REGULAR ARTICLE

Conservation agriculture, increased organic carbonin the top-soil macro-aggregates and reducedsoil CO2 emissions

Mariela Fuentes & Claudia Hidalgo &

Jorge Etchevers & Fernando De León &

Armando Guerrero & Luc Dendooven &

Nele Verhulst & Bram Govaerts

Received: 13 August 2011 /Accepted: 23 November 2011 /Published online: 30 December 2011# Springer Science+Business Media B.V. 2011

AbstractBackground and aims Conservation agriculture, thecombination of minimal soil movement (zero or re-duced tillage), crop residue retention and crop rota-tion, might have the potential to increase soil organicC content and reduce emissions of CO2.Methods Three management factors were analyzed:(1) tillage (zero tillage (ZT) or conventional tillage(CT)), (2) crop rotation (wheat monoculture (W),maize monoculture (M) and maize-wheat rotation(R)), and (3) residue management (with (+r), or with-out (−r) crop residues). Samples were taken from the0–5 and 5–10 cm soil layers and separated in micro-aggregates (< 0.25 mm), small macro-aggregates (0.25

to 1 mm) and large macro-aggregates (1 to 8 mm). Thecarbon content of each aggregate fraction wasdetermined.Results Zero tillage combined with crop rotation andcrop residues retention resulted in a higher proportionof macro-aggregates. In the 0–5 cm layer, plots with acrop rotation and monoculture of maize and wheat inZT+r had the greatest proportion of large stable mac-ro-aggregates (40%) and highest mean weighted di-ameter (MWD) (1.7 mm). The plots with CT had thelargest proportion of micro-aggregates (27%). In the5–10 cm layer, plots with residue retention in both CTand ZT (maize 1 mm and wheat 1.5 mm) or withmonoculture of wheat in plots under ZT without

Plant Soil (2012) 355:183–197DOI 10.1007/s11104-011-1092-4

Responsible Editor: Johan Six.

M. Fuentes : F. De LeónLaboratorio de Fisiología y Tecnología de Cultivos,Universidad Autónoma Metropolitana-Xochimilco,Calzada del Hueso 1100, Col. Villa Quietud,México 04960 D.F., Mexico

C. Hidalgo : J. EtcheversLaboratorio de Fertilidad,Colegio de Postgraduados, IRENAT,Km 36.5 Carretera México-Texcoco,Montecillo CP 56230, Mexico

A. GuerreroLaboratorio de Suelos, Plantas y Aguas, Campus Tabasco,Colegio de Postgraduados,Supera-Anuies, Mexico

L. DendoovenCinvestav,Av. Instituto Politécnico Nacional 2508,México C.P. 07360 D.F., Mexico

N. Verhulst :B. Govaerts (*)International Maize and Wheat ImprovementCentre (CIMMYT),Apdo. Postal 6-641, 06600 Mexico, D.F., Mexicoe-mail: b.govaerts@cgiar.org

residues (1.4 mm) had the greatest MWD. The 0–10 cm soil layer had a greater proportion of smallmacroaggregates compared to large macro-aggregatesand micro-aggregates. In the 0–10 cm layer of soilwith residues retention and maize or wheat, the great-est C content was found in the small and large macro-aggregates. The small macro-aggregates contributedmost C to the organic C of the sample. For soilcultivated with maize, the CT treatments had signifi-cantly higher CO2 emissions than the ZT treatments.For soil cultivated with wheat, CTR-r had significantlyhigher CO2 emissions than all other treatments.Conclusion Reduction in soil disturbance combinedwith residue retention increased the C retained in thesmall and large macro-aggregates of the top soil due togreater aggregate stability and reduced the emissions ofCO2 compared with conventional tillage without residuesretention and maize monoculture (a cultivation systemnormally used in the central highlands of Mexico).

Keywords Aggregate stability . Soil CO2 emissions .

Zero tillage

Introduction

Declining SOC contents in agro-ecosystems are impor-tant in the global C budget. Agronomic practices, in-cluding tillage, residues management and crop rotation,are crucial determinants of the quantity of carbonretained in the soil (Allmaras et al. 2004; Fuentes et al.2009; West and Post 2002). Generally, the top soil offields under ZT with residue retention has a larger soilorganic carbon (SOC) content and a lower soil organicmatter (SOM) decomposition rate than soil with con-ventional tillage (CT) (Diekow et al. 2005; Fuentes et al.2009; Jantalia et al. 2007; Six et al. 2002). When con-sidering the whole soil profile, results have been lessconsistent (Govaerts et al. 2009a). Some researchshowed that soils with zero or reduced tillage had great-er SOC stocks than soils with CT in the first 30 cm soillayer (e.g. Bayer et al. 2000; Halvorson et al. 2002;Huggins et al. 2007; Rasmussen and Smiley 1997;Yang and Kay 2001), while other studies reported theopposite (Black and Tanaka 1997; Blanco-Canqui andLal 2008) and some research found no significant differ-ences (Angers et al. 1997; Dolan et al. 2006). Croprotation can affect SOC because the SOC contentdepends on the type of crop in the rotation, the quality

and quantity of crop residues and/or root development(Wright and Hons 2005). Since crop residues are pre-cursors of the SOM pool, returning more crop residuesto the soil is associated with an increase in SOC con-centration (Govaerts et al. 2009a).

The SOC distribution in soil aggregates largely deter-mines the sequestration or release of C within a givenagricultural system (Blanco-Canqui and Lal 2007; Six etal. 2000). Six et al. (2002) stated that a greater accumu-lation of SOC in ZT than in CTcan be related to a lack ofsoil disturbance and a better preservation of aggregates inZT compared to CT. Soil with ZT and crop residueretention has a greater proportion of macro-aggregates(>250 μ) with a higher organic C content than CTwithout residues (Tantely et al. 2008). Tantely et al.(2008) also reported that the macro-aggregates containedmore organic C than the micro-aggregates (<250 μ). Thedifferences in C concentration within the fraction ofmicroaggregates occluded in macroaggregates betweenmanagement systems can be linked to differences in theamount, stability and turnover of the macroaggregate-occluded-microaggregates (Denef et al. 2007). Themacroaggregate-occluded-microaggregates have aslower turnover due to the protective environment ofthe macroaggregates. This slower turnover allows great-er protection of the coarse particulate organic matter(cPOM) and a greater stabilization of mineral-bound Cdecomposition products in the macroaggregate-occluded-microaggregates (Denef et al. 2007). Macro-aggregates have a higher particulate organic matter(POM) content than other soil aggregates. Since POMis more susceptible to degradation than the other organicmatter fractions, the organic C content in the macro-aggregates is an indicator of the stability of the aggre-gates and the retention or loss of C as affected bydifferent management practices.

The SOC content and its distribution in the soilprofile as a result of management practices affectemissions of CO2 (Osozawa and Hasegawa 1995).Different effects of soil management on emissions ofCO2 have been reported. Some studies showed thatsoil with CT emitted more CO2 than the same soilswith ZT systems (Chatskikh et al. 2008; Hutchinson etal. 2007; Oorts et al. 2007; Schlesinger 1999; Ussiriand Lal 2009). However, Hendrix et al. (1988) foundthe opposite. Other authors reported no significantdifferences in emissions of CO2 between soils withZT and CT (Elder and Lal 2008; Nouchi andYonemura 2008).

