Sciences, University of Illinois atUrbana-Champaign, Urbana, IL, USAPublished online: 06 Aug 2013.

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REVIEW ARTICLE

A wide view of no-tillage practices and soil organic carbon sequestration

Xueming Yanga*, Craig F. Drurya and Michelle M. Wanderb

aGreenhouse & Processing Crops Research Centre, Agriculture & Agri-Food Canada, Harrow, ON, Canada; bDepartment of

Natural Resources & Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, USA

(Received 28 May 2013; final version received 12 June 2013)

Many believe that conservation tillage practices could increase the sequestration of atmospheric carbon dioxideinto agricultural soils and this sequestered carbon may partially offset the greenhouse gas effect and thus reducethe impact of global warming. Recent advances in soil carbon (C) and greenhouse gas analysis have made itpossible to evaluate the impacts of conservation tillage on C sequestration from various perspectives. Althoughconservation tillage favors soil and water conservation, there are biased estimates of C sequestration associatedwith conservation tillage, and it is particularly an issue for a ‘‘pure’’ no-tillage (NT) system. Accordingly, thispaper presents an overview of the progress achieved in evaluating C sequestration in no-till (the extreme type ofconservation tillage) and conventional tillage production systems. In addition to extended discussion of how soilsampling and calculations could influence the estimates of C gains or losses in no-till versus conventional tilledsoil, this review will also focus on following aspects, including (1) the impact of NTon crop yields which governsorganic C inputs to soil from crop residue, (2) the impact of NT on soil organic C mineralization which is amajor pathway of soil C output, and (3) the roles of the initial levels of C stocks and soil erosion rates which arecrucial for estimating soil C sequestration under different tillage systems. Many soil C studies have indicatedthat the impacts of NTon soil C sequestration are compounded by many factors and should not be generalized.

Keywords: no-tillage; carbon sequestration; carbon inputs; crop yields; carbon mineralization; soil erosion; soildeposition

Introduction

The tremendous losses of 42�78 Pg of carbon (C)

from soils resulting from the conversion of natural

vegetation to intensive agricultural production (Lal

2004), the larger global soil organic carbon (SOC)

pool (1500 Pg) relative to the atmospheric C pool

(760 Pg) (Schlesinger & Andrews 2000), and the loss

of soil C resulting from the plowing of soils

(Reicosky & Archer 2007) all contribute to the belief

that the adoption of conservation tillage, particularly

no-tillage (NT), could reverse this process and

sequester atmospheric CO2-C into agricultural soils

which could offset the greenhouse gas effect (Bruce

et al. 1999; Lal 2004). In the USA alone, the

adoption of no-till may have the potential to seques-

ter 275�763 Tg CO2 annually, and this alone could

represent approximately 4% to 11% of the US total

greenhouse gas emissions in 1999 (Bruce et al. 1999;

Duiker & Lal 1999). Based on these estimates, it was

projected that a huge amount of atmospheric CO2-C

could be sequestrated in agricultural soils with the

conversion of all conventionally managed croplands

to conservation tillage over the next 50 years (Pacala

& Socolow 2004; Lal 2011a). The cost of mitigating

climatic change by sequestering atmospheric CO2-C

into the soil through the adoption of NT is argued to

be substantially lower than the costs associated with

reducing industrial C emissions (Marland et al.

2001). Although the Chicago Climate Exchange for

the trading of greenhouse gases in the USA was shut

down for lack of legislative interest, scientists and

policy makers are still promoting no-till farming as a

recommended strategy for world food security and,

at the same time, no-till practices are still being

considered a greenhouse gas mitigation strategy (Lal

2010, 2011b).

*Corresponding author. Email: Xueming.Yang@agr.gc.ca

Acta Agriculturae Scandinavica, Section B - Soil & Plant Science, 2013

Vol. 63, No. 6, 523�530, http://dx.doi.org/10.1080/09064710.2013.816363

# 2013 Her Majesty the Queen in Right of Canada, as represented by the Minister of Agriculture and Agri-Food, Canada. Michelle M Wander hereby waives

her right to assert copyright, but not her right to be named as co-author in the article.

