Agriculture, Ecosystems and Environment 133 (2009) 114–122

Soil carbon sequestration and stratification in a cereal/leguminous crop rotationwith three tillage systems in semiarid conditions

J.L. Hernanz a,*, V. Sanchez-Giron b, L. Navarrete c

a Departamento de Ingenierıa Forestal, ETSI Montes, Universidad Politecnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spainb Departamento de Ingenierıa Rural, ETSI Agronomos, Universidad Politecnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spainc Instituto Madrileno de Investigacion y Desarrollo Agrario, Finca El Encıın, Alcala de Henares, P.O. Box 127, Madrid, Spain

A R T I C L E I N F O

Article history:

Received 24 March 2009

Received in revised form 11 May 2009

Accepted 12 May 2009

Available online 16 June 2009

Keywords:

Long-term effect

Tillage system

Soil organic carbon

Stratification ratio

Semiarid condition

A B S T R A C T

The stratification of soil organic matter at different depths is common under conservation tillage and

especially under no-tillage. The degree of stratification, or stratification ratio (SR), can be used as an

indicator of soil quality because surface organic matter is essential to erosion control, water infiltration,

and the conservation of nutrients. In the semiarid regions of the Mediterranean which are devoted to

rain-fed crop production, soil has low organic carbon content because of the high mineralization rates of

soil organic matter and the lack of crop residue after periods of drought. Twenty-year effects (1985–

2006) of tillage systems on soil organic carbon (SOC) stratification were studied in a Vertic Luvisol with a

loam texture. SOC was expressed in carbon concentration (Cc) and in equivalent soil mass (esm). The

tillage treatments used were conventional tillage (CT), minimum tillage (MT), and no-tillage (NT). These

treatments were under winter wheat (Triticum aestivum L.), vetch (Vicia sativa L.) and pea (Pisum sativum

L.) rotation (W-VP). Similarly, several stratification ratios (SRs) were assessed as indicators of SOC’s time

evolution. Before the start of the experiment the crop rotation was cereal-fallow (C-F). In each treatment

and replication four soil layers of the same thickness (10 cm) were sampled to obtain soil bulk density

(BD) and Cc. After 20 years the study revealed that the adoption of a W-VP rotation was at least as

important as the shift from CT to NT in the increase of stocked SOC in the soil profile. This last treatment

was the tillage system with the highest SOC, whereas no significant differences were encountered

between MT and CT. The average SOC was 14% higher in NT than in MT and CT. This trend has been

systematically observed practically since 1996 to the present in all treatments. The steady state of SOC

sequestration was reached after 11 years of starting the experiment in NT and 12 years in CT and MT.

SOC, expressed as Cc and esm, showed the highest stratification in NT, second highest in MT and lowest

in CT. In NT, stocked SOC increased from 1996 to 2005 in the top layer but it declined systematically in

the bottom layer.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

For different reasons, conservation tillage systems possessattributes that have never ceased to enlighten during the last 60years. Smart and Bradford (1999), for example, stated that windand water erosion are reduced, and water content in the soil profileis increased, if mulch is maintained on the soil surface. Thepresence of mulch allows the soil to retain water and it insulatesthe soil surface against temperature extremes. Mulch alsoalleviates soil compaction and there is typically an increase ininfiltration rates that result in decreased runoff containingfertilizers, herbicides and pesticides. Conservation tillage improveslabour efficiency in comparison with conventional tillage because

* Corresponding author. Tel.: +34 91 336 71 20; fax: +34 91 543 95 57.

E-mail address: joseluis.hernanz@upm.es (J.L. Hernanz).

0167-8809/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.05.009

it reduces the number of field operations to prepare the seedbed.The reduction of field operations saves costs related to fuel, labour,and farm equipment (Hernanz et al., 1995; West and Marland,2002; Lal, 2004a; Zentner et al., 2004). In practically all theseattributes the carbon (C) element, either indirectly or directly,plays a relevant role. It constitutes an essential part of the cropresidues that protect the top soil against erosive agents. Thisprotection has enhanced the shift from conventional tillage toconservation tillage since World War II (Harrold and Edwards,1972; Phillips and Phillips, 1984; Griffith et al., 1986; Smika andUnger, 1986; Unger, 1994; Holland, 2004).

In the 1990s the term ‘sustainable agriculture’ rose insignificance out of the many techniques used for crop production.During that time soil quality indicators were established; amongthem, researchers consider soil organic matter as the most crucial(Cannell and Hawes, 1994; Larson and Pierce, 1994; Arshad andMartin, 2002; Nortcliff, 2002; Shukla et al., 2006).

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122 115

Since the European Union initiated the common agriculturepolicy in 1992; European farmers, even those practicing dry-landfarming in semiarid conditions, have been concerned aboutreducing their production costs of fuel, machinery, fertilizersand pesticides (Crosson et al., 1986; Zentner et al., 2002; Sanchez-Giron et al., 2004, 2007; Nail et al., 2007). The Kyoto Protocol,signed by nearly all the countries in the world, states an agreementbetween the countries to reduce CO2 emissions and othergreenhouse gasses (GHGs). Compared to traditional tillagetechniques, conservation tillage can play a key role in decreasingCO2 emissions and in increasing the pool of C sequestered in thesoil (Deen and Kataki, 2003; Freibauer et al., 2004; Lal, 2004b,c;Baker et al., 2007).

Nowadays biomass and bio-fuel, obtained from the drymatter of the so-called energetic crops, are common alternativesto non-renewable energy sources. However, it is very importantto take into consideration soil sustainability, particularly whencrop residues are used (Lal and Bruce, 1999; Andrews, 2006).Reicosky (2005) made reference to this when he said, ‘‘Carbon isthe C that starts Conservation’’. Three main strategies for soilorganic carbon (SOC) sequestration in the biosphere are: (i)enhancing the re-growth of perennial vegetation throughconversion of cropland to grassland or forestland; (ii) increasingthe net primary productivity through intensification of agricul-tural inputs: fertilizer, irrigation and manuring, which wouldincrease the amount of residue returned to the soil; and (iii)adopting conservation tillage, especially no-tillage (Puget andLal, 2005).