184 Plant Soil (2012) 355:183–197

Determining the effect of management practices onSOC content, C distribution in the aggregate fractionsand the emissions of CO2 from the soil will help tounderstand the processes that affect C sequestration.Therefore, the objectives of this research were to de-termine the effect of 16 y of ZT compared to CTcombined with different crop rotations (monocultureand rotation) and crop residue management (with andwithout residues) on the retention and emission of C ofthe soil considering: i) aggregate size distribution andstability, ii) the contribution of C within differentaggregate fractions of the top-soil and iii) the emis-sions of CO2 from the soil.

Materials and methods

Experimental site

The study was conducted at CIMMYT’s experimentalstation in El Batán, situated in the semi-arid, subtrop-ical highlands of Central Mexico (19° 31′ North, 98°50′ West, 2259 altitude), in a Haplic Phaeozem(Clayic) (IUSS, Working Group WRB 2006) or fine,mixed, thermic Cumulic Halplustoll (Soil Survey Staff2003) with a particle size distribution of 380, 370 and250 gkg-1 of clay, silt and sand respectively. Thestation has an average temperature of 14°C with600 mm y-1 rainfall, with about 520 mm falling be-tween May and October. The rainy season has typicallyshort, intense rain showers followed by dry spells andevapotranspiration exceeds rainfall throughout the year(yearly potential evapotranspiration is 1900 mm)(Govaerts et al. 2005).

The experiment was set up in 1991, with 64 plots of7.5×22 m. The slope was 0.3% (north to west). Thirtytwo treatments were applied in a randomized completeblock design with two repetitions (blocks). In thisresearch, only the sixteen core treatments were includ-ed (so thirty two plots were sampled). These treat-ments have not been changed since the start of theexperiment and consist of combinations tillage prac-tice (zero tillage [ZT] or conventional tillage [CT]),crop residue management (with [+r] and without [−r]retention) and crop rotation (monoculture of maize(Zea mays L.) [M], monoculture of wheat (Triticumaestivum L.) [W] or rotation of both crops [R]). Eachphase of the rotation was present each year.

The soil preparation in CT consisted of harrowing at20 cm depth, with a disc harrow starting some days afterharvest and repeated when needed for weed control (atleast once) during the dry season. To prepare the seedbed a spike tooth harrow was used once. The ZT plotswere sown directly with maize or wheat using anAlmaco® seeder and an Aitcheson® machine respec-tively, both using disc openers for seed placement.Sowing was done in May and the harvest in Octoberfor wheat and November for maize. In the treatmentswith residue retention (+r) all the residues of the formercrop were kept on the field; in CT plots the residueswere incorporated by tillage and in ZT they were left onthe soil surface. In the –r treatments (residues removed),most of the aerial residues were removed simulatingfarmers’ practice. Both crops were fertilized at the rateof 120 kg de N ha−1 using urea, with all N applied towheat at the first node growth stage (broadcast) and tomaize at the 5–6 leaf stage (surface-banded).

Soil sampling and analysis

Undisturbed samples were collected at four locations ofeach plot from the 0–5 cm and 5–10 cm layer inSeptember 2007. The C content in the aggregates wasmeasured with a C autoanalyzer (TOC-5050A –TotalOrganic Carbon, Shimadzu©). Inorganic C in the soilwas negligible so total Cwas considered as the organic C.Soil samples (0.1 to 0.8 g) grounded and sieved through a0.15 mm mesh were subjected to a dry combustion at900°C for 3 to 4 min. The emitted CO2 was registered bymeans of an infrared sensor and considered as organic C.

Wet aggregate stability was determined on a 20 gdried soil sub-sample (8 mm) (Barthès et al. 2000;Kemper and Rosenau 1986; Limón-Ortega et al.2002). The sample was slaked in water for 30 min,and then wet sieved through a column of sieves with amesh opening of 4.75, 2.00, 1.00, 0.50, 0.25 and0.05 mm, submerged in a cylinder of distilled waterand driven up and down at 60 cycles per minute. Thefractions held in the sieves were collected and dried at105°C for 18 h. The proportion of aggregates of differ-ent sizes and the mean weighted diameter (MWD) persample were determined. The amount of organic C wasdetermined in all fractions obtained with a C autoana-lyzer. The C contribution of each fraction to the total Cof the sample was calculated by grouping the fractionsby size as follows: (a) <0.25 mm (micro-aggregates), (b)

Plant Soil (2012) 355:183–197 185

0.25 to 1 mm (small macro-aggregates) and (c) 1 to8 mm (large macro-aggregates).

In situ emission of CO2, soil water content, airtemperature and precipitation

The emissions of CO2 and soil water content were deter-mined monthly, while additional measurements weredone when soil was fertilized, tilled or planted. Eightsamples were collected along two virtual lines drawn inthe central part of the plot (at 2.5 m from each border) atintervals of 4 m. The CO2 emission was measured with aportable non-dispersive infrared gas analyzer (EGM-4CO2) and a soil respiration chamber which contains anair suction pump. The portable chambers were placed onthe bare soil, i.e. no plants. An internal solenoid (builtinto the CFX-1 Soil Respiration Chamber) switches thegas stream from reference and analysis for 30 s intervals.The increase in the concentration of CO2 in the air abovethe soil was calculated giving the concentration of CO2 inmg kg−1 and rates of CO2 in gm

−2 h−1. Soil water contentwas determined gravimetrically at 0–20 cm.

Statistical analysis

The experiment was a randomized complete block(RCB) design with two replicates. The effect of treat-ment on SOC content of the total sample and of theaggregates and its distribution were analyzed statisti-cally with the General Linear Model (GLM) procedurefor analysis of variance with significance set at 5%(SAS Institute 1994). The following class factors wereconsidered: rotation, tillage type, residue managementand repetition. The CO2 and water data of the exper-iment were analyzed using a linear mixed modelwhere the repeated measurements of each treatmentin every month were modeled with an unstructuredvariance-covariance matrix using PROC MIXED(SAS Institute 1994).

Results

Aggregate distribution and stability

In the 0–5 cm soil layer, the greatest proportion of largemacro-aggregates was found in ZTW+r (47%), ZTW-r(43%) and ZTR+r (41%), followed by ZTM+r (33%),while the lowest proportion was found in CT-r with

monoculture of wheat (10.5%), monoculture of maize(7.5%) and crop rotation (11.7%). The proportion ofsmall macro-aggregates was the largest in CTW-r(64.2%) and CTR-r (61%) and the lowest in ZTW+r(32%), ZTW-r (34%) and ZTR+r (41%). The greatestpercentage of micro-aggregates was found in CTM-r(36.0%) and the lowest in ZTR+r and ZTM+r (both19%). The greatest MWD was found in the ZTW+r(1.88 mm), ZTW-r (1.70 mm) and ZTR+r (1.68 mm)treatments and the lowest in the CTM-r treatment(0.52 mm) (Fig 1a). Residue management had a highlysignificant (P<0.01) effect on the percentage of largemacro-aggregates in soil under maize and maize-wheatrotation and on the percentage of micro-aggregates insoil under monoculture of maize. Tillage had a highlysignificant (P<0.01) effect on the percentage of largemacro-aggregates for all rotations and on the percentageof small andmicro-aggregates in soil under monocultureof maize. However, there was no significant interactioneffect of residues and tillage on the proportion of differ-ent aggregates size (Table 1).