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NT is primarily initiated to protect soils from

erosion, to conserve moisture and reduce production

costs, and NT is associated with additional environ-

mental benefits including a richer soil biota (Holland

2004). With better planters, herbicides, and accu-

mulated experience, NT began to be widely adopted

in the 1980s in the USA and then in Australia, South

America, and Canada. Today, approximately 23% of

the total cropland in the USA is planted using NT

(Triplett & Dick 2008). Some studies assert that no-

till farming has already increased soil C contents

relative to levels that would have existed under

conventional farming (e.g., moldboard plowing);

they have estimated C sequestration rates of 0.31�0.82 Mg C ha�1 yr�1 in the USA and across the

world (West & Post 2002; Spargo et al. 2008;

Franzluebbers 2010; Mishra et al. 2010). However,

other studies showed that C accumulation under NT

often occurs only in the top few centimeters of soil

and this C increase is often offset by the depletion of

C in the lower soil depths (Hunt et al. 1996; Yang &

Wander 1999; Dolan et al. 2006). This is a serious

issue and the value of no-till agriculture for increas-

ing C in soils has been questioned in recent studies

(Ogle et al. 2012). For instance, NT has been found

to increase C concentrations in the upper layers of

some soils, but it did not store more C than

conventional management when the whole soil

profile was considered (VandenBygaart and Angers

2006; Blanco-Canqui & Lal 2008; Novak et al.

2009; Luo et al. 2010; VandenBygaart et al. 2011).

The higher C stock in NT system reported in many

studies may simply be due to an artifact of the

sampling protocol that has led to biased results

(Baker et al. 2007). Comparing C sequestration

from NT and conventional tillage requires a thor-

ough and objective assessment (Angers et al. 1997;

Puget & Lal 2005; Yang et al. 2008b). Failure to

conduct appropriate assessments can jeopardize the

credibility of the scientific community and its ability

to promote other land use and management prac-

tices that may effectively mitigate rising atmospheric

concentrations of greenhouse gases (Baker et al.

2007). In a meta-analysis of the effects NT versus

conventional tillage on SOC stocks, Angers and

Eriksen-Hamel (2008) found, on average, 4.9 Mg

ha�1 more SOC under NT than full-ploughed tillage

even though great SOC content often occurred near

the bottom of the plow layer under full-plough

tillage. Other recent studies found that the soil

organic C stocks observed with NT compared to

conventional tillage were not as large as expected

(Virto et al. 2012) and the evaluation of the relative

carbon balance for NT and conventional tillage

depends upon complex inter-relationships between

soil and climate factors which are still not completely

understood (Soane et al. 2012). Although some

recent studies have documented that shallow sam-

pling and other reasons could introduce a bias to

estimating C sequestration in NT soils (Baker et al.

2007; Lal 2011a), there are other factors such as

carbon input and carbon mineralization (C output)

and soil erosion and re-deposition that should also

be considered when assessing the impact of NT

practices on C sequestration in agricultural soils.

Besides describing how soil sampling and calcula-

tions could influence the estimates of carbon seques-

tration induced by tillage practices, we will also

evaluate the impacts of NT on crop biomass produc-

tion (soil C inputs) and organic carbon mineraliza-

tion (soil C output), as well as the impact of soil

initial levels of C stocks and soil erosion and

sediment re-deposition on soil organic C gains and

losses.

Soil sampling and calculations of carbon stock

in soil

Recent trials that measured soil C at depth have

shown that soil type (Blanco-Canqui & Lal 2008;

