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Agriculture, Ecosystems and Environment 139 (2010) 224–231

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journa l homepage: www.e lsev ier .com/ locate /agee

an no-tillage stimulate carbon sequestration in agricultural soils?meta-analysis of paired experiments

hongkui Luoa,b, Enli Wangb,∗, Osbert J. Sunc

State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, ChinaCSIRO Land and Water, Clunies Ross Street, Black Mountain, Canberra, ACT 2601, AustraliaMOE Key Laboratory for Silviculture and Conservation and College of Forest Science, Beijing Forestry University, Beijing 100083, China

r t i c l e i n f o

rticle history:eceived 12 July 2010eceived in revised form 3 August 2010ccepted 4 August 2010vailable online 9 September 2010

eywords:oil carbon changeonventional tillageonservation tillageoil profileropping systemertilizationlimate

a b s t r a c t

Adopting no-tillage in agro-ecosystems has been widely recommended as a means of enhancing carbon(C) sequestration in soils. However, study results are inconsistent and varying from significant increaseto significant decrease. It is unclear whether this variability is caused by environmental, or managementfactors or by sampling errors and analysis methodology. Using meta-analysis, we assessed the responseof soil organic carbon (SOC) to conversion of management practice from conventional tillage (CT) tono-tillage (NT) based on global data from 69 paired-experiments, where soil sampling extended deeperthan 40 cm. We found that cultivation of natural soils for more than 5 years, on average, resulted in soilC loss of more than 20 t ha−1, with no significant difference between CT and NT. Conversion from CT toNT changed distribution of C in the soil profile significantly, but did not increase the total SOC except indouble cropping systems. After adopting NT, soil C increased by 3.15 ± 2.42 t ha−1 (mean ± 95% confidenceinterval) in the surface 10 cm of soil, but declined by 3.30 ± 1.61 t ha−1 in the 20–40 cm soil layer. Overall,adopting NT did not enhance soil total C stock down to 40 cm. Increased number of crop species inrotation resulted in less C accumulation in the surface soil and greater C loss in deeper layer. Increasedcrop frequency seemed to have the opposite effect and significantly increased soil C by 11% in the 0–60 cm

soil. Neither mean annual temperature and mean annual rainfall nor nitrogen fertilization and durationof adopting NT affected the response of soil C stock to the adoption of NT. Our results highlight thatthe role of adopting NT in sequestrating C is greatly regulated by cropping systems. Increasing croppingfrequency might be a more efficient strategy to sequester C in agro-ecosystems. More information on theeffects of increasing crop species and frequency on soil C input and decomposition processes is needed

ing o

to further our understand

. Introduction

Soil cultivation for agricultural production is one of the mostotable land use change that has led to significant losses of car-on (C) from soil (Mann, 1986; Davidson and Ackerman, 1993; Guond Gifford, 2002). To offset or mitigate the stimulating effect ofemission on global warming, conservation agricultural practices

CAPs) are recommended to potentially increase C stock in agri-ultural soils (West and Post, 2002; Lal, 2004; Smith, 2004; Luot al., 2010). Globally, agricultural soils are estimated to potentially

equester 0.4–0.8 Pg C per year by adopting CAPs, which represents3.3–100% of the total potential of C sequestration in world soilsLal, 2004). Among all CAPs options, conversion from conventionalillage (CT) to no-tillage (NT) was considered to be one of the poten-

∗ Corresponding author. Tel.: +61 2 6246 5964; fax: +61 2 6246 5965.E-mail address: Enli.Wang@csiro.au (E. Wang).