In the 5–10 cm layer, the ZTW+r, CTW+r andZTW-r treatments had the highest proportion of largemacro-aggregates (38%, 35% and 34% respectively).Conventional tillage without residues under monocul-ture of maize and maize-wheat rotation and ZTM-r hadthe lowest proportion of largemacro-aggregates (11.5%,12.5% and 13.7% respectively). All the treatments withmonoculture of maize (regardless residues managementor type of tillage) had the greatest proportion of smallmacro-aggregates (average 62%) (Fig. 1b). The greatestMWD was found in ZTW+r (1.53 mm), CTW+r(1.40 mm) and ZTW-r (1.47 mm) and the lowest inCTR-r (0.70 mm), CTM-r (0.71 mm) and ZTM-r(0.72 mm) (Fig. 1b). Residue management had a signif-icant (P<0.05) effect on large macro-aggregates for allcrop rotations and on small macro-aggregates in soilwith monoculture of wheat and rotation. Tillage signif-icantly (P<0.05) affected the percentage of large macro-aggregates in soil with wheat monoculture and maize-wheat rotation. The interaction effect of residue man-agement and tillage on the proportion of different aggre-gates size was not significant (Table 1).

C distribution in the aggregate fractionsand contribution to total C

In the 0–5 cm layer of soil with residue retention(regardless of crop rotation or tillage), the organic C

186 Plant Soil (2012) 355:183–197

content in the large macro-aggregates was greater(average 2.4%) than in soil where residue wasremoved (average 1.3%), except in the CTR+rtreatment where it was similar (1.5%) to treatmentswithout residues and in the ZTW-r (2.1%) where itwas similar to treatments with residues (Table 2).In the small macro-aggregates, the greatest organicC was found for ZT+r regardless of crop rotations(average 2.32%), while the lowest organic C wasfound in soil without residues cultivated withmaize and wheat-maize in rotation (average 1.3%)(Table 2). No differences between treatments werefound for the micro-aggregates. Residue managementhad a significant (P<0.05) effect on C content in largeand small macro-aggregates for soil cultivated withmaize. Tillage had no clear effect on the C in the largeand small macro-aggregates. The interaction effect ofresidue and tillage was only significant for the organic C

in the small macro-aggregates in soil cultivated withmaize (Table 1).

In the 5–10 cm soil layer, the organic C in the smallmacro-aggregates was significantly greater in plotswhen residue was retained (average 1.6%) than whenit was removed (average 1.3%) (Tables 1 and 2). Nosuch effect, however, was found in the micro-aggregates and large macro-aggregates, except in theCT treatment with rotation where the C content in thelarge macro-aggregates was higher when residue wasretained than when it was removed. Tillage had noeffect on the C in the large macro-aggregates and itonly affected significantly (P<0.05) the C content insmall macro-aggregates and micro-aggregates in soilcultivated with wheat. The interaction between residueand tillage significantly (P<0.05) affected C in largeand small macro-aggregates in treatments with mono-culture of wheat (Table 1).

Agg

rega

te d

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Mean w

eight diameter (m

m)

Agg

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b) 5-10 cm soil layer

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ZTR+r

CTR

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2.5Large macroaggregatesSmall macroaggregatesMicroaggregatesMWD

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Fig. 1 Aggregate distribu-tion and mean weighteddiameter (MWD) in a) the0–5 cm and b) 5–10 cmlayer. Soil with zero tillage(ZT) or conventional tillage(CT), maize monoculture(M), wheat monoculture(W) and rotation (R), withresidues (+r) or withoutresidues (−r) at CIMMYT’slong-term tillage sustain-ability trial at El Batán(Mexico) for 16 y. Bars areplus one STD

Plant Soil (2012) 355:183–197 187

Table 1 The effect of residue, tillage and their interaction onthe percentage of large macro-, small macro- and microaggre-gates, the C in large macro-, small macro- and microaggregatesand the contribution of the C in large macro-, small macro- and

microaggregates to the overall C content soils cultivated withmaize or wheat as monoculture, or a rotation of wheat and maizein CIMMYT’s long-term tillage sustainability trial, El Batán(Mexico) for 16 years

0–5 cm soil layer 5–10 cm soil layer

Maize Wheat Rotation Maize Wheat Rotation

Treatment F value P value F value P value F value P value F value P value F value P value F value P value

Percentage large macroaggregates

Residue (R) 45.02 <0.0001 2.06 0.1771 21.66 <0.0001 11.56 0.0053 5.98 0.0309 9.12 0.0053

Tillage (T) 29.31 0.0040 9.12 0.0107 28.09 <0.0001 1.28 0.2791 5.60 0.0357 5.78 0.0231

R*T 0.94 0.6549 0.89 0.3629 0.03 0.8644 0.01 0.9116 1.80 0.2049 2.28 0.1423

Percentage small macroaggragetes

Residue (R) 3.27 0.0957 5.98 0.0309 19.21 0.0001 0.29 0.6020 5.60 0.0357 6.24 0.0186

Tillage (T) 0.33 0.5746 5.60 0.0357 18.27 0.0002 0.49 0.4970 3.23 0.0974 1.16 0.2905

R*T 2.03 0.1801 1.80 0.2049 0.21 0.6470 1.43 0.2552 1.51 0.2422 0.06 0.8083

Percentage microaggregates

Residue (R) 15.54 0.0020 0.30 0.5945 2.53 0.1230 2.00 0.1828 0.60 0.4520 0.12 0.7303

Tillage (T) 4.93 0.0464 1.49 0.2459 7.36 0.0113 0.05 0.8296 1.89 0.1942 0.66 0.4234

R*T 1.50 0.2446 0.00 0.9671 0.43 0.5180 0.95 0.3493 0.09 0.7701 2.57 0.1204

Percentage C in the large macroaggregates

Residue (R) 18.33 0.0004 2.11 0.1622 17.73 0.0001 7.67 0.0118 1.01 0.3264 7.70 0.0081

Tillage (T) 1.38 0.2545 1.43 0.2450 4.25 0.0451 0.37 0.5567 2.27 0.1476 0.08 0.7752

R*T 0.93 0.3460 0.01 0.9043 1.70 0.1994 0.27 0.6117 5.39 0.0309 0.79 0.3779

Percentage C in the small macroaggragetes

Residue (R) 45.82 <0.0001 0.60 0.4549 31.98 <0.0001 24.78 0.0003 16.92 0.0014 19.59 0.0001

Tillage (T) 3.69 0.0789 7.02 0.0212 3.91 0.0578 2.25 0.1596 17.43 0.0013 0.98 0.3308

R*T 8.58 0.0126 0.03 0.8598 2.25 0.1445 1.23 0.2889 10.54 0.0070 0.37 0.5456

Percentage C in the microaggregates

Residue (R) 1.79 0.2524 0.00 0.9633 3.03 0.1072 0.00 0.9900 2.21 0.2116 0.36 0.5619

Tillage (T) 0.81 0.4182 4.44 0.1029 0.04 0.8474 4.81 0.0935 12.71 0.0235 0.01 0.9371

R*T 2.41 0.1952 1.20 0.3354 0.09 0.7731 0.61 0.4769 2.62 0.1810 0.00 0.9496

Contribution of the C in the large macroaggregates

Residue (R) 23.97 0.0081 2.28 0.2057 17.01 0.0014 6.31 0.0659 2.99 0.1587 6.11 0.0293