Christopher et al. 2009), landscape position (Van-

denBygaart et al. 2002), and sampling depth

(Hermle et al. 2008; Yang et al. 2008b; Syswerda

et al. 2011) can influence the estimates regarding C

gains or losses. Kravchenko and Robertson (2011)

concerned that many soil C studies which reported

no significant treatment effects are examples of

Type-II error where the designs of the experiment

made it almost impossible to statistically detect even

large differences in C stocks. The study of Christo-

pher et al. (2009) is exactly the case of the Type-II

error because the experimental design clearly put

their results in serious doubt due to lack of power to

detect change between the comparisons (high varia-

bility, lack of true control, and minimal number of

samples) (VandenBygaart 2009). Although Type-II

error is common in the assessment of management

effects on SOC content, VandenBygaart and Allen

(2011) discussed few approaches that help soil

scientists reduce Type-II errors to acceptable levels,

including effectively controlling experimental varia-

tion among plots, determining the required number

of replicate plots needed to ensure that power is

acceptably high, and exploit ‘‘hidden replication’’

through the use of factorial experiments.

In many studies, the changes in soil C stock are

estimated for a volume of soil which typically extends

to a given depth, such as 1 m defined by Interna-

tional Panel on Climate Change (IPCC) (2000).

However, this volume-based calculation does not

account for differences in soil masses induced by

different bulk densities between tillage treatments

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(i.e., mass of soil in each depth increment) which

may otherwise confound the results. There was a

range of outcomes when the bulk density (BD) of

no-till soils was compared to other conservation or

conventional tillage systems including studies where

NT had greater bulk densities than conventional

tillage, studies where no differences in BD were

reported and other studies where NT had lower BD

values than conventional tillage (Lampurlanes &

Cantero-Martinez 2003). Since BD can change

with land management practice, BD must be taken

into account when SOC stocks are determined so

that land management practices are compared on the

same ‘‘mass’’ basis (Don et al. 2011). Furthermore,

BD can also vary over and between years as a result

of differences in soil moisture (especially for shrink-

ing/swelling soils), crop growth, and due to the type,

frequency and timing of field operation. Although

BD influences soil C stock over the entire profile, it

is particularly crucial at the soil surface where the

majority of the SOC is stored and the majority of the

SOC stocks are compared.

The equivalent soil mass concept was proposed to

overcome the bias of the unit area-based calculations

(Ellert & Bettany 1995). The equivalent soil mass is

defined as the reference soil mass per unit area

chosen in a layer and equivalent C mass is the C

stock stored in the same equivalent soil mass. In a soil

system whether a higher (based on the soil with a

higher soil BD) or a lower (based on the soil with a

lower BD) equivalent soil mass should be defined for

a particular comparison is dependent upon both the

direction of soil BD changes as well as whether soil C

concentrations are uniform in the soil (Lee et al.

2009). Although many studies have implemented the

equivalent soil mass technique to calculate soil C

stocks (Yang & Wander 1999; Bayer 2003; Deen &

Kataki 2003; Liang et al. 2006; VandenBygaart &

Angers 2006; Mishra et al. 2010; Shi et al. 2010),

only a few indicate whether or not the above-

mentioned issues were considered. Moreover, no

matter what equivalent soil mass is identified, the

concentration of C in ‘‘the additional layer’’ of soil

required to reach the equivalent soil mass is often

unknown and is just estimated using this technique.

It is straightforward to use the average C concentra-

tion of the deepest depth or of the layer below the one

layer (mass) that is being estimated if the C concen-

tration in these lower soil layers has been measured.

Just a few studies have enriched or diluted soil C

concentrations in the additional layer by considering

how soil C concentration would change with the

gradient in soil depth (Staricka et al. 1991; Angers

et al. 1995; Yang et al. 2008a; Lee et al. 2009).

Conclusions concerning the C stock changes in soils

can be altered depending upon the availability of C

concentrations from one horizon or another (Yang &

Wander 1999). For more accurately quantifying SOC

in an equivalent soil mass, Gifford and Roderick

(2003) proposed cumulative mass coordinates meth-

odology, which does not involve repeat sampling

trips, nominal specification of the location of bound-

aries between soil horizons, or independent sampling

for determining soil bulk densities. Based on the

method of Gifford and Roderick (2003), Wendt and

Hauser (2013) developed a new equivalent mass

procedure for calculating SOC stocks in multiple soil

layers and showed that it can be implemented with-

out BD sampling. Obviously, without careful con-

sideration of differences in soil BD and enrichment

(or dilution) of soil C concentrations as they vary

within the soil profile, calculations of soil C stocks

can over- or under-estimate C reserves.