In some semiarid cropping areas of the Mediterranean basinwhich are devoted to rain-fed crop production, rainfall is themain restrictive factor to high production. Rainfall is erratic,scarce, and can cause severe erosion in agricultural soils duringperiods of intensive storms (ECAF, 1999; Mrabet et al., 2001).Relative to SOC, the climatic consequences for these rain-fedcrops are: (i) high mineralization rates of SOM enhanced by hightemperatures during wet periods; (ii) lack of residues to coverthe soil surface after periods of drought which affect cropproduction; (iii) some tillage practices are performed immedi-ately after harvesting to bury residues by disking, and even topartially prepare the seedbed in dry conditions; (iv) manyfarmers still practice bare fallowing for 1 year which decreaseslevels of SOC; and (v) cereal crop residues are currently baledafter harvesting (Lopez-Fando and Almendros, 1995; Lopez andArrue, 1997; Hernanz et al., 2002; Sanchez-Giron et al., 2004;Ozpinar, 2006; Madejon et al., 2007; Munoz et al., 2007; De Vitaet al., 2007). In a long-term perspective, (>10 years) SOC storagein the soil may increase by implementing conservation tillagepractices such as no-tillage (Kern and Johnson, 1993; Paustianet al., 1997; Reeves, 1997; West and Post, 2002; Franzluebbers,2005; Johnson et al., 2005; Liebig et al., 2005; Martens et al.,2005).

Changing from conventional tillage (CT) to no-tillage (NT)affects SOC dynamics due to soil type, cropping systems, residuemanagement, and climate (Franzluebbers et al., 1998; Dou et al.,2007). Based on regression analyses, West and Post (2002)reported that within 15–20 years, C sequestration rates can beexpected to have a delayed response, reach peak sequestrationrates in 5–10 years, and decline to close to a rate of zero. Kern andJohnson (1993) concluded that changing from CT to NT sequestersthe greatest amount of C in the top 8 cm of the soil, a lesser amountat the depth of 8–15 cm, and no significant amount below 15 cm.However, in some cases no significant differences were encoun-tered between CT, MT and NT in relation to the SOC stored either inthe top soil layer or in deeper layers after several years ofexperimentation following different crop rotations such as springwheat-fallow rotation, corn-soybean rotation and soybean-barley

rotation (Lopez-Bellido et al., 1997; Halvorson et al., 2002; Al-Kaisiet al., 2005).

Sequestration duration can be separated into two compo-nents: flow duration and stock duration. Flow durationrepresents the time period of active sequestration, or the timein which annual changes in soil C are occurring. Stock durationrepresents the time period following active sequestration, andcan be defined as the period of passive sequestration, or the timein which previously sequestered C remains sequestered (Westand Six, 2007).

The stratification of soil organic matter at different depths iscommon in many natural ecosystems and managed grasslands andforests, as reflected by horizon distribution, as well as whendegraded cropland is restored with conservation tillage (Fran-zluebbers, 2002). The surface of the soil is the vital interface thatreceives much of the fertilizers and pesticides applied to thecropland; it receives the intense impact of rainfall, and partitionsthe flux of gases in and out of the soil. For these reasons the degreeof stratification, or stratification ratio (SR), can be used as anindicator of soil quality, because surface organic matter is essentialto erosion control, water infiltration, and conservation of nutrients(Franzluebbers, 2002).

The main objectives of this work were: (i) to evaluate SOCvariations in three tillage systems over a period of 20 years,starting in 1985, in a wheat-legume rotation for forage (vetch andpeas); and (ii) to assess and compare several stratification ratios asindicators of the time evolution of SOC.

2. Materials and methods

2.1. Site description and treatment

A 20 year (1985–2005) tillage experiment was conducted at ElEncın Experimental Station, located in Alcala de Henares (Latitude,408 290N; longitude, 38 220W), Madrid, Spain, which belongs toInstituto Madrileno de Investigacion y Desarrollo Regional Agrario(IMIDRA). The soil is a Vertic Luvisol with a loam texture, and in the0–60 cm depth it contains 450 g kg�1 sand (2–0.05 mm), 340 g kg�1

silt (0.05–0.002 mm) and 210 g kg�1 clay (<0.002 mm). The site is610 m above sea level with an average annual temperature of13.1 8C. The annual precipitation averages 430 mm, with a max-imum precipitation of 670 mm and a minimum of 230 mm. Table 1contains air temperatures, monthly average precipitation andmonthly average potential evapotranspiration (1957–2000) at ElEncın Experimental Station. The aridity index, i.e. the ratio of annualmean precipitation annual mean evapotranspiration, is 0.56reflecting a semiarid climate.

Prior to the establishment of the experiment, CT was appliedto the field following a cereal fallow (C-F) rotation. Theexperiment was started in 1985 following a winter wheat(cultivar ‘Talento’)–vetch (cultivar ‘Senda’) rotation (W-VP).Since 1995, the winter wheat cultivars ‘Albares’ and ‘Marius’have been sown instead of the winter wheat cultivar ‘Talento’.Since 2002, forage pea (cultivar ‘Gracia’) was substituted forvetch. Three tillage systems were compared: conventionaltillage (CT), minimum tillage (MT), and no-tillage (NT). CTconsisted of fall ploughing to an average depth of 25 cm,followed by one or two passes with a spring tine cultivator (10–15 cm depth) for seedbed preparation. Drilling was performedwith a reversible tine opener conventional drill (trade mark: Gil-GT with 3.0 m working width). MT substituted chisel ploughing(15–20 cm average depth) for mouldboard ploughing, whileseedbed preparation and drilling were carried out according toCT. In the NT treatment, sowing was performed with an invertedT furrow-opener no-till drill (trade mark: Gil-SNL with 3.0 mworking width). In all treatments wheat crop residues were

Table 1Average monthly air temperatures, precipitation and potential evapotranspiration

(PET) (1957–2000) at El Encın Experimental Station (IMIDRA) (CAM, 2000).