Tillage (T) 1.72 0.2601 6.56 0.0625 17.45 0.0013 1.40 0.3022 6.65 0.0614 6.47 0.0257

R*T 0.10 0.7636 0.26 0.6348 0.00 0.9852 0.00 0.9819 0.47 0.5304 0.34 0.5709

Contribution of the C in the small macroaggragetes

Residue (R) 0.05 0.8326 2.56 0.1846 11.87 0.0048 0.48 0.5275 0.82 0.4166 1.08 0.3185

Tillage (T) 0.59 0.4859 6.68 0.0610 17.29 0.0013 0.82 0.4164 0.91 0.3939 0.22 0.6459

R*T 0.43 0.5496 0.82 0.4171 0.03 0.8721 0.00 0.9786 0.02 0.8960 0.62 0.4451

Contribution of the C in the microaggregates

Residue (R) 28.64 0.0059 0.06 0.8200 5.93 0.0315 8.57 0.0429 2.37 0.1989 1.47 0.2494

Tillage (T) 0.51 0.5141 1.04 0.3655 3.12 0.1026 0.00 0.9595 10.32 0.0325 2.81 0.1192

R*T 0.16 0.7074 0.02 0.8865 0.04 0.8501 0.00 0.9928 1.11 0.3516 1.06 0.3228

188 Plant Soil (2012) 355:183–197

In both the 0–5 cm and the 5–10 cm layer, the C inthe small macro-aggregates contributed most to thetotal organic C for all treatments (average 55%), ex-cept for ZTW+r where the major contribution camefrom the large macro-aggregates (average 50%)

(Fig. 2). In maize monoculture and residue removal(ZTM-r and CTM-r) and CTR-r, the micro-aggregatescontributed more C to the total organic C than in theother treatments (average of 0 to 10 cm CTM-r 32.5%,ZTM-r 29.5% and CTR-r 27.5% respectively) (Fig. 2).

Table 2 Organic C (%) in different aggregates of the 0–5 and 5–10 cm layers of soil cultivated with maize or wheat as monoculture, ora rotation of wheat and maize at CIMMYT’s long-term tillage sustainability trial El Batán (Mexico) for 16 years

Maize monoculture Wheat monoculture Maize-Wheat crop rotation

CT a ZT b LSD c F value P value CT ZT LSD F value P value CT ZT LSD F value P value

Large macroaggregates in the 0–5 cm

+R d 2.04 2.71 1.30 1.31 0.2789 2.16 2.52 0.79 0.99 0.3438 1.50 2.43 0.63 3.58 0.0718

−R e 1.01 1.07 0.50 0.08 0.7772 1.65 2.09 1.23 0.62 0.4508 1.35 1.48 0.32 0.69 0.4144

LSD 0.58 1.27 1.24 0.77 0.42 0.57

F value 15.95 8.32 0.86 1.58 5.98 11.76

P value 0.0025 0.0163 0.3756 0.2378 0.0230 0.0024

Small macroaggregates in the 0–5 cm

+R d 1.87 2.55 0.67 6.35 0.0453 1.76 2.19 0.63 2.79 0.1457 1.84 2.21 0.43 3.43 0.0853

−R e 1.32 1.18 0.19 3.43 0.1235 1.60 2.09 0.57 4.40 0.0807 1.39 1.44 0.16 0.49 0.4968

LSD 0.33 0.61 0.48 0.70 0.20 0.42

F value 2.79 4.40 0.72 0.13 23.08 15.75

P value 0.1457 0.0807 0.4300 0.7325 0.0003 0.0014

Micromacroaggregates in the 0–5 cm

+R d 1.62 1.06 1.85 1.73 0.3190 1.12 1.61 1.16 3.28 0.2119 1.57 1.55 0.78 0.00 0.9582

−R e 0.96 1.11 0.71 0.83 0.4594 1.43 1.28 0.62 1.17 0.3924 1.21 1.29 0.38 0.31 0.5955

LSD 1.42 1.38 0.59 1.18 0.54 0.68

F value 4.06 0.02 1.37 0.41 2.71 0.85

P value 0.1815 0.8904 0.3620 0.5885 0.1510 0.3927

Large macroaggregates in the 5–10 cm

+R d 1.74 1.76 0.48 0.01 0.9396 1.40 1.99 0.43 9.25 0.0124 1.77 1.69 0.38 0.23 0.6385

−R e 1.14 1.35 0.66 0.48 0.5036 1.60 1.48 0.53 0.28 0.6111 1.24 1.41 0.47 0.58 0.4547

LSD 0.66 0.48 0.60 0.53 0.46 0.38

F value 4.11 3.70 0.56 0.28 5.69 2.17

P value 0.0701 0.0833 0.4706 0.6111 0.0261 0.1548

Small macroaggregates in the 5–10 cm

+R d 1.63 1.60 0.29 0.08 0.7908 1.79 1.49 0.11 41.62 0.0007 1.72 1.60 0.24 1.00 0.3338

−R e 1.31 1.09 0.29 3.36 0.1164 1.49 1.45 0.16 0.32 0.5912 1.37 1.34 0.18 0.10 0.7575

LSD 0.28 0.30 0.15 0.13 0.24 0.18

F value 8.06 17.30 22.75 0.46 10.18 9.67

P value 0.0296 0.0060 0.0031 0.5216 0.0065 0.0077

Micromacroaggregates in the 5–10 cm

+R d 1.42 0.86 1.60 2.26 0.2713 1.81 1.02 0.24 197.13 0.0050 1.33 1.30 0.65 0.01 0.9354

−R e 1.27 1.01 0.24 22.47 0.0417 1.34 1.04 1.28 0.98 0.4262 1.22 1.22 0.39 0.00 0.9880

LSD 1.59 0.29 1.29 0.18 0.61 0.45

F value 0.16 4.55 2.45 0.24 0.17 0.20

P value 0.7245 0.1667 0.2578 0.6756 0.6971 0.6667

a CT: Conventional tillage, b ZT: zero tillage, c LSD: Least significant difference (P<0.05), d +R: crop residue retained, e −R: cropresidue removed

Plant Soil (2012) 355:183–197 189

Monthly CO2 fluxes and soil water content

The emission of CO2 from soil cultivated withmaize and wheat increased during the rainy sea-son, i.e. from June to September and after harvest(Fig. 3). In February and May in treatments culti-vated with maize, the CO2 emission was the high-est from the CT+r treatments with maize-wheatrotation (0.39 gC m−2 h−1 and 0.45 gC m−2 h−1

for February and May, respectively) and monocul-ture (0.36 gC m−2 h−1 and 0.42 gC m−2 h−1 forFebruary and May, respectively), and twice as highas from soil with ZT (the highest in both monthswas 0.17 gC m−2 h−1). At the second measure-ment in November, the highest CO2 emissions werefrom the CTR+r and CTR-r treatments (0.44 gC m−2 h−1) while in the ZT treatment the CO2 emissionwas 0.04 gC m−2 h−1. At the first measurement in

December, the highest CO2 emission values werefound in the CTM+r (0.17 gC m−2 h−1) and ZTM+r (0.19 gC m−2 h−1) treatments. At the secondmeasurement of December, all the CT treatmentsemitted more CO2 (average 0.065 gC m−2 h−1)than the ZT treatments (average 0.05 gC m−2 h−1). Forsoil cultivated with maize, the CT treatments hadsignificantly higher CO2 emissions than the ZTtreatments. For soil cultivated with wheat, CTR-rhad significantly higher CO2 emissions than allother treatments.