167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.agee.2010.08.006

n the potential ability of C sequestration in agricultural soils.© 2010 Elsevier B.V. All rights reserved.

tially efficient strategies (Smith et al., 1998; Paustian et al., 2000;Six et al., 2004) with the rate of C sequestration of 100–1000 kg ha−1

per year (Lal, 2004). However, this view is based on the informationfrom most studies on carbon change in the surface soil (<30 cm),which ignores the possible management-induced redistributionof soil C in different soil depths. It may simply be an artifact ofsampling methodology, and has been challenged by several recentstudies that emphasized the changed soil C distribution under NTas compared with CT (Baker et al., 2007; Angers and Eriksen-Hamel,2008; Lal, 2009).

As the changes of agricultural practices, such as tillage and croptypes, influence C input (Lampurlanes and Cantero-Martinez, 2003;Martínez et al., 2008), C distribution and C decomposition processes

(Bearé et al., 1994; Six et al., 1999) in different soil layers, soil Cdynamics in all affected soil depth, rather than only the surface soil,should be investigated. Lal (2009) suggests that soil sampling depthshould be extended at least to 1 m for assessment of management-induced changes of soil C stock. Several studies have reported soil

Z.Luoet

al./Agriculture,Ecosystem

sand

Environment

139 (2010) 224–231225

Table 1Summary of data for the studies in the meta-analysis of soil organic carbon (SOC).

Code Location Duration(years)

Replicates Crop system Residuemanagements

Tillagedepth (cm)

N fertilizer(kg N ha−1)

Samplingdepth (cm)

Reference

1 Kanawha, IA, USA 7 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)2 Sutherland, IA, USA 7 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)3 Nashua, IA, USA 7 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)4 Armstrong, IA, USA 7 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)5 Crawfordsville, IA, USA 7 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)6 Ames, IA, USA 3 3 Corn, soybean Retained 25 135 60 Al-Kaisi et al. (2005)7 Selvanera, Lleida, Spain 18 3 Wheat, barley, rapeseed Retained 50 NA 40 Álvaro-Fuentes et al. (2008)8 Agramunt, Lleida, Spain 15 4 Wheat, barley Retained 30 NA 40 Álvaro-Fuentes et al. (2008)9 Penaflor, Zaragoza, Spain 16 3 Barley Retained 35 NA 40 Álvaro-Fuentes et al. (2008)

10 Penaflor, Zaragoza, Spain 16 3 Barley with fallow Retained 35 NA 40 Álvaro-Fuentes et al. (2008)11 Harrington, PEI, Canada 8 4 Wheat, barley, soybean Retained 20 NA 60 Angers et al. (1997)12 La Pocatière, Qué, Canada 6 4 Barley Removed 25 NA 60 Angers et al. (1997)13 Normandin, Qué, Canada 3 4 Barley Retained 25 NA 60 Angers et al. (1997)14 Ottawa, ONT, Canada 5 4 Corn Retained 25 NA 60 Angers et al. (1997)15 Ottawa, ONT, Canada 5 4 Wheat Retained 25 NA 60 Angers et al. (1997)16 Delhi, ONT, Canada 4 4 Corn Retained 25 NA 60 Angers et al. (1997)17 Harrow, ONT, Canada 11 2 Corn Retained 15 NA 60 Angers et al. (1997)18a Fremont, OH, USA 15 3 Corn, soybean Retained NA 225 60 Blanco-Canqui and Lal (2008)19a Troy, PA, USA 20 3 Corn NA NA 50b 60 Blanco-Canqui and Lal (2008)20 PEI, Canada 9 4 Wheat, barley, soybean Retained 25 NA 40 Carter (1996)21 PEI, Canada 15 4 Many Retained 20 73.7 60 Carter (2005)22a 111C, IN, USA 10 4 Corn, soybean Retained NA NA 60 Christopher et al. (2009)23a 114B, IN, USA 23 4 Corn, soybean Retained NA 108 60 Christopher et al. (2009)24a 122, IN, USA 10 4 Corn, soybean Retained NA NA 60 Christopher et al. (2009)25a 99, OH, USA 5 4 Corn, soybean, wheat Retained NA 20 60 Christopher et al. (2009)26a 111A, OH, USA 18 4 Corn, soybean Retained NA 76 60 Christopher et al. (2009)27a 111B, OH, USA 20 4 Corn, soybean, wheat Retained NA 101 60 Christopher et al. (2009)28a 111D, OH, USA 5 4 Corn, soybean Retained NA 44 60 Christopher et al. (2009)29a 126, OH, USA 15 4 Corn, soybean, rye Retained NA 29.9c 60 Christopher et al. (2009)30 Warwick, Queensland,