Month temperature (8C) Rainfall (mm) PET (mm)

Minimum Maximum

January 0.0 10.5 42.7 11.6

February 0.8 12.6 39.2 16.5

March 2.4 16.0 25.5 31.3

April 4.4 18.0 45.7 44.0

May 7.8 22.6 50.6 76.5

June 11.7 27.8 29.2 112.1

July 14.4 32.3 14.0 145.7

August 13.9 31.8 13.3 130.8

September 11.3 27.1 30.3 88.5

October 7.3 20.4 49.9 52.2

November 3.0 14.2 48.9 22.7

December 0.8 10.6 40.4 13

Average 6.5 20.3

Total 429.7 744.9

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122116

baled until 1990; after which, they were chopped and spreadon the soil surface. Tillage treatments were applied in arandomized block design with four replications and plot sizeof 20 m � 30 m.

Average sowing rates were 160 kg ha�1 for winter wheat and100 kg ha�1 for vetch and pea. Crop fertilization was accomplishedin the same way for all treatments. From 1986 to 1995 the averagerates applied were, 68 kg N ha�1, 21 kg P ha�1 and 13 kg K ha�1 forwheat. On the other hand, vetch crop was fertilised with24 kg N ha�1, 31 kg P ha�1 and 20 kg K ha�1. In 1992–1993, thewinter wheat was not fertilised prior to sowing because of lowproduction of the previous crop. Since 1996 average rates were85 kg N ha�1, 12 kg P ha�1 and 22 kg K ha�1 for winter wheat and30 kg N ha�1, 14 kg P ha�1 and 25 kg K ha�1 for vetch and pea. Inthe NT system, weeds and volunteers were sprayed with0.72 kg ha�1 glyphosate. All cereal plots were sprayed with apost-emergence application at an average rate of 3 L ha�1 ofioxynil + mecoprop + bromoxynil (75 + 375 + 75 g a.i. L�1).

2.2. Soil sampling

Once harvesting was completed and before tilling in autumn,SOC was assessed, following the procedure of Walkley andBlack (Allison, 1965), collecting four soil samples from eachtreatment and replication (four blocks) at 10 cm depthintervals (0–10, 10–20, 20–30, and 30–40 cm) in the years1991, 1996, 1998, 2000, 2002–2005. All four samples weremixed to produce a composite sample from each treatment,layer and block.

To obtain the SOC stocks, soil bulk density was measured, at thesame depth intervals and years as for SOC assessing, by coresampling prior to tillage in autumn. Three replications wereconsidered in each treatment, depth and block. However, in 1985before the experiment was initiated two soil bulk densityreplicates were measured in each block at each of the formerdepth intervals. Initial SOC conditions were determined in each ofthe two core sample replicates extracted for bulk densitymeasurement.

When the cereal crop was mature in early July, two swaths perplot, 1.40 m wide and 30 m long, were combine-harvested in eachplot. The vetch and pea crops were harvested in May when theywere flowering. Forage was estimated by hand-mowing fourdelimited areas in each plot, size 0.5 m2. Likewise estimates werederived from weighing the above-ground material after beingdried at 65 8C for 24 h.

2.3. Variables analyzed

The variables analyzed were soil organic carbon concentration,(Cc) (kg C Mg�1); soil bulk density (BD) (Mg m�3); and SOC stocks(Mg ha�1), both for each layer and the whole amount in the overalllayers. This latter was calculated in terms of equivalent soil mass(esm) following the procedure of Ellert and Bettany (1995). In thisstudy esm was the mass of the heaviest soil layer which was mostsusceptible to the influence of management. For comparisons ofSOC the standard reference soil mass used was that of the heaviestsoil layer analyzed (1670 Mg ha�1 for NT at 30–40 cm in 2000), andthe mass of SOC was calculated for layers L1 (top)–L4 (bottom),each of them having different depths but the same esm. Thestratification ratio (SR) is defined as the ratio between a soilproperty measured at the soil surface and the same propertymeasured at a lower depth (Franzluebbers, 2002). Three SRs weredefined concerning the sampling layers:

- SRCc(1/4), i.e. the ratio of Cc in the 0–10 cm layer and in the 30–40 cm layer.

- SResm(1/4), i.e. the ratio of esm stocked SOC in layer L1 and esmstocked SOC in layer L4.

- SRALesm(1), i.e. the ratio of esm stocked SOC in layer L1 and esmstocked SOC averaged across the four layers considered.

2.4. Statistical design

The statistical design was a split-plot in time and space for BD,Cc and SOC stocks with tillage systems as main effects, samplingyears and soil layers as sub-treatments. Overall SOC and thedifferent SRs considered were assessed in a split-plot in time withtillage systems as the main effect and sampling years as sub-treatment.

Differences between means were assessed by a least significantdifference (LSD) test (P < 0.01). All statistical analyses wereperformed using STATISTICA software (Statsoft, 2003).

3. Results and discussion

3.1. Soil bulk density

Soil bulk density data can be seen in Fig. 1. In the 9 yearsstudied, significant differences (P < 0.01) were encounteredbetween the means of tillage systems, where NT > MT > CT.When the tillage by depth interaction was analyzed, the highestdifferences between treatment means appeared in the top layer(0–10 cm). In the 10–20 cm layer CT was significantly lower thanMT and NT. However, no significant differences appeared amongthe two latter tillage systems, although CT was lower than MT andNT in 20–30 cm layer.