For the treatments with maize, the ZT+r treat-ment showed the greatest soil water contents fromFebruary to May and August to December. In July,the greatest soil water content was detected in theCTR+r and ZTR+r treatments (Fig. 4). The soilwater content was affected by tillage and residuemanagement from February to April and August to

SOC

con

trib

utio

n to

tota

l C (

%)

SOC

con

trib

utio

n to

tota

l C (

%)

a) 0-5 cm soil layer

b) 5-10 cm soil layer

0

20

40

60

80

100

0

20

40

60

80

100

CTM

+r

ZTM+r

CTM

-r

ZTM-r

CTW

+r

ZTW+r

CTW

-r

ZTW-r

CTR

+r

ZTR+r

CTR

-r

ZTR-r

CTM

+r

ZTM+r

CTM

-r

ZTM-r

CTW

+r

ZTW+r

CTW

-r

ZTW-r

CTR

+r

ZTR+r

CTR

-r

ZTR-r

Large macroaggregates

Small macroaggregates

Microaggregates

Fig. 2 The contribution ofthe C in the different aggre-gates to total organic carbonof the soil sample in a) the0–5 cm and b) the 5–10 cmlayer. Soil with zero tillage(ZT) or conventional tillage(CT), maize monoculture(M) wheat monoculture (W)and rotation (R), with resi-dues (+r) or without residues(−r) at CIMMYT’s long-term tillage sustainabilitytrial at El Batán (Mexico)for 16 y. Bars are plus oneSTD

190 Plant Soil (2012) 355:183–197

December. For the treatments with wheat, the largestwater contents were found in the ZT+r treatments(Fig. 4). The soil water content was affected by residuemanagement and tillage from February to May andAugust to November.

Discussion

Aggregate distribution and stability, and C distributionin the aggregate fractions

Aggregate stability depends on various factors, i.e.texture, clay mineralogy, cation content, aluminum

and iron oxides and soil organic matter (Bronickand Lal 2005; Le Bissonnais 1996). Several stud-ies in different soils and climates showed a posi-tive correlation between soil organic matter andthe structural stability of both macro and micro-aggregates (Mohanty et al. 2007; Shukla et al. 2006;Wander and Bollero 1999). Organic matter stabilizesaggregates by at least two different mechanisms: (1)by increasing the inter-particle cohesion within aggre-gates thereby decreasing their breakdown and (2) byincreasing their hydrophobicity and thus decreasingtheir breakdown by slaking (Eynard et al. 2006; LeBissonnais 1996). Malhi and Lemke (2007) showed inan 8 year study that residue retention lead to a lower

(g C

m-2

h-1

)

January February March April May June July August Octobre November December January

b) Zero tillage

0.0

0.6

1.2

1.8

2.4

3.0

a) Conventional tillage

0.0

0.6

1.2

1.8

2.4

3.0Maize+residueMaize-residueWheat+residueWheat-residueRotation+residueRotation-residue

Fig. 3 The emission of CO2

from soil subjected to zerotillage and conventionaltillage, maize and wheatmonoculture and rotation,with residues and withoutresidues in CIMMYT’slong-term tillage sustain-ability trial at El Batán(Mexico) for 16 y. Barsare±one STD

Plant Soil (2012) 355:183–197 191

proportion of fine (<0.83 mm diameter) and a greaterproportion of large (>38.0 mm) dry aggregates, aswell as a larger MWD compared to treatmentswithout residue retention regardless of the type oftillage. They concluded that there was no benefi-cial effect on soil aggregation when tillage wasstopped. In contrast, our results confirmed earlierobservations at this site that ZT+r increased ag-gregate stability compared to CT (Fuentes et al.2009; Govaerts et al. 2009b). Soils with CT+r hada low MWD compared to plots with ZT+r, whichindicates that despite the incorporation of residuesthere was a negative effect on soil stability withtillage. The commonly used practice in the study

area of CT-r with monoculture of maize resulted inpoor structural stability.

The increased aggregate stability in ZT+r com-pared to ZT-r and CT practices resulted in increasedinfiltration (Govaerts et al. 2009b) and soil watercontent, especially during drought periods (Verhulstet al. 2011). Since water is an important limiting factorfor crop growth in the study area, the increased watercontent in ZT+r ensures high and stable yields for thismanagement practice compared to practices involvingtillage or ZT-r (Govaerts et al. 2005; Verhulst et al.2011).

Fuentes et al. (2009) evaluated chemical and phys-ical soil quality in 2003 and 2004 in the same

(%)

January February March April May June July August Octobre November December January

b) Zero tillage

0

5

10

15

20

25

30

35

a) Conventional tillage

0

5

10

15

20

25

30

35 Maize+residueMaize-residueWheat+residueWheat-residueRotation+residueRotation-residue

Fig. 4 Water content of soilsubjected to zero tillage andconventional tillage, maizeand wheat monoculture androtation, with residues andwithout residues in CIM-MYT’s long-term tillagesustainability trial at ElBatán (Mexico) for 16 y.Bars are±one STD

192 Plant Soil (2012) 355:183–197

experiment. They reported that ZT+r treatments had ahigher MWD than the ZT−r and CT treatments. In thispaper, a detailed study of aggregation is made andlinked to soil C stocks and CO2 emissions. Six et al.(1999) reported that CT causes disruption of soilaggregates, especially macro-aggregates (>0.25 mm).Our results showed that small macro-aggregates(0.25–1 mm) were more abundant in soils with CTand with or without residue than large macro-aggregates and micro-aggregates (Fig. 1). It can behypothesized that the negative effect of tillage wasgreater for large macro-aggregates than small macro-aggregates. The abundance of small macro-aggregatesmight be the result of the breaking up of large macro-aggregates and/or the physical and chemical character-istics of small macro-aggregates makes them moreresistant to break up by tillage.

Six et al. (2002) found a greater accumulation oforganic C in the top-soil of systems with ZT comparedto CT due to a better preservation of aggregates in ZT.The C not exposed is longer retained in the soil (Six etal. 2004a). It has been reported that the stability of asoil can be related to the proportion of large macro-aggregates, normally containing most of the C in thesoil (Six et al. 2004b). In our study, ZT and residueretention increased the proportion of large macro-aggregates in most of the treatments.

The ZT+r treatment had the highest proportion ofwater stable large macro-aggregates, organic C andMWD (Fig. 1). A possible explanation for the highproportion of large macro-aggregates in treatmentswith residue retention is that they are formed aroundfresh organic matter, while micro-aggregates containolder organic matter (Balesdent et al. 2000; Denef etal. 2001a). In the majority of the treatments, the smallmacro-aggregates contained less C than the largemacro-aggregates (Table 2). This behavior can beexplained by the concept of aggregate hierarchy(Oades 1984; Tisdall and Oades 1982) which statedthat large macro aggregates tend to be richer in organicmatter compared to smaller aggregates because freshorganic matter is the precursor in the formation ofmacro-aggregates. Elliott (1986) showed that increas-ing organic C content is related to increasing aggregatesize. It is important to note that the organic matter inmacro-aggregates is labile while the one in smalleraggregates is more stable. Therefore, tillage operationsgenerate a larger loss of organic matter in large macro-aggregates than in small macro-aggregates.