Australia13 4 Wheat, barley Retained 10 NA 120 Dalal (1989)

31 ONT, Canada 25 4 Corn, soybean Retained 18 NA 60 Deen and Kataki (2003)32 Rosemount, MN, USA 23 3 Corn, soybean Removed NA 0 45 Dolan et al. (2006)33 Rosemount, MN, USA 23 3 Corn, soybean Removed NA 200 45 Dolan et al. (2006)34 Rosemount, MN, USA 23 3 Corn, soybean Retained NA 0 45 Dolan et al. (2006)35 Rosemount, MN, USA 23 3 Corn, soybean Retained NA 200 45 Dolan et al. (2006)36 Luancheng, China 5 3 Wheat, cornd Retained 20 268 90 Dong et al. (2009)37 West Lafayette, IN, USA 27 4 Corn, soybean Retained 25 222 100 Gál et al. (2007)38 Lowveld, Zimbabwe 5 3 Wheat, cottond Retained 25 122 60 Gwenzi et al. (2009)39a Tänikon, Switzerland 19 4 Wheat, maize, canola Retained 25 150 40 Hermle et al. (2008)40 Waseca, MN, USA 14 4 Corn Retained 30 225 45 Huggins et al. (2007)41 Waseca, MN, USA 14 4 Soybean Retained 30 225 45 Huggins et al. (2007)42 Waseca, MN, USA 14 4 Corn, soybean Retained 30 225 45 Huggins et al. (2007)43 Narrabri, NSW, Australia 9 4 Cotton Retained 30 140 60 Hulugalle and Entwistle (1997)44a Narrabri, NSW, Australia 5 4 Cotton Retained 30 120 60 Hulugalle (2000)45a South Charleston, OH, USA 41 4 Corn NA NA NA 80 Jarecki and Lal (2005)46a Hoytville, OH, USA 16 3 Corn, soybean, oat NA NA NA 80 Jarecki and Lal (2005)47 Córdoba, Spain 6 4 Wheat Retained 30 100 90 López-Bellido et al. (1997)48 Londrina, Brazil 21 3 Manyd Retained 20 NA 40 Machado et al. (2003)49 Qué, Canada 13 4 Corn, soybean Retained 20 40 60 Poirier et al. (2009)50 Qué, Canada 12 4 Corn, soybean Retained 20 40 60 Poirier et al. (2009)51 Qué, Canada 11 4 Corn, soybean Retained 20 40 60 Poirier et al. (2009)52 Bushland, TX, USA 10 3 Wheat Retained NA 45 65 Potter et al. (1997)53 Bushland, TX, USA 10 3 Wheat Retained NA 0 65 Potter et al. (1997)

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Environment 139 (2010) 224–231

C changes after adopting NT based on the soil data sampled deeperthan 30 cm depth. Synthesizing the results from 47 experimentswith sampling depth deeper than 0.3 m, Angers and Eriksen-Hamel(2008) showed that NT led to significant C increase in surface soil,while full-inversion tillage (FIT) resulted in more C accumulationnear or at the bottom of the plow layer (23 cm). They also showedthat the greater SOC content at depth under FIT did not completelyoffset the gain under NT in the surface layer, leading to a highertotal C stock under NT than FIT. However, results from 12 pairedNT and CT experiments across three states in USA indicated that theoverall change of soil C stock in the surface 60 cm of soil ranged from22.8 to −20.3 t ha−1 after adopting NT for 5–23 years (Christopheret al., 2009). There is clearly inconsistency and uncertainty in theresults so far, which vary from significant increase to significantdecline (Sisti et al., 2004; Álvaro-Fuentes et al., 2008; Christopheret al., 2009). Further, other management practices, such as croppingsystem type, fertilization application and irrigation, interact withlocal soil type and climate conditions to impact on soil C. It is notclear whether the inconsistency in the current results is caused byenvironmental, or management factors or by sampling strategiesand analysis methodology.