Significant differences between BD values at the 0–10 cm, 10–20 cm and 20–30 cm layers were observed in CT; however, BD atthe 30–40 cm layer was not significantly different to that in the20–30 cm layer. A similar trend was observed for MT where BD at10–20 cm was different to that at 0–10 cm and 30–40 cm. Withreference to NT top layer was statistically different in relation tobottom layer.

Treatment by year interactions were not significantly differentin most years. BD values ranged from 1.47 Mg m�3 to 1.57 Mg m�3

for CT; from 1.50 Mg m�3 to 1.58 Mg m�3 for MT; and from1.51 Mg m�3 to 1.59 Mg m�3 for NT. There are many studies in theliterature that report higher BD values in NT than in CT, not only inthe upper most soil layers but in the whole soil profile as well (0–30 cm) (Schjonning and Rasmussen, 2000; Van den Bygaart et al.,1999; Lampurlanes and Cantero-Martınez, 2003; Hao et al., 2000;Bescansa et al., 2006; Gal et al., 2007).

Fig. 1. Average values of the treatment � depth interaction and treatment means

for soil bulk density. (a) Means followed by the same lower case letter are not

significantly different between tillage treatments at the same depth (p < 0.01). (b)

Means followed by the same upper case letter are not significantly different

between depths for the same treatment (p < 0.01). CT, conventional tillage; MT,

minimum tillage; NT, no-tillage.

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122 117

Our results are consistent with those of Franzluebbers et al.(1995) who reported no significant differences in BD at a soil depthof 12.5–20 cm after chisel ploughing to 25 cm in comparison withNT in a silty clay loam soil. Dam et al. (2005) observed in a loamysand soil that bulk density was affected by tillage practices, butonly within the first 10 cm where CT and MT reduced bulk densityrelative to NT. For these authors, residues did not affect bulkdensity because the latter did not exhibit any temporal trends in an11-year experiment. In a silty loam soil, Yang and Wander (1999)

Table 2Soil organic carbon concentrations (Cc) (g C kg�1) in each tillage treatment compared a

Year Depth (cm) CT MT NT

SOC (g C kg�1)

1991 0–10 6.2 baAb Bc 7.0 bA CD 8.8 aA

10–20 6.0 aA B 6.1 aAB ABC 5.6 aB

20–30 5.5 aAB BC 5.1 aBC AB 4.9 aB

30–40 4.6 aB C 4.5 aC AB 4.5 aB

Mean 5.6 a CD 5.7 a BC 6.0 a

1996 0–10 7.6 bA A 8.1 bA BC 10.3 aA

10–20 7.3 aA A 7.0 aAB AB 7.1 aB

20–30 7.0 aA A 6.0 aB A 6.6 aB

30–40 5.8 aB AB 4.5 bC AB 4.8 abC

Mean 6.9 ab A 6.4 b A 7.2 aA

1998 0–10 7.5 cA A 8.9 bA AB 11.3 aA

10–20 6.6 aAB AB 6.5 aB ABC 7.3 aB

20–30 6.2 aBC AB 5.6 aB A 6.2 aBC

30–40 5.1 aC ABC 3.9 aC AB 5.0 aC

Mean 6.3 b AB 6.2 b AB 7.5 a

2000 0–10 7.2 cA AB 8.6 bA AB 12.5 aA

10–20 6.3 aA AB 5.8 aB C 5.7 aB

20–30 6.1 aAB AB 5.4 aB AB 5.1 aB

30–40 5.0 aB BC 5.1 aB A 4.9 aB

Mean 6.2 b BC 6.2 b AB 7.1 a

Mean tillage � depth (1996–2005)

0–10 7.2 cA 8.9 bA 12.6 aA

10–20 6.6 aB 6.5 abB 6.1 bB

20–30 6.1 aB 5.2 bC 5.3 bC

30–40 5.2 aC 4.3 bD 4.6 bD

Mean tillage

(1996–2005)

6.3 b 6.2 b 7.2 a

CT, conventional tillage; MT, minimum tillage; NT, no-tillage.a Means in each row followed by the same lower case letter are not significantly difb Means in each column followed by the same upper case letter are not significantlyc Means in each column followed by the same upper case letter are not significantly

observed no significant differences between CT, MT and NT at 20–30 cm soil depth. CT was mouldboard ploughing to a depth of25 cm.

3.2. SOC concentrations and SOC stock

Results of SOC expressed in Cc are shown in Table 2, and SOCexpressed in esm can be seen in Table 3. Average values presentedin these tables correspond to the period 1996–2005 where theequilibrium of SOC is reached practically in all tillage systems.Significant differences (P < 0.01) were encountered between mainfactors and their respective interactions. NT was the tillage systemwith the highest SOC, whereas no significant differences wereencountered between MT and CT. Average Cc and esm SOC was 15%higher in NT than in MT and CT.

In the top layer (0–10 cm) average Cc SOC in NT was 75% and42% higher than in CT and MT, respectively, whereas in the L1 layerNT resulted in an average esm SOC 70% and 42% higher than in CTand MT, respectively. MT in comparison with CT was 23% higher inCc SOC and 20% higher in esm SOC. In the 10–20 cm layer, and inthe L2 layer, the highest average Cc and esm SOC was observed inCT followed by MT and NT. Both Cc and esm SOC in the 20–30 cmsoil layer were significantly higher in CT than in NT and MT, but inthe L3 layer CT and NT were significantly higher than MT. CT wasthe tillage system with the highest Cc and esm SOC in the 30–40 cm layer and in the L4 layer. Our results are consistent withthose of Angers et al. (1997) who observed higher Cc SOC in CT thanin NT as soil depth increased.

The interaction treatment � soil depth � years for the SOCsequestered and expressed in Cc and esm can be seen in Tables 2and 3, respectively. Significant differences appeared in the top

t different depths from 1985 to 2005. ‘‘El Encın’’ Experimental Station.