The proportion of small macro-aggregates wasgreater than that of the other two aggregate frac-tions. Consequently, the small macro-aggregatescontributed more C to the total C. However, inthe 0–5 cm layer of treatments with monocultureof wheat with ZT+r, the greatest contribution of Cto the total C was from the large macro-aggregates(Fig. 2). Tisdall and Oades (1982) explained thatroots are involved in stabilizing macro-aggregates.Therefore, agricultural management practice partlycontrols the formation of macro-aggregates becauseit influences crop growth. Gregory (2006) showedthat wheat roots promoted aggregate formation to alarger extent than maize roots, which was alsoreflected in the results of this study.

The proportion of the micro-aggregates in all treat-ments was small and they had the lowest organic Ccontent. However, micro-aggregate formation (Gale etal. 2000; Six et al. 1998) and micro-aggregates withinthe macro-aggregates (Denef et al. 2001b; Six et al.2000) can play an important role in C storage andstabilization in the long term. The formation ofmicro-aggregates occurs in advanced stages of organicC decomposition, so the organic matter in the micro-aggregates is more stable or recalcitrant compared tothe organic C found in other aggregates, thereby fa-voring aggregate stability and C retention (Six et al.2004a). Additionally, the organic C is physically pro-tected in the micro-aggregates within macro-aggregates(Denef et al. 2004).

Stewart et al. (2008) stated that the C sequestrationcapacity of a soil is determined mainly by the protec-tion of C in the aggregates. Soil C stocks change withtillage and management practices (Govaerts et al.2009a). Fuentes et al. (2010) reported for the sameexperiment as this study that the SOC content inthe 0–10 cm layer was affected by tillage and resi-due management. The highest SOC content wasfound in the 0–5 cm layer of the ZT+r comparedother treatments (Table 3). The SOC stock (calculat-ed on equivalent soil mass basis) in the 0–10 cmshowed a similar tendency (Table 3). The soils withZT+r (both for monoculture and rotation), showedhigher percentages of SOC and SOC stock thanCT+r and CT−r (Table 3). Consequently, the com-bination of ZT with residue retention is what makesaggregates more stable, protects C and thus increasesC sequestration and not zero tillage or residue reten-tion separately.

Plant Soil (2012) 355:183–197 193

CO2 fluxes and soil water

The CO2 emissions in-situ varied with time. An increasein the emissions of CO2 was observed when fertilizerwas applied. However, it also coincided with cropgrowth and the heaviest rainfall, which might have alsocontributed to increased emissions. Therefore, it is dif-ficult to pinpoint the exact reason why the emission ofCO2 increased right after fertilizer application (i.e. fer-tilizer, rainfall or plant growth) as no unfertilized plotswere included in the experiment. The CO2 emissionsincreased after harvest. This has been reported andattributed to an increased activity of heterotrophicorganisms associated with the decomposition of freshroots (Franzluebbers et al. 1995).

In soil cultivated with maize, tillage increased theemissions of CO2 compared to ZT. At ploughing, thesoil disturbance in the CT treatment increased theemissions of CO2 as compared to the ZT treatments.Ussiri and Lal (2009) suggested that mechanical till-age accelerated the decomposition of C. Our studyindicated that ZT increased the SOC pool probablyby slowing the decomposition of new maize andwheat-derived C and protecting older more recalcitrantC against decomposers.

Several experiments have shown that the top soilwith ZT and retention of residues had a larger SOC

content and lower decomposition rates of SOC thanthe top soil with CT leading to a reduction in theemissions of CO2 for ZT (Diekow et al. 2005;Fuentes et al. 2009; Jantalia et al. 2007; Six et al.2002). The decomposition of plant residues is slowerin the ZT+r treatment due to reduced soil-residuecontact. Mechanical tillage stimulates decompositionof organic material by aerating the soil, breaking upaggregates and incorporating crop residues into thesoil, thereby increasing the contact between soilmicroorganisms and crop residue (Ussiri and Lal2009). In ZT, the contact between micro-organismsand residues is delayed and organic material becomesphysically protected in aggregates that are not brokenup by tillage (Stewart et al. 2008). Additionally, soilaggregates are more stable in ZT than in CT (Ball et al.1999; Fuentes et al. 2009; Govaerts et al. 2006).Consequently, SOC is better protected in stable aggre-gates against microbial decomposition than in lessstable aggregates as in CT. Better protected SOCreduces C mineralization and thus CO2 emissions.

The soil in ZT+r had greater soil water contentsthan in CT with or without residue retention(Shaver et al. 2002; Ussiri and Lal 2009). Cropresidue on the soil surface forms a barrier againstevaporation thereby maintaining the water storedin the plant root zone (Lichter et al. 2008). The

Table 3 The soil organic carbon (g kg−1 dry soil) in soilscultivated with maize and wheat, subjected to: ZTM+r/−r0Zerotillage monoculture with residues or without, ZTR+r/−r0Zerotillage rotation with residues or without, CTM+r/−r0Conventional

tillage monoculture with residues or without and CTR+r/−r0Con-ventional tillage rotation with or without residues, in CIMMYT’slong-term tillage sustainability trial, El Batán (Mexico) for 16 years(Fuentes et al. 2010)

Soil organic carbon (g kg−1) Soil organic carbon stock asequivalent soil mass (g m−2)

Maize Wheat Maize Wheat

Treatment 0–5 cm 5–10 cm 0–5 cm 5–10 cm 0–10 cm 0–10 cm

ZTM+r 24.1 14.4 23.4 14.8 2220.1 2422.4

ZTR+r 22.0 14.9 22.6 15.0 2292.3 2433.1

CTM+r 17.6 16.4 17.3 16.0 1884.5 1921.4

CTR+r 17.3 16.2 19.3 16.1 2013.7 1787.6

ZTM-r 12.7 12.3 17.9 13.8 1498.7 2116.1

ZTR-r 14.2 12.6 14.6 14.0 1772.8 1730.6

CTM-r 13.4 13.0 15.5 15.0 1478.3 1651.4

CTR-r 13.2 13.0 14.5 14.1 1486.1 1491.1

LSD a 2.0 1.7 2.7 2.0 257.8 306.5

a P<0.05 level based on least square difference grouping (LSD)

194 Plant Soil (2012) 355:183–197

differences in water content were due mainly tothe residue management while emissions of CO2

were mainly due to tillage. Lee et al. (2009) founda linear relation between CO2 emission and soil-water content in plots with CT and cultivated withmaize, but no relationship was found for sunflowerand chickpea. They concluded that CO2 emissionis highly variable and depends on the crop, tillageand season. Our study showed that the relationbetween CO2 emissions and soil water-content var-ied in each production system. In CT+r withmaize or wheat both were highly correlated (P<0.001, r2 0.70). In the ZT+r treatments, CO2 emis-sion and soil water content were significantly cor-related (P<0.05) with r2 of 0.50 similar to that ofCT-r and in ZT-r with r2 of 0.40.