Meta-analysis is a powerful tool to synthesize site-specific, tem-porally variable results and to draw general conclusions at regionaland global scales (Gurevitch and Hedges, 1999; Gurevitch et al.,2001). In this study, we conducted a meta-analysis of publisheddata on the responses of soil organic C to conversion from con-ventional tillage to no-tillage managements in 69 paired croplandexperiments. The objective was to clarify whether adoption of NTresulted in an increase in overall soil C stock, or only in changesin the C distribution in the soil profile. We also analyzed howlocal climate conditions (i.e., rainfall and temperature), durationof the NT application, N fertilization rate and the type of croppingsystems regulated the responses of soil C stock to the adoptionof NT.

2. Materials and methods

2.1. Data sources

We collected data from 69 paired CT and NT experiments inpeer-reviewed research papers that reported the change in soilorganic C (SOC) contents of different soil depth. Details of theselected studies and references are shown in Table 1. We used thefollowing criteria to select paired experiments:

(1) In some studies, there were several types of tillage treatments,in which cases we chose the tillage treatment that disturbedsoil the most (e.g., moldboard plowing) as CT.

(2) We specifically focused on studies involving soil samplingdeeper than 40 cm. In addition, only the studies that providedata to estimate soil C content on area basis were selected forcomparison purpose.

(3) Further, we only included the studies that had the similar soiltype, aspect, and managements (e.g., cropping system, fertiliza-tion, and irrigation) in both the CT and NT treatments. In eachindividual study the CT and NT treatments must be conductedat the same site with same experimental duration.

If a study reports soil C content in soils under adjacent natural veg-etation (natural soils), we also used it to compare the difference in

C stock change between cultivated (CT and NT) and natural soils.Other information included mean annual rainfall and mean annualtemperature at the site, duration of the experiments, type of crop-ping systems (number of crop types during the experiment, i.e.,crop diversity, and number of crops per year, i.e., crop frequency),

s and Environment 139 (2010) 224–231 227

rp

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B

wdm

C

2

tstmss

S

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td

M

T

S

wei

sit

M

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w

wm

D

we

Fig. 1. Mean difference (MD) of soil carbon contents at different soil depth in crop-land soils under conventional tillage (solid circles) and no-tillage (open circles) as

Z. Luo et al. / Agriculture, Ecosystem

esidue management (removed or retained), tillage depth, soil sam-ling depth, and N application rate.

In seven of the studies, only C concentration (Cc, %) was reported.n that case, soil C content (Ct, Mg ha−1) in corresponding soil layer

as calculated as:

t = BD × Cc × D, (1)

here BD is soil bulk density (Mg m−3), and D is the thickness ofhe soil layer (m). For all these seven studies, BD was not availablend was roughly estimated according to (Adams, 1973):

D = 100(COM/0.244) + ((100 − COM)/1.64)

, (2)

here 0.244 is the bulk density of soil organic matter, 1.64 the bulkensity of soil mineral matter, and COM is the content of soil organicatter (%), and was estimated as:

OM = 1.72 × Cc. (3)

.2. Data analysis

Meta-analysis was used to determine changes in soil C con-ent after adoption of NT at different soil layers. Because differentampling layers were used in the collated experiments, we haveo define soil layers based on the most sampled depths for the

eta-analysis. We extracted the mean, standard deviation (SD) andample size (n) of soil C contents in each experiment. If only thetandard errors (SE) were given, we converted them to SD by:

D = SE√

n. (4)

here we could not calculate SD, we reassigned the SD as 1/10 ofhe mean (Luo et al., 2006).