Year Depth

(cm)

CT MT NT

E 2002 0–10 7.1 cA AB 9.4 bA A 13.8 aA A

C 10–20 6.5 abA AB 7.2 aB A 5.4 bB C

C 20–30 6.3 aA AB 5.1 bC AB 5.1 bB BC

A 30–40 6.2 aA A 3.7 bD B 4.3 bB A

B Mean 6.6 ab AB 6.3 b A 7.2 a A

D 2003 0–10 6.9 cA AB 9.2 bA AB 13.1 aA AB

AB 10–20 6.1 aAB B 5.7 aB C 5.8 aB C

A 20–30 6.0 aAB ABC 5.0 aB AB 4.9 aBC C

A 30–40 4.9 aB BC 4.6 aB AB 4.6 aC A

A Mean 6.0 b BC 6.1 b AB 7.1 a A

CD 2004 0–10 7.2 cA AB 9.4 bA A 14.2 aA A

A 10–20 7.0 aA AB 6.9 aB ABC 5.2 bB C

AB 20–30 5.5 aB BC 4.2 bC B 4.5 abB C

A 30–40 4.9 aB BC 3.8 aC B 4.1 aB A

A Mean 6.2 b BC 6.1 b AB 7.0 a A

BC 2005 0–10 7.2 cA AB 8.5 bA AB 13.2 aA AB

C 10–20 6.5 aA AB 6.7 aB ABC 5.9 aB BC

BC 20–30 6.0 aA ABC 4.8 aC AB 4.9 aBC C

A 30–40 4.7 aB BC 4.1 aC AB 4.6 aC A

A Mean 6.1 b BC 6.0 b AB 7.1 a A

Initial conditions

1985 0–10 6.4 aA AB 6.4 aA D 6.4 aA F

10–20 5.9 aAB B 5.9 aAB BC 5.9 aAB C

20–30 4.8 aBC C 4.8 aBC AB 4.8 aBC C

30–40 4.2 aC C 4.2 aC AB 4.2 aC A

Mean 5.3 a D 5.3 a C 5.3 a C

ferent between tillage treatments at the same depth (P < 0.01).

different between depths for the same treatment and year (P < 0.01).

different between years at the same tillage treatment and depth (P < 0.01).

Table 3Soil organic carbon stocked in equivalent soil mass (esm SOC) (Mg C ha�1) in each tillage treatment compared at different soil depths from 1985 to 2005. ‘‘El Encın’’

Experimental Station.

Year Layer CT MT NT Year Layer CT MT NT

SOCesm (Mg C ha�1)

1991 L1 10.0 baAb Cc 11.4 bA D 14.1 aA G 2002 L1 11.8 cA AB 15.5 bA A 22.2 aA AB

L2 9.7 aA BC 9.8 aAB BC 9.3 aB B L2 10.8 aA ABC 11.5 aB A 9.0 bB B

L3 8.8 aAB BC 8.3 aBC AB 8.1 aBC C L3 10.5 aA AB 8.0 bC AB 8.3 bBC BC

L4 7.5 aB ABC 7.3 aC A 7.5 aC A L4 8.8 aB A 6.1 bD A 6.9 bC A

Mean 9.0 a CD 9.2 a BC 9.7 a B Mean 10.5 b AB 10.3 b A 11.6 a A

1996 L1 12.6 bA A 13.3 bA C 16.7 aA F 2003 L1 11.4 cA ABC 14.8 bA ABC 21.2 aA BC

L2 12.1 aA A 11.3 aB AB 11.7 aB A L2 10.2 aAB BC 9.4 aB C 9.6 bB B

L3 11.2 aA A 9.3 bC A 10.3 abB A L3 9.6 aB AB 8.3 aBC AB 8.2 aBC C

L4 8.6 aB AB 7.1 aD A 7.5 aC A L4 7.5 aC ABC 7.3 aC A 7.3 aC A

Mean 11.1 ab A 10.3 b A 11.5 a A Mean 9.6 b BC 9.9 b AB 11.6 a A

1998 L1 12.3 cA A 14.5 bA ABC 18.6 aA E 2004 L1 12.0 cA AB 15.2 bA AB 23.1 aA A

L2 10.9 aAB ABC 10.6 aB ABC 12.0 aB A L2 11.2 aA AB 10.7 aB ABC 8.6 bB B

L3 9.8 aB AB 8.8 aC A 10.0 aC AB L3 8.9 aB BC 6.9 bC B 7.5 abBC C

L4 7.7 aC ABC 6.3 aD A 7.9 aD A L4 7.3 aB ABC 6.2 aC A 6.7 aC A

Mean 10.2 b AB 10.0 b AB 12.1 a A Mean 9.9 b BC 9.8 b AB 11.4 a A

2000 L1 11.9 cA AB 13.8 bA BC 20.3 aA DE 2005 L1 11.8 cA AB 13.8 bA BC 21.2 aA BC

L2 10.4 aAB BC 9.5 aB C 9.4 aB B L2 10.6 aAB ABC 10.6 aB ABC 9.6 aB B

L3 9.5 aB AB 8.8 aBC A 8.5 aBC BC L3 9.3 aB BC 7.8 aC AB 8.0 aBC C

L4 7.3 aC ABC 7.8 aC A 7.7 aC A L4 7.1 aC BC 6.6 aC A 7.2 aC A

Mean 9.8 b BC 10.0 b AB 11.5 a A Mean 9.7 b BC 9.7 b AB 11.5 a A

Mean tillage � depth (1996–2005) Initial conditions

L1 12.0 cA 14.4 bA 20.5 aA 1985 L1 10.6 aA BC 10.6 aA D 10.6 aA H

L2 10.9 aB 10.5 abB 10.0 bB L2 9.4 aAB C 9.4 aAB C 9.4 aAB B

L3 9.8 aC 8.3 bC 8.7 bC L3 7.7 aBC C 7.7 aBC AB 7.7 aBC C

L4 7.7 aD 6.8 bD 7.3 abD L4 6.5 aC C 6.5 aC A 6.5 aC A

Mean 8.5 a D 8.5 a C 8.5 a C

Mean tillage

(1996–2005)

10.1 b 10 b 11.6 a

CT, conventional tillage; MT, minimum tillage; NT, no-tillage. Equivalent soil mass is 1670 Mg ha�1 for each soil layer.a Means in each row followed by the same lower case letter are not significantly different between tillage treatments at the same soil layer (P < 0.01).b Means in each column followed by the same upper case letter are not significantly different between soil layers for the same tillage treatment and year (P < 0.01).c Means in each column followed by the same upper case letter are not significantly different between years at the same tillage treatment and same soil layer (P < 0.01).