Conclusion

Reduction in soil disturbance combined with residueretention increased the C retained in the small and largemacro-aggregates of the top soil due to greater aggregatestability and reduced the emissions of CO2 comparedwith conventional tillage without residues retention andmaize monoculture (a cultivation system normally usedin the central highlands of Mexico). The retention ofresidues increased the C in aggregates of the top-soil andthe reduction in soil disturbance resulted in a decrease inemissions of CO2. A crop rotation of maize and wheatreduced emissions of CO2 as compared to wheat mono-culture. Zero tillage with residue retention and mono-culture of maize or rotation with wheat was the mostattractive system to maximize C retention in the aggre-gates of the top-soil, under the experimental conditions.Our results showed that the retention of C in the top-soildepends mainly on the C content in the small and largemacro-aggregates of the 0–10 cm soil layer while ag-gregate stability depends primarily on the large macro-aggregates.

Acknowledgments Mariela Fuentes received a PhD fellow-ship from CONACYT. Fieldwork was done in a long-term trialestablished by Dr. R.A. Fisher at CIMMYT’s El Batán researchstation. The research was supported by CIMMYT and its stra-tegic donors and forms part of the strategic research networkdeveloped in the frame of MasAgro (Modernización sustentablede la agricultura tradicional), component ‘Desarrollo sustent-able con el productor’. The authors thank M. Martinez, A.Martinez, and H. González-Juárez for help with the field work.

References

Allmaras RR, Linden DR, Clapp CE (2004) Corn-residuetransformations into root and soil carbon as related to nitro-gen, tillage, and stover management. Soil Sci Soc Am J68:1366–1375

Angers DA, Recous S, Aita C (1997) Fate of carbon and nitrogenwater-stable aggregates during composition of 13C 15N-labelled wheat straw in situ. Eur J Soil Sci 48:295–300

Balesdent J, Chenu C, Balabane M (2000) Relationship of soilorganic matter dynamics to physical protection and tillage.Soil Till Res 53:215–230

Ball B, Scott A, Parker JP (1999) Field N2O, CO2 and CH4

fluxes in relation to tillage, compaction and soil quality inScotland. Soil Till Res 53:29–39

Barthès B, Azontonde A, Boli BZ, Prat C, Roose E (2000)Field-scale run-off and erosion in relation to topsoil aggre-gate stability in three tropical regions (Benin, Cameroon,Mexico). Eur J Soil Sci 51:485–495

Bayer C, Mielniczuk J, Amado TJC, Martin-Neto L, FernandesSV (2000) Organic matter storage in a sandy clay loamAcrisol affected by tillage and cropping systems in south-ern Brazil. Soil Till Res 54:101–109

Black AL, Tanaka DL (1997) A conservation tillage-cropping study in the Northern Great Plains of theUnited States. In: Paul EA, Paustian K, Elliot ET,Cole CV (eds) Soil organic matter in temperate agro-ecosystems long-term experiments in North America.CRC, New York, pp 335–342

Blanco-Canqui H, Lal R (2007) Soil structure and organiccarbon relationships following 10 years of wheat strawmanagement in no-till. Soil Till Res 95:240–254

Blanco-Canqui H, Lal R (2008) No-tillage and soil-profile car-bon sequestration: An on farm assessment. Soil Sci SocAm J 72:693–701

Bronick CJ, Lal R (2005) Soil structure and management: areview. Geoderma 124:3–22

Chatskikh D, Olesen JE, Hansen EM, Elsgaard L, Petersen BM(2008) Effects of reduced tillage on net greenhouse gasfluxes from loamy sand soil under winter crops inDenmark. Agr Ecosyst Environ 128:117–126

Denef K, Six J, Bossuyt H, Frey SD, Elliott ET, Merckx R,Paustian K (2001a) Influence of dry-wet cycles on theinterrelationship between aggregate, particulate organicmatter, and microbial community dynamics. Soil BiolBiochem 33:1599–1611

Denef K, Six J, Paustian K, Merckx R (2001b) Importance ofmacroaggregate dynamics in controlling soil carbon stabi-lization: short-term effects of physical disturbance inducedby dry-wet cycles. Soil Biol Biochem 33:2145–2153

Denef K, Six J, Merckx R, Paustian K (2004) Carbon seques-tration in microaggregates of no-tillage soils with differentclay mineralogy. Soil Sci Soc Am J 68:1935–1944

Denef K, Zotarellia L, Boddey RM, Six J (2007) Microaggregate-associated carbon as a diagnostic fraction for management-induced changes in soil organic carbon in two Oxisols. SoilBiol Biochem 39:1165–1172

Diekow J, Mielniczuk J, Knicker H, Bayer C, Dick DP, Kogel-Knabner I (2005) Soil C and N stocks as affected bycropping systems and nitrogen fertilization in a southern

Plant Soil (2012) 355:183–197 195

Brazil Acrisol managed under no-tillage for 17 years. SoilTill Res 81:87–95

Dolan MS, Clapp CE, Allmaras RR, Baker JM, Molina JA(2006) Soil organic carbon and nitrogen in a Minnesotasoil as related to tillage, residue and nitrogen management.Soil Till Res 89:221–231

Elder JW, Lal R (2008) Tillage effects on gaseous emissionsfrom an intensively farmed organic soil in North CentralOhio. Soil Till Res 98:45–55

Elliott ET (1986) Aggregate structure and carbon, nitrogen, andphosphorus in native and cultivated soils. SSSAJ 50:627–633

Eynard A, Schumacher TE, Lindstrom MJ, Malo DD, Kohl RA(2006) Effects of aggregate structure and organic C onwettability of Ustolls. Soil Till Res 88:205–216

Franzluebbers AJ, Hons FM, Zuberer DA (1995) Tillage andcrop effects on seasonal dynamics of soil CO2 evolution,water content, temperature, and bulk density. Appl SoilEcol 2:95–109

Fuentes M, Govaerts B, De León F, Hidalgo C, Sayre KD,Etchevers J, Dendooven L (2009) Fourteen years of apply-ing zero and conventional tillage, crop rotation and residuemanagement systems and its effect on physical and chem-ical soil quality. Eur J Agron 30:228–237

Fuentes M, Govaerts B, Hidalgo C, Etchevers J, Gónzalez-Martín I, Hernández-Hierro JM, Sayre KD, Dendooven L(2010) Organic carbon and stable 13C isotope in conserva-tion agriculture and conventional systems. Soil BiolBiochem 42:551–557

Gale WJ, Cambardella CA, Bailey TB (2000) Root-derivedcarbon and the formation and stabilization of aggregates.Soil Sci Soc Am J 64:201–207

Govaerts B, Sayre KD, Deckers J (2005) Stable high yields withzero tillage and permanent bed planting? Field Crops Res94:33–42

Govaerts B, Sayre K, Deckers J (2006) A minimum data set forsoil quality assessment of wheat and maize cropping in thehighlands of Mexico. Soil Till Res 87:163–174

Govaerts B, Verhulst N, Castellanos-Navarrete A, Sayre KD,Dixon J, Dendooven L (2009a) Conservation agricultureand soil carbon sequestration; between myth and farmerreality. Crit Rev Plant Sci 28:97–122

Govaerts B, Sayre KD, Goudeseune B, De Corte P, Lichter K,Dendooven L, Deckers J (2009b) Conservation agricultureas a sustainable option for the central Mexican highlands.Soil Till Res 103:222–230