For each soil layer in the studies, the mean of soil C content underhe CT (xCT) and NT (xNT) treatments were used to calculate a meanifference (MD) in soil C contents between the treatments:

D = xNT − xCT. (5)

he standard deviation of each MD (SDmean) was calculated as:

Dmean =√

(nNT − 1) × SD2NT + (nCT − 1) × SD2

CTnNT + nCT − 2

, (6)

here nNT and nCT are the sample size of CT and NT experiments inach study, SDNT and SDCT the standard deviation of soil C contentn CT and NT experiments, respectively.

In all studies, the results with smaller SD are more reliable andhould carry more weight in the estimation of the mean MD, annverse variance weighted mean MD (MD) was calculated accordingo the method of Gurevitch et al. (2001):

D =∑k

i=1(wi × MDi)∑ki=1wi

, (7)

here MDi is the MD in the ith individual study (i = 1, 2, 3, . . ., k), wi

s the weighting factor calculated using the DerSimonian and Lairdethod (DerSimonian and Laird, 1986):

i = 1

SD2mean + D

, (8)

here D is the variance induced by random effects. In fixed effectsodel, D equals to 0. In random effects model, D is calculated as:⎧⎨ ⎫⎬

= max⎩ Q − (k − 1)∑k

i=1(w∗i) −

(∑ki=1(w∗

i)2/

∑ki=1(w∗

i)) , 0⎭ , (9)

here w∗i

is the weighting factor on the assumption that the fixedffects model is suitable and Q is the statistical value examining the

compared with in adjacent natural soils. Horizontal bars show the 95% confidenceinterval; numbers of observations are the same for the tillage treatments and givenin parenthesis.

heterogeneity of variance. A MD of 0 indicates that the adoptionof NT had no effect on SOC as compared with CT; a positive MDindicates that NT leads to soil C accumulation, while a negative MDindicates that NT leads to soil C loss.

The standard deviation of MD was calculated as:

SDMD

=√

1∑ki=1wi

. (10)

Then, the 95% confidence interval (CI) for the MD was given as:

CI = MD ± 1.96 × SDMD

. (11)

If the 95% CI of MD in each soil layer does not overlap with 0, thechange of soil C content after adoption of NT is then consideredsignificant (P < 0.05).

The same calculation process was applied to compare the dif-ferences in soil C content between cultivated soils and adjacentnatural soils (Fig. 1). The random effects model of meta-analysiswas conducted using MIX software (http://www.mix-for-meta-analysis.info; Bax et al., 2006).

To analyze the responses of soil C in different soil depths to adop-tion of NT, the cumulative soil C contents (i.e., total soil carbon downto a certain depth) in both the NT and CT treatments down to a spe-cific soil depth (from all the experiments) were calculated, whichenables the derivation of relative change (soil C contents in NT rel-ative to that in CT) in total soil C contents down to specific depthfor each experiment (Fig. 3A). We then separated the experimentsinto 6 groups based on the deepest sampling depth, i.e., up to 40,50, 80, 100 and 120 cm, and calculated the average relative change

in total soil C for all sampling depths in each group (Fig. 3B). Regres-sion analysis was applied to examine the relationship between therelative changes of cumulative soil C content and soil depth usingall the data points in Fig. 3B.

228 Z. Luo et al. / Agriculture, Ecosystems and Environment 139 (2010) 224–231

Fav

amfs(mcaccss

smac

3

sa5ciNia

tNi0t

Fig. 3. The relative change of cumulative soil C content with soil depth after the con-version from conventional tillage to no-tillage. (A) summarizes all samples; numbersrefer to code in Table 1. (B) shows the average cumulative change in soil C to the

ig. 2. Mean difference (MD) of carbon contents of soils under conventional tillagend no-tillage. Horizontal bars show the 95% confidence interval; numbers of obser-ations are shown in the parenthesis.