Fig. 2. Time evolution of the SOC per hectare expressed as esm of the four soil layers.

Equivalent soil mass is 1670 Mg ha�1 for each soil layer. (a) Means in each column

followed by the same lower case letter are not significantly different between tillage

treatments (P � 0.01). (b) Means in each row followed by the same upper case letter

are not significantly different between years for the same treatment (P � 0.01). CT,

conventional tillage; MT, minimum tillage; NT, no-tillage.

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122118

layer for Cc and esm SOC in 6 out of 9 years (1998–2005) whereNT > MT > CT. In 2 out of 9 years (1991 and 1996) NT > MT = CT inCc and esm SOC. In layers 10–20 cm and L2 there were nosignificant differences in Cc and esm SOC between tillage systemsexcept in 2002 and 2004 when MT = CT > NT. In the bottom twolayers no significant differences were observed between tillagesystems in most years except in 2002 where CT > MT = NT, both inCc and esm.

It can be seen from Table 2 that average Cc SOC declinedsignificantly in each tillage system as soil depth increased. Thissame pattern was observed in esm SOC (Table 3), although CT wasslightly less stratified than MT and NT. In CT and MT the SOCsequestered in the soil profile increased significantly from 1991 to1996 and from this year onwards SOC was almost constant in everysoil layer. NT showed a higher variability in terms of SOCsequestered in the soil profile than CT and MT, particularly inthe top layer. In the latter, SOC increased continuously from 1991to 2002. Since this last year, SOC in the top layer has been almostconstant (Tables 2 and 3).

In each of the tillage treatments compared, C sequestration in thesoil can be analyzed according to the following three different timeperiods of our experiment (Fig. 2). The first period started in 1985and ended in 1991. In this time period, crop residues were baled andrelative to the low SOC sequestered results in each tillage system canbe considered as short-term because it included the first years whenthe C-F crop rotation was substituted for the W-VP crop rotation.Levels of Cc SOC in the soil increased at average rates of20� 5.5 g kg�1 C year�1 in NT; 11 � 5 g kg�1 C year�1 in MT; and8 � 4 g kg�1 C year�1 in CT. MT and NT average SOC rates were3 � 4 g kg�1 C year�1 and 12 � 5.8 g kg�1 C year�1, respectively,higher than the CT SOC rate. Shifting crop rotation from C-F to W-VP

resulted in an average esm SOC rate increase of0.52� 0.21 Mg C ha�1 year�1. Considering the tillage systems com-pared, CT resulted in an average esm SOC rate of0.30� 0.22 Mg C ha�1 year�1, MT in an average rate of0.45� 0.29 Mg C ha�1 year�1 and NT in 0.8� 0.21 Mg C ha�1 year�1.However, no statistical differences were encountered between tillagesystems. MT and NT resulted in average esm SOC increase in the rates of0.15� 0.1 Mg C ha�1 year�1 and 0.50� 0.16 Mg C ha�1 year�1 com-pared to CT, respectively. Concerning rain-fed crops in the semiaridconditions of Spain, Lopez-Fando and Almendros (1995) reported a 29%increase of Cc SOC in NT compared to CT after 3 years ofexperimentation. Similar results were obtained in Spain by Lopez-

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122 119

Bellido et al. (1997), Bescansa et al. (2006) and Martın-Rueda et al.(2007).

The second period, ranged from 1991 to 1996 (Fig. 2). Inthese years no statistical differences were encountered betweenstocked SOC in CT and NT, but NT was significantly higherthan MT. Average rates of the increase of Cc SOC, for this period,were 46 � 14.6 g kg�1 C year�1, 25 � 12.8 g kg�1 C year�1, and40 � 9.4 g kg�1 C year�1 for CT, MT and NT, respectively. For esmstocked SOC the average rates of increase were1.72 � 0.16 Mg C ha�1 year�1, 0.85 � 0.17 Mg C ha�1 year�1, and1.45 � 0.28 Mg C ha�1 year�1 for CT, MT and NT, respectively.Nevertheless, from 1985 to 1996 those latter rates were0.94 � 0.2 Mg C ha�1 year�1in CT, 0.63 � 0.2 Mg ha�1 year�1in MTand 1.1 � 0.13 Mg C ha�1 year�1 in NT.

The third period (Fig. 2), ranged from 1996 to 2005. Equilibriumwas reached in terms of carbon sequestration because no statisticaldifferences were observed for the stocked SOC in the whole soilprofile in NT. However CT and MT showed no significant differencesbetween 1991 and 2005 reaching the significant highest values in1996. Average esm SOC were 10.1 Mg ha�1, 10 Mg ha�1, and11.6 Mg ha�1 for CT, MT and NT, respectively (Table 3). In the first11 years, compared to C-F rotation (1985), average values (1996–2005) of W-VP rotation resulted in average rate increases of esm SOCof 0.57 � 0.21 Mg C ha�1 year�1 in CT, 0.53� 0.25 Mg C ha�1 year�1 inMT, and 1.12� 0.13 Mg C ha�1 year�1 in NT. Similarly, average rateincreases of Cc SOC were 17 � 4.6 g kg�1 C year�1 in CT,15� 7 g kg�1 C year�1 in MT, and 32� 7 g kg�1 C year�1 in NT. Theseresults showed that adoption of crop rotations which include a legumeresulted in similar stocked SOC in the soil profile compared to theincreases of stocked SOC observed when CT is substituted for NT. On theother hand MT followed the same trend as CT.