Gregory PJ (2006) Roots, rhizosphere and soil: the route to abetter understanding of soil science? Eur J Soil Sci 57:2–12

Halvorson AD, Wienhold BJ, Black AL (2002) Tillage, nitro-gen, and cropping systems effects on soil carbon seques-tration. Soil Sci Soc Am J 66:906–912

Hendrix PF, Han CR, Groffman PM (1988) Soil respiration inconventional and no-tillage agroecosystems under differentwinter cover crop rotations. Soil Till Res 12:135–148

Huggins DR, Allmaras RR, Clapp CE, Lamb JA, Randall GW(2007) Corn soybean sequence and tillage effects on soilcarbon dynamics and storage. Soil Sci Soc Am J 71:255–258

Hutchinson JJ, Campbell CA, Desjardins RL (2007) Someperspectives on carbon sequestration in agriculture. AgricMeteorol 142:288–302

Institute SAS (1994) SAS user’s guide. SAS Inst, Cary

Jantalia CP, Resck DVS, Alves BJR, Zotarelli L, Urquiaga S,Boddey RM (2007) Tillage effect on C stocks of a clayeyOxisol under a soybean-based crop rotation in the BrazilianCerrado region. Soil Till Res 95:97–109

Kemper W, Rosenau R (1986) Aggregate stability and sizedistribution. In: Klute A, Campbell G, Jackson R,Mortland M, Nielsen D (eds) Methods of soil analysis.Part I. ASA and SSSA, Madison, pp 425–442

Le Bissonnais Y (1996) Aggregates stability and assessment ofsoil crustability and erodibility: I. Theory and methodolo-gy. Eur J Soil Sci 47:425–437

Lee J, Hopmans JW, van Kessel C, King A, Evatt KJ, Louie D,Rolston D, Six J (2009) Tillage and seasonal emissions ofCO2, N2O and NO across a seed bed and at the field scale in aMediterranean climate. Agric Ecosyst Environ 129:378–390

Lichter K, Govaerts B, Six J, Sayre KD, Deckers J, Dendooven L(2008) Aggregation and C and N contents of soil organicmatter fractions in the permanent raised-bed planting systemin the Highlands of Central Mexico. Plant Soil 305:237–252

Limón-Ortega A, Sayre K, Drijber R, Francis C (2002) Soilattributes in a furrow-irrigated bed planting system innorthwest México. Soil Till Res 63:123–132

Malhi SS, Lemke R (2007) Tillage, crop residue and N fertilizereffects on crop yield, nutrient uptake, soil quality andnitrous oxide gas emissions in a second 4-yr rotation cycle.Soil Till Res 96:269–283

Mohanty M, Painuli DK, Misra AK, Ghosh PK (2007) Soilquality effects of tillage and residue under rice–wheatcropping on a Vertisol in India. Soil Till Res 92:243–250

Nouchi I, Yonemura S (2008) CO2, CH4 and N2O fluxes fromsoybean and barley double cropping in relation to tillage inJapan. Phyton-Ann Rei Bot 45:327–338

Oades JM (1984) Soil organic matter and structural stability:mechanisms and implications for management. Plant Soil76:319–337

Oorts K, Merchkx R, Gréhan E, Lebreuche J, Nicolardot B(2007) Determinants of annual fluxes of CO2 and N2O inlong-term no-tillage and conventional tillage systems innorthern France. Soil Till Res 95:133–148

Osozawa S, Hasegawa S (1995) Daily and seasonal changes insoil carbon dioxide concentration and flux in Andisol. SoilSci 160:117–124

Rasmussen PE, Smiley RW (1997) Soil carbon and nitrogenchange in log-term agricultural experiments at Pendleton,Oregon. In: Paul EA, Paustian K, Elliot ET, Cole CV (eds)Soil organic matter in temperate agroecosystems long-termexperiments in North America. CRC, NewYork, pp 353–360

Schlesinger WH (1999) Carbon sequestration in soils. Science248:2095

Shaver TM, Peterson GA, Ahuja LR, Westfall DG, Sherrod LA,Dunn G (2002) Surface soil properties after twelve years ofdryland no-till management. Soil Sci Soc Am J 66:1292–1303

Shukla MK, Lal R, Ebinger M (2006) Determining soil qualityindicators by factor analysis. Soil Till Res 87:194–204

Six J, Elliott ET, Paustian K, Doran JW (1998) Aggregation andsoil organic matter accumulation in cultivated and nativegrassland soils. Soil Sci Soc Am J 62:1367–1377

Six J, Elliott ET, Paustian K (1999) Aggregate and soil organicmatter dynamics under conventional and no-tillage sys-tems. Soil Sci Soc Am J 63:1350–1358

196 Plant Soil (2012) 355:183–197

Six J, Elliott ET, Paustian K (2000) Soil macroaggregate turnoverand microaggregate formation: a mechanism for C seques-tration under no-tillage agriculture. Soil Biol Biochem32:2099–2103

Six J, Feller C, Denef K, Ogle SM, de Morales Sá JC, Albrecht A(2002) Soil organic matter, biota and aggregation in temper-ate and tropical soils-effects of no tillage. Agronomie22:755–775

Six J, Ogle SM, Breidt F, Conant R, Mosier A, Paustian K(2004a) The potential to mitigate global warming withno-tillage management is only realized when practiced inthe long term. Glob Change Biol 10:144–160

Six J, Bossuyt H, Degryze S, Denef K (2004b) A history ofresearch on the link between (micro)aggregates, soil biota,and soil organic matter dynamics. Soil Till Res 79:7–31

Soil Survey Staff (2003) Keys to Soil Taxonomy. United StatesDepartment of Agriculture, Natural Resources ConservationService. Washington DC, pp 332

Stewart C, Plante A, Paustian K, Conant R, Six J (2008) SoilCarbon Saturation: Linking concept and measurable car-bon pools. SSSAJ 72:379–392

Tantely M, Razafimbelo AA, Oliver R, Chevallier T, Chapuis-Lardy L, Feller C (2008) Aggregate associated-C andphysical protection in a tropical clayey soil under Malagasy

conventional and no-tillage systems. Soil Till Res 98:140–149

Tisdall JM, Oades JM (1982) Organic matter and water-stableaggregates in soils. J Soil Sci 62:141–163

Ussiri DAN, Lal R (2009) Long-term tillage effects on soilcarbon storage and carbon dioxide emissions in continuouscorn cropping systems from an alfisol in Ohio. Soil Till Res104:39–47

Verhulst N, Nelissen V, Jespers N, Haven H, Sayre KD, Raes D,Deckers J, Govaerts B (2011) Soil water content, maizeyield and its stability as affected by tillage and crop residuemanagement in rainfed semi-arid highlands. Plant Soil344:73–85

Wander MM, Bollero GA (1999) Soil quality assessment oftillage impacts in Illinois. Soil Sci Soc Am J 63:961–971

West TA, Post WM (2002) Soil organic carbon sequestrationrates by tillage and crop rotation: A global data analysis.Soil Sci Soc Am J 66:1930–1946

Wright AL, Hons FM (2005) Soil Carbon and nitrogen storagein aggregates from different tillage and crop regimes. SoilSci Soc Am J 69:141–147

Yang XM, Kay BD (2001) Rotation and tillage effects on soilorganic carbon sequestration in a typic Hapludalf in south-ern Ontario. Soil Till Res 59:107–114

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