To analyze the effects of cropping systems on soil C dynamicsfter the conversion from CT to NT, we further grouped the experi-ents into single and double cropping systems (referred to as crop

requency), and separated the experiments with single croppingystems into three sub-groups based on the number of crop typescrop diversity) involved in the experiment. The studies that had

ore than three crop types are grouped into the sub-group of threerop types. For each group and sub-group, the MD was calculatedt each soil layer down to 60 cm (Fig. 4A and B), and the relativehange of cumulative soil C down to a specific soil depth was alsoalculated (Fig. 4C and D). t-Tests were applied to accumulativeoil C change at 60 cm depth to see whether the relative change isignificantly different from zero.

We also applied regression analysis to examine the relation-hip of the relative change of cumulative soil C down to 60 cm withean annual rainfall, mean annual temperature, N application rate,

nd the duration of experiments. All the statistical analysis wasonducted using R 2.8.0 (R Development Core Team, 2008).

. Results

Soil C content under cropping systems with both CT and NTignificantly (P < 0.05) declined in comparison with that under thedjacent natural systems (Fig. 1). The decline was highest in the topcm of the soil, and was higher under CT (10.19 ± 3.50 t ha−1) asompared with NT (7.51 ± 3.03 t ha−1). The decline decreased withncreasing soil depth and negligible at 60 cm depth for both CT andT. After more than 5 years of cultivation (Table 1), the total decline

n soil C to 60 cm depth in the CT and NT soils were comparable,nd were 22.85 t ha−1 and 21.31 t ha−1, respectively.

Conversion from CT to NT had a significant impact on the dis-

ribution of soil C with soil depth (P < 0.05; Fig. 2). Compared to CT,T led to an increase of 3.15 ± 2.42 t ha−1 (MD ± 95% CI) in soil C

n the surface 10 cm of soil, but a decline of 2.40 ± 1.05 t ha−1 and.90 ± 0.60 t ha−1 in the 20–30 and 30–40 cm soil layers, respec-ively. Below 40 cm, there was no significant difference in soil C

specified depth. In (B), different symbols show the group of data sampled to thesame soil depth. Sampling depths were 45, 65 and 75 cm and were grouped intothe 50, 60 and 80 cm, respectively. The solid fitted line (B) shows the relationshipbetween the average relative cumulative change and soil depth.

content between the CT and NT treatments. However, there waslarge variability in the results among different studies (Fig. 3A).Considering the change of total soil C content down to a specificsoil depth, an exponential decay function well fitted the changesof cumulative soil C content with increasing soil depth (Fig. 3B).When all data were pooled together, on average, adoption of NTincreased soil C accumulation in the surface 30 cm of soil. Whendeeper soil layers (>40 cm depth) were included, the total soil Ccontent kept almost stable, with an insignificant increase of ∼2.8%(P = 0.094 for the constant value 0.028 when fitting the exponentialdecay function, Fig. 3B).

The types of cropping systems affected the impacts of NT onsoil C change with depth (Fig. 4). NT with double cropping systemscaused more C accumulation in the surface 0–20 cm soil layers andless C decline in the 20–30 cm soil layer (Fig. 4A). When the totalsoil C to 60 cm depth is considered, NT led to significant increase insoil C (10.94%) only when double cropping was practiced (Fig. 4C).In all the single cropping systems, with increasing number of cropspecies in rotation, NT led to less increase in soil C in the surface10 cm of soil and more decline in soil C at the 20–30 cm depth. As aresult, net change of total soil C stock would change from positiveto negative with increasing depth to 60 cm (Fig. 4D). For all thesingle cropping systems, the relative change of total soil C to the60 cm depth (4.69%, 3.20% and −2.94% for monoculture, two andthree crop types respectively) did not significantly differ from zero(Fig. 4D).