Our results are consistent with those of many other authors whohave observed the increase of stocked SOC over time for NT incomparison with MT and CT for different soils, climatic conditions,crops, rotations and management (Kern and Johnson, 1993; Paustianet al., 1997; Franzluebbers et al., 1998; Peterson et al., 1998;Stockfisch et al., 1999; Sperow et al., 2003; Al-Kaisi et al., 2005;Moreno et al., 2006; Wang and Dalal, 2006; Masri and Ryan, 2006;Lopez-Fando et al., 2007). On the other hand, our results showed thatshifting from C-F rotation to W-VP rotation was similar relevant interms of stocked SOC increase than shifting from CT to NT and evento MT. Observations made by Ryan (1998) and Liu et al. (2006)indicated that the inclusion of some legumes in the crop rotation,such as vetch, resulted in higher levels of organic matter content. Italso resulted in higher N concentrations in the soil, in comparisonwith continuous cereal cropping and cereal-fallow rotation, whichmay further improve soil productivity.

Several studies have suggested that SOC contents increaserapidly during the first 10 years following conversion to NTpractices. However, this period is usually followed by anothercharacterized by slower SOC increases (Yang and Wander, 1999;Franzluebbers and Arshad, 1996; Puget and Lal, 2005). The highestvalues were obtained with NT, in comparison with MT and CT, sothat NT follows a different dynamic in carbon transformation fromcrop residues to soil carbon than the other treatments (Jastrowet al., 2007). The trend of SOC evolution over time in all treatments(Fig. 2) was similar to that observed by West and Six (2007).

Results of SOC distribution in depth showed that under MT andNT systems SOC content was higher in the surface 0–10 cm soillayer compared to CT, while at the depth of 20–30 cm this trendwas reversed. The trend in SOC concentration is a function of thedepth of tillage, and hence the depth of residue incorporation, dueto the specific tillage operation (Deen and Kataki, 2003; Mrabetet al., 2001). Tebrugge and During (1999) reported that mould-board ploughing incorporated 65% of crop residues at a soil depthof 15–25 cm, whereas the remaining 35% were incorporated at a

depth of 0–15 cm. About 70% of residues are buried in 0–15 cm ofdepth in MT when either a cultivator or a chisel is used for primarytillage. In our case, 40% of surface residues were buried aftersowing in 0–10 cm of depth in NT and the other 60% remained inthe soil surface. In CT and MT the C content of surface residues arequickly oxidized to CO2 once they have been incorporated into thesoil profile (Alvaro-Fuentes et al., 2007; Lal and Kimble, 1997).However, in NT the pools of SOC below the sowing depth are longlasting because the C contained in the roots is exposed to a slowerdecomposition process as the soil profile is not inverted whentilled, like in CT and to a less extent in MT. Furthermore, these twotillage systems expose a greater portion of the soil to freezing–thawing and wetting–drying cycles resulting in a macro-aggregate(53–250 mm) breakdown that promotes the soil’s susceptibility todegradation (Conant et al., 2007). Soil structure plays a dominantrole in the physical protection of SOC by controlling microbialaccess to substrates, microbial turnover processes, and decom-poser food web interactions (Post et al., 2004; Liu et al., 2006).

3.3. Stratification ratios

The values obtained for the different SRs compared in this workcan be seen in Table 4. SRCc(1/4) depends only on the Cc of the soillayers considered. SResm(1/4) depends on Cc, BD and thickness of thesoil layers that take part in the calculation of esm. SRALs explainthe top soil effect in SOC stratification compared to SOC in all soillayers. In other words, SRALs reflect the percentage of SOC in thetop soil relative to the total SOC in all the soil layers. In our work,for example, we have considered four homogeneous layers in esmSOC. Whenever SRAL equals the value of one and the number oflayers considered is four, as it is in our case, the SOC contained inthe top layer is 25% of the total SOC sequestered in the soil. Thedifferent SRs and SRALs established in this work give additionalinformation to the SR defined by Franzluebbers (2002) as a soilquality indicator. Similarly, we have considered SResm(1/4) > 2indicates an improvement in soil quality produced by changingfrom C-F to W-VP, as well as changing from CT to MT and NT.

Significant differences were observed between tillage systemsin average values for all the SRs considered (Table 4) whereNT > MT > CT. It can be seen in Table 4 that systematically all SRswere significantly higher in NT than in the other two tillagesystems since 1996, and all SR values for NT were �2 indicating animprovement of the soil quality. Similarly, from 2002 onwards allthe SRs were significantly higher in MT than in CT Similar findingsfor SRCc(1/n) have been obtained by Franzluebbers (2002), Mrabet(2002), Jarecki et al. (2005), Moreno et al. (2006), Noellemeyer et al.(2006), Franzluebbers et al. (2007), Lopez-Fando et al. (2007), andAlvaro-Fuentes et al. (2008). On the other hand, Jarecki et al. (2005)observed a SR < 2 for NT when it was calculated in soil volumebasis. However, in most of the cases SRCc(1/n) values are notcomparable because the nth layer is not always the same. For thisreason, in order to be able to compare different SR values it isnecessary to establish a standardised procedure to obtainhomogeneous results from different sites.

The time evolution of the different SRs in each tillage systemrevealed no significant differences in CT for SRCc(1/4) and SResm (1/4).MT exhibited a higher variability than CT. Results for NT showed aprogressive and significant increase in the values of SR from 1985to 2000, but from 2000 to 2005 both variables remained with smalldifferences. Considering that from 1996 to 2005 no significantdifferences were observed in the SOC pools in NT, it can beconcluded that while the top layer increases its SOC, this variable isdecreasing in the bottom layer. Similar findings were observed byDıaz-Zorita and Grove (2002).