NT and soil carbon input

For agricultural soils, the C inputs primarily come

from the crop residues (shoots and roots) which

remain in soil after harvest. For this reason, if one

tillage system produced more residues over a second

system, then soil organic C content may be higher in

the former system. The amount of crop residue

remaining in soil is often closely linked to crop yields.

An extensive literature review on the influence of

tillage on corn and soybean yields in the USA and

Canada reported the average difference in corn and

soybean yield between no-till and conventional

tillage was 0.5% lower for corn and 0.7% greater

for soybean from no-till than from conventional

tillage on an area weighted yield basis (DeFelice et

al. 2006). The same review found clear geophysical

and environmental patterns that no-till compared

with conventional tillage tended to have greater

yields in the south and west regions, similar yields

in the central USA, and typically lower yields in the

northern USA and Canada. Another meta-analysis

found that crop productivity can be reduced with

adoption of no-till, particularly in cooler and/or

wetter climatic conditions (Ogle et al. 2012). Besides

crop yields, crop harvest index (ratio of harvested

yield to the amount of crop aboveground residue

remaining in the soil) plays another key role in the

quantity of crop residues returned to the soil. In

the central Corn Belt of America, it was found that

the corn harvest index was higher for no-till than

conventional tillage (0.57 vs. 0.52) with normal

rate of nitrogen application (Kwaw-Mensah &

Al-Kaisi 2006). Higher harvest index for no-till

than conventional managed corn was also found in

Pakistan (Khan et al. 2007). The crops with higher

harvest index have lower crop residues and therefore

lower C returns to soils relative to crops with lower

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harvest index. In contrast, tillage treatments did not

result in significant differences in the harvest indices

for other major crops, including soybean (Spaeth

et al. 1984), sorghum, and wheat in the Southern

USA (Jones & Popham 1997).

The tillage practices can also influence crop root

growth which affects C inputs to the soil. NT led to

higher root density in the upper soil layer

(0�5 cm) but lower root density in the 5�60 cm

layer compared to conventional moldboard plough

tillage, which resulted in no significant difference in

total root mass between no-till and moldboard

plough in the top soil layer (0�28.2 cm) (Cheng et

al. 1990). Another classic study indicated that corn

roots developed more extensively and to a greater

depth where the soil was plowed annually than where

soil was not tilled or tilled to only a 5-cm depth, and

the differences in root mass in tilled soil relative to

no-till soil could be greater than 5% (Barber 1971).

Analysis of wheat root distribution and quantity

showed notable enrichment in the surface layer of a

no-till soil; however, there was no significant differ-

ence in root biomass between the two tillage treat-

ments (Wulfsohn et al. 1996). Similar trends with

reduced root growth at depth with NT were also

found in corn (Qin et al. 2005) and in wheat (Qin et

al. 2004) in European soils. Since NT typically has

had lower crop yields, a higher harvest index and

shallower roots than conventional tillage, the C input

would also be expected to be lower in these soils.

NT and SOC mineralization

Lower organic C contents in tilled compared to no-

tilled surface soils have been often attributed to

greater CO2 emissions in the former, especially after

tillage operations as the carbon would be mixed into

the soil and the soil would be aerated (Reicosky &

Lindstrom 1993; Reicosky et al. 1997). However,

more CO2 emissions from moldboard plough than

from no-till soils are generally based on short-term

fluxes (typically over a growing season), but not

necessarily over an annual basis (Fortin et al. 1996).

On a yearly basis, ploughing the soil appears to affect

the time of CO2 release rather than the amount of

CO2 produced when compared to no-till practices

(Hendrix et al. 1988). Yearly CO2 emissions were

also found higher in no-till than in tilled soil

(Franzluebbers et al. 1995; Alvarez et al. 1998). In

soils under maize�wheat rotation, the cumulated

CO2 over the 331 days of measurement were 31609

269 and 40649138 kg CO2-C ha�1 from conven-

tional and no-tilled plots, respectively, and the

authors believed that a large proportion of the CO2

emissions in no-till was probably due to the decom-

position of old weathered residues (Oorts et al.