After conversion from CT to NT, no apparent relationships werefound between changes of total soil C content to 60 cm soil andclimatic condition (i.e., rainfall, temperature), duration of the adop-tion of NT (≥3 years), and the N fertilization rate (Fig. 5).

4. Discussion

Cultivation of natural ecosystems has led to loss of soil organiccarbon (SOC), and adoption of no-tillage has been suggested to bean effective way to increase soil C stock (Lal, 2004; Smith, 2004).Our meta-analysis of global data from 69 paired experiments indi-

cates that cultivation for more than 5 years, on average, results inSOC loss of more than 20 t ha−1 in the top 60 cm soil profile, withno significant differences between CT and NT (Fig. 1). Conversionfrom CT to NT significantly altered the vertical distribution of soilC in the soil profile, resulting in increased soil C in the 0–10 cm

Z. Luo et al. / Agriculture, Ecosystems and

Fig. 4. Upper panels: The effects of crop frequency (A) and crop diversity (B) on themean difference (MD) of soil C stock between NT and CT croplands in 0–5, 5–10,20–30, 30–40, 40–50 and 50–60 cm depth intervals based on available data (Note:there are not enough data in the last two specific depth intervals in monoculture,three types, and double cropping systems). The results were shown in 40–60 soilprofile if possible. Horizontal bars show the upper envelope of the 95% confidenceinterval if the mean value is greater than 0, and the lower envelope if the mean valueis less than 0. All the studies that had more than three crop types are included inthe calculation of the MD of rotation with three types of crops. Lower panels: Theeffects of crop frequency (C) and crop diversity (D) on the averaged relative changeoad

swaopbdmc(tsicdiCcW

increasing litter diversity (thus the diversity of C substrate) could

f cumulative soil C stock down to different soil depth (0–5, 0–10, 0–20, 0–30, 0–40nd 0–60 cm) after the conversion from conventional tillage to no-tillage. Samplingepths of 45, 50, 60 and 65 cm were grouped into 0–60 cm.

oil and a decline in soil C in the 10–40 cm soil. This is consistentith the findings of Angers and Eriksen-Hamel (2008). Our results

lso showed that conversion from CT to NT did not increase theverall SOC stock in most cases, except for those with double crop-ing systems (two crops per year). These results were obtainedy taking into account the uncertainty caused by variations in theata from the cited experiments using the standard meta-analysisethod. The method used and the results on overall change in soil C

aused by NT are different from those of Angers and Eriksen-Hamel2008). The finding confirms the concerns of Baker et al. (2007)hat “the widespread belief that conservation tillage favors carbonequestration may simply be an artifact of sampling methodology”n many cases. Our results further clarify the differences in soil Change caused by NT as influenced by cropping frequency and cropiversity, which may be one of the causes for the inconsistency

n previous results (Sisti et al., 2004; Álvaro-Fuentes et al., 2008;hristopher et al., 2009). In all the single cropping systems (onerop per year), crop types affected the vertical distribution of soil C.

ith more diverse crop types in the rotation, conversion from CT to

Environment 139 (2010) 224–231 229

NT led to less C increase in surface soil and a larger C decrease in the10–40 cm soil, and consequently a net decline in total soil C ratherthan an increase. The pattern of change in soil C after conversionto NT did not seem to be related to climate (i.e., temperature andrainfall) or nitrogen application rate. No consistent trend of changewas found with the duration of NT practice (since the conversion).