SRALesm(1) presented a similar trend as the other two SRs abovementioned. However, it is a better indicator as it is more able to

Table 4Stratification ratios (SR) in each tillage treatment compared from 1985 to 2005. ‘‘El Encın’’ Experimental Station.

Year SRCc(1/4) (kg C kg�1 C�1) SResm(1/4) (Mg C ha�1/Mg C ha�1) SRALesm(1) (MgC ha�1/MgC ha�1)

CT MT NT CT MT NT CT MT NT

1985 1.52 aa Ab 1.52 a F 1.52 a F 1.64 a A 1.64 a E 1.64 a F 1.24 a A 1.24 a D 1.24 a E

1991 1.37 b A 1.63 ab EF 1.94 a EF 1.34 b A 1.59 ab E 1.88 a EF 1.11 b A 1.24 b D 1.45 a D

1996 1.32 b A 1.86 ab DEF 2.28 a DE 1.49 b A 1.91 ab CDE 2.32 a DE 1.14 c A 1.30 b CD 1.45 a D

1998 1.47 b A 2.36 a ABC 2.42 a CDE 1.60 b A 2.35 a ABC 2.49 a CD 1.21 b A 1.45 a AB 1.54 a D

2000 1.53 b A 1.69 b EF 2.55 a CD 1.67 b A 1.78 b DE 2.64 a CD 1.22 c A 1.38 b BC 1.77 a C

2002 1.15 c A 2.56 b A 3.24 a AB 1.34 c A 2.54 b A 3.23 a AB 1.13 c A 1.51 b AB 1.91 a AB

2003 1.40 c A 1.98 b BCDE 2.87 a BC 1.52 c A 2.04 b BCDE 2.91 a BC 1.17 c A 1.49 b AB 1.84 a BC

2004 1.47 c A 2.48 b AB 3.46 a A 1.64 c A 2.47 b AB 3.46 a A 1.22 c A 1.56 b A 2.01 a A

2005 1.53 c A 2.08 b ABCD 2.88 a BC 1.67 c A 2.12 b ABCD 2.93 a BC 1.22 c A 1.43 b ABC 1.84 a BC

Mean 1.42 c 2.02 b 2.57 a 1.55 c 2.05 b 2.61 a 1.18 c 1.40 b 1.67 a

CT, conventional tillage; MT, minimum tillage; NT, no-tillage.a Means in each row followed by the same lower case letter are not significantly different between tillage treatments in each year (P < 0.01).b Means in each column followed by the same upper case letter are not significantly different between years for the same tillage treatment (P < 0.01).

Fig. 3. Relation between esm SRAL, and esm SR. Esm, equivalent soil mass; SR(1/4),

stratification ratio is defined as the ratio of organic carbon in the top soil layer and

the organic carbon in the bottom soil layer; SRAL is defined as the ratio of organic

carbon in the top soil layer and the average organic carbon in the four layers

considered.

J.L. Hernanz et al. / Agriculture, Ecosystems and Environment 133 (2009) 114–122120

discriminate because in 6 out of 9 years there were significantdifferences between tillage systems and NT > MT > CT. Franzlueb-bers (2002) stated that a SOC stratification ratio > 2 is a goodindicator of dynamic soil quality, independent of soil type andclimatic regime, because a ratio > 2 is uncommon in degradedsoils. A regression analysis was performed between SRALesm(1) andSResm(1/4). It can be seen in Fig. 3 that SR reaches the value 2 whenSRAL is 1.4. This fact implies that the SOC stocked in the top soil is(100/4)1.4 = 35% of the total SOC stocked in the soil profile.Similarly, about 30% of SOC is stocked in the top layer for CT,whereas 35% and 42% is stocked for MT and NT, respectively. Forthis last treatment the highest value, 50%, was reached in 2004.

4. Conclusions

After 20 years with this field trial, the adoption of a W-VProtation instead of a C-F rotation was at least as important as theshift from CT to NT in terms of the increase of stocked SOC in thesoil profile. MT showed a similar behaviour as CT. The increase instocked SOC has been produced in three time periods: (i) from1985 to 1991 all tillage treatments showed similar increasing ratesof stocked SOC; (ii) from 1991 to 1996 all tillage treatmentsincreased the SOC content, but the increases observed in NT and CTwere significantly higher than those in MT; and (iii) from 1996 to2005 when stocked SOC almost reached a constant valuepractically in the three tillage systems compared. Stocked SOCreached equilibrium, or steady state, after 11 years since startingthe experiment in NT. However, this equilibrium was reached after12 years in CT and MT.

Carbon concentration and esm SRs showed the higheststratification in NT followed by MT and CT. In NT, stocked SOCin the top layer increased from 1996 to 2005 whereas stocked SOCin the bottom layer declined systematically. SRAL, expressed inesm, were introduced in this work as an alternative way to assessthe SOC distribution in the soil profile. These ratios providedinformation about the extent to which the top layer affects thestocked SOC in the whole soil profile. It is necessary to carry out athorough study of the different variables assessing SOC stratifica-tion in order to define a procedure capable of comparing the valuesobtained in different sites and locations, and the values obtainedfor soil layers with different thickness.

Acknowledgements

The authors wish to thank the Comision Interministerial deCiencia y Tecnologıa (CICYT) of the Spanish Ministry of Educationand Science for the financial support given along 20 years of study(Grants No: PR84-0495; AGR90-0088; AGF96-1138-C02-01;AGL2001-3822-C02-01; AGL2002-04186-C03-01; and AGL2007-65698-C03-01). Thanks should be given to IMIDRA for theirsupport during these last 30 years of field experimentation at ‘‘ElEncın’’ experimental Station.

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