2007). Although no-till cropping systems may pro-

tect carbon through less soil disturbance (Reicosky &

Lindstrom 1993) and increased aggregation (Six

et al. 2000) relative to conventional tillage, CO2

emissions in no-till and conventional tillage systems

vary with soil climatic conditions and the amounts

and location of crop residues and soil organic matter.

Thus, the concept that no-till practices always

protect SOC from mineralization relative to mold-

board ploughing should not be generalized.

Lack of information on the initial level of C

stocks or about erosion

NT effectively reduces soil erosion (Harrold &

Edwards 1974; Derpsch et al. 1986; Schuller et al.

2007); accordingly, NT preserves organic C in soil in

situ by preventing movement of C-rich topsoil.

Paired comparisons that estimate C sequestration

rates using the conventional plot as the reference

form the basis for most frequently cited review

papers which report that no-till sequestered C.

There was an average increase of 0.22 Mg C ha�1

yr�1 in soil under no-till compared to conventional

tillage based on 39 paired tillage experiments ranging

in duration from 5 to 20 years (Paustian et al. 1997);

an average C sequestration rate of 0.48 Mg C ha�1

yr�1 reaching equilibrium in 15�20 years based on a

global data (West & Post 2002); and an increase of

0.33 Mg C ha�1 yr�1 when conventional tillage is

converted to NT in 56 paired plots (Puget & Lal

2005). Olson (2010) also found 8.4 Mg ha�1 more

organic C retained in the NT than in the conven-

tional tillage after 20 years with a sequestration rate

of 0.48 Mg C ha�1 yr�1; however, he concluded

that these were relative amounts and that no C

sequestration actually occurred in the NT plots

because the C level decreased over time comparing

to the initial baseline value of C content, and the rate

of C decrease was greater in the conventional tilled

soil compared to the no-till soil. This relative gain in

C (8.4 Mg C ha�1) in NT compared with conven-

tional moldboard plough tillage over a 20-year

period was associated with the same amount (8.4

Mg C ha�1) of C loss in soil erosion from the

moldboard plough compared with the no-till soils.

Clearly, the initial C levels present at the time of each

experiment would need to be known in order to

estimate C sequestration.

The C loss associated with erosion contributes to

the soil atmospheric C balance. Significant amount of

SOC was found in depositional areas in the Dijle

catchment of Belgium (Van Oost et al. 2012) and in

different agroecosystems across Canada (VandenBy-

gaart et al. 2012) and these erosion-induced C

sinks were associated with significant erosion and

526 X. Yang et al.

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redistribution of SOC from eroded areas. Failure to

correctly assess the role of soil erosion in the carbon

cycle may lead to an unrealistic view of the potential

benefits of no-till farming (Van Oost et al. 2004). Soil

carbon has been found to be significantly greater in

riparian soils than in upland soils suggesting that

riparian system may capture the eroding soil and

associated C which moves with the water runoff

(Ritchie & McCarty 2003). Their study indicated

C storage in the agricultural riparian system was

4 times greater than that of upland soils (0�30 mm).

Following the argument that NT preserves soil

carbon in situ, an average annual C loss of 325 kg

C ha�1 yr�1 from farmland through soil erosion could

be estimated based on the average national erosion rate

of 13 Mg soil mass ha�1 yr�1 on agricultural land

(Crosson 1995; U.S.D.A. 2000) and the average soil

organic C content 25 g C kg�1 in the upper 5 cm soil in

the USA (Waltman et al. 2010). This number is

equivalent to the estimate of 330 Mg C ha�1 yr�1

carbon sequestration (Puget & Lal 2005) and could

offset a significant amount of C sequestration rates

projected by other models (West & Post 2002). The

annual C loss in fields under moldboard plough could

be greater than 325 kg ha�1 yr�1 because the soil

erosion rate from moldboard plough field is greater than

the average erosion rate (13 Mg ha�1 yr�1). Since the

eroded soil organic C from the moldboard plough soils

was trapped in sediments and could be a portion of the

‘‘C sink’’ in the global C budget (Smith et al. 2001), the

C sequestration in no-till soil relative to in moldboard

plough soil should not be simply calculated as the

apparent difference between two soils but should take

into consideration C stored in the eroded sediment

which may be at the bottom of the slope or in an

adjacent field.