Tillage-induced change in soil C distribution with soil depth islikely a result of two causes: redistribution of surface soil C andchanged root growth. Firstly, the surface soil layer has most ofthe C, and plowing in conventional tillage moves the crop residueand surface soil C into deeper soil layers. Secondly, plowing alsoloosens the soil down to the depth of 15–35 cm (sometimes down to50 cm, Table 1), changes the soil physical conditions, and promotesmore crop root growth in those loose soil layers thereby increas-ing C input through root senescence at corresponding soil layer.Conversion to no-tillage leads to increased soil cover, reduced soildisturbance and increased soil strength. It does not only discourageroot growth into deeper soil layers (Lampurlanes and Cantero-Martinez, 2003; Qin et al., 2004; Martínez et al., 2008), but alsoreduces the downwards movement of surface soil C. In addition,residues on the ground surface under NT can cause decrease in soiltemperature in the top soil in summer, therefore lead to reducedsoil C decomposition (Duiker and Lal, 2000). It can also increasemoisture through reduced evaporation in the top soil, leading tochanges in crop root growth and other soil processes related toSOC decomposition in the top soil layer.

Although tillage may increase residue incorporation into soiland move surface soil C into deeper layers, it is also consideredto stimulate the oxidation of SOC. There are two explanations forthe latter: (1) tillage fragments macro-aggregates and increases thesurface area for soil microbes to attack and decompose the origi-nally physically aggregate-protected soil C (Bearé et al., 1994; Sixet al., 1999), (2) incorporated crop residues lead to more favorablephysical conditions (e.g., water and thermal condition) for theirdecomposition in soils than on the soil surface (Aulakh et al., 1991;Coppens et al., 2007), and provides nutrients and energy for micro-bial growth and therefore further enhances decomposition of soil C,including inert C (Fontaine et al., 2007). These two opposing effectsof residue incorporation (i.e., C input increase vs. the stimulation ofdecomposition) may in part counteract each other. Hence, the netoverall effect of tillage on soil C stocks may remain moderate and beregulated by crop systems that determine the quantity and qualityof crop residues (as carbon input into the soil), and by soil condi-tions that determine the decomposition process of the incorporatedcrop residues.

The change in the vertical distribution of soil C caused by tillagefurther highlights the need for deeper soil sampling in soil C study(Baker et al., 2007; Angers and Eriksen-Hamel, 2008). Many agri-cultural plants can grow their roots down to 100–150 cm or evendeeper and root biomass constitutes an important part of soil C. Lal(2009) suggests that it is important to extend soil sampling depthat least down to 1 m. Our analysis indicates that sampling needs tobe done at least down to 0.4 m in order to account for the tillage-induced soil C change. For other types of studies, such as thoseinvolving impact of plant types on soil C, sampling down to therooting depth of the target plants may be needed to avoid bias inresults.

The effects of crop diversity (increased number of crop species)on soil C change after adopting NT may contribute to the variabil-ity in the quantity and quality of aboveground crop residues androot growth and distribution in the soil profile. Studies showed that

increase soil microbial biomass and decomposition rate (Bardgettand Shine, 1999; Gartner and Cardon, 2005), which may helpexplain the net decline in soil C in CT treatments with more diversecrop types after conversion to NT (Fig. 4D). Increase in cropping

230 Z. Luo et al. / Agriculture, Ecosystems and Environment 139 (2010) 224–231

F he top( -tillag

fls2occepCa

A

CA

R

A

A

Á

A

A

A

ig. 5. The relationship between the relative changes of cumulative soil C stock in tA) mean annual rainfall, (B) mean annual temperature, (C) duration of adopting no

requency, such as double cropping per year (Fig. 4A and C), canead to more annual overall production of residues and roots thaningle cropping, thereby increasing soil C stock (West and Post,002; Luo et al., 2010). Further, the increased residue cover can notnly reduce soil erosion, but also affect soil structure and nutrientycling. The complicated patterns of soil C change under differentropping systems highlight that cropping systems must be consid-red when assessing soil C dynamics under various managementractices. The underlying mechanism causing the variation in soilchange under different cropping systems needs to be further

ddressed with more detailed data.

cknowledgement

The funding support from the Joint PhD Program under theSIRO-MOE (Ministry of Education, China) Scientific Exchangegreement is greatly acknowledged.

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