No-till practices have been the greatest single

contributor to soil erosion control (Hatfield 2000);

hence, it is reasonable to believe that pioneering

tillage field trials were established on sites where soils

were most vulnerable to erosion. The successful

expansion of NT farming to encompass about

105 million ha of cropland worldwide (Derpsch &

Friedrich 2009), this technique has been applied in

areas where erosion is not problematic, including

areas with cold and wet climates with heavy textured

soils. NT practices may not perform well under these

soil conditions (Reynolds et al. 2003; Teasdale et al.

2007), and often has not increased C sequestration

relative to that observed under moldboard plough

practices (Angers et al. 1995; Hunt et al. 1996;

Yang & Wander 1999; Dolan et al. 2006; Blanco-

Canqui & Lal 2008; Novak et al. 2009). These

results may suggest that C sequestration in other

no-till managed soils may not be due to reduced

C mineralization but instead, are due to lower losses

of soil C associated with soil erosion. Differences

would be most pronounced in areas which are prone

to erosion. Studies in eastern Canada (Angers et al.

1995; VandenBygaart et al. 2003) clearly indicate

that potential to sequester C in cold, humid climates

may be limited, and the studies in the USA (Dolan

et al. 2006) indicate that moldboard tillage could

actually provide a means to sequester C in cooler

climates and with fine textured soils since inversion

of residues remained in the near surface to the depth

in cool, wet portions of the profile which could

inhibit decomposition. Maybe even intermittent

tillage and NT (say 5�10 years of NT followed by

single tillage events to place accumulated C in near

surface to depth where cooler and wetter) could

provide a means of sequestration (Bert VandenBy-

gaart, personal communications).

Concluding remarks

Although the impact of no-till practices on yields

compared with conventional tillage varies across soils

and climatic regions, NT has been widely adopted

because it reduces labor, fuel and machinery costs,

conserves water, and reduces soil erosion. This

unquestionably has contributed to soil quality and

agricultural sustainability. However, it may not be

appropriate to attribute all of the higher C content in

the surface of no-till soil to either increased C input

or reduced mineralization (output) relative to con-

ventionally tilled soils, particularly when the mold-

board plough is used as some of these effects may be

due to differences in soil erosion. It is also misleading

to use the C content in the conventional tilled soil as

the reference to assess the effect of NT on C

sequestration. It is far more appropriate to use the

initial C status of the soil before NTwas initiated and

to determine the fate of the eroded C from con-

ventionally tilled land to see if it is truly lost or just

re-deposited downslope or in riparian areas.

This does not mean that agricultural soils are no

longer an important C sink; instead, we need to look

beyond NT practice to fully address C sink issues.

Along with advances in field management and crop

variety improvement, crop yields are expected to

continue increasing which could also increase the C

input from crop residues to soils and have a positive

effect on soil C stocks (Berntsen et al. 2006). This

effect applies to all agricultural soils irrespective of

the tillage practices applied. Current estimates of the

C sequestration potential for agricultural soils may

be far less than actual values because the amount of

C buried in the landscape and/or deposited in

riparian areas from the tilled fields is currently not

being included. Basically, a landscape analysis of the

C in soil must take into consideration all inputs and

Acta Agriculturae Scandinavica, Section B - Soil & Plant Science 527

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outputs as some apparent loss from one agricultural

field may be a gain in another field. Including the

physical, chemical, and biological processes would

provide a more complete assessment which would

also include the fate of soil and carbon translocated

from tillage practices or transported through water or

wind erosion as well as the increased sedimentation

rates in the low-lying ecosystem.

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