Plant Soil
DOI 10.1007/s11104-011-1092-4
REGULAR ARTICLE
Conservation agriculture, increased organic carbon
in the top-soil macro-aggregates and reduced
soil 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
# Springer Science+Business Media B.V. 2011
Abstract
Background and aims Conservation agriculture, the
combination of minimal soil movement (zero or reduced tillage), crop residue retention and crop rotation, might have the potential to increase soil organic
C 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 without (−r) crop residues). Samples were taken from the
0–5 and 5–10 cm soil layers and separated in microaggregates (< 0.25 mm), small macro-aggregates (0.25
to 1 mm) and large macro-aggregates (1 to 8 mm). The
carbon content of each aggregate fraction was
determined.
Results Zero tillage combined with crop rotation and
crop residues retention resulted in a higher proportion
of macro-aggregates. In the 0–5 cm layer, plots with a
crop rotation and monoculture of maize and wheat in
ZT+r had the greatest proportion of large stable macro-aggregates (40%) and highest mean weighted diameter (MWD) (1.7 mm). The plots with CT had the
largest proportion of micro-aggregates (27%). In the
5–10 cm layer, plots with residue retention in both CT
and ZT (maize 1 mm and wheat 1.5 mm) or with
monoculture of wheat in plots under ZT without
Responsible Editor: Johan Six.
M. Fuentes : F. De León
Laboratorio 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. Etchevers
Laboratorio de Fertilidad,
Colegio de Postgraduados, IRENAT,
Km 36.5 Carretera México-Texcoco,
Montecillo CP 56230, Mexico
A. Guerrero
Laboratorio de Suelos, Plantas y Aguas, Campus Tabasco,
Colegio de Postgraduados,
Supera-Anuies, Mexico
L. Dendooven
Cinvestav,
Av. Instituto Politécnico Nacional 2508,
México C.P. 07360 D.F., Mexico
N. Verhulst : B. Govaerts (*)
International Maize and Wheat Improvement
Centre (CIMMYT),
Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico
e-mail: [email protected]
Plant Soil
residues (1.4 mm) had the greatest MWD. The 0–
10 cm soil layer had a greater proportion of small
macroaggregates compared to large macro-aggregates
and micro-aggregates. In the 0–10 cm layer of soil
with residues retention and maize or wheat, the greatest C content was found in the small and large macroaggregates. The small macro-aggregates contributed
most C to the organic C of the sample. For soil
cultivated with maize, the CT treatments had significantly higher CO2 emissions than the ZT treatments.
For soil cultivated with wheat, CTR-r had significantly
higher CO2 emissions than all other treatments.
Conclusion Reduction in soil disturbance combined
with residue retention increased the C retained in the
small and large macro-aggregates of the top soil due to
greater aggregate stability and reduced the emissions of
CO2 compared with conventional tillage without residues
retention and maize monoculture (a cultivation system
normally used in the central highlands of Mexico).
Keywords Aggregate stability . Soil CO2 emissions .
Zero tillage
Introduction
Declining SOC contents in agro-ecosystems are important in the global C budget. Agronomic practices, including tillage, residues management and crop rotation,
are crucial determinants of the quantity of carbon
retained in the soil (Allmaras et al. 2004; Fuentes et al.
2009; West and Post 2002). Generally, the top soil of
fields under ZT with residue retention has a larger soil
organic carbon (SOC) content and a lower soil organic
matter (SOM) decomposition rate than soil with conventional tillage (CT) (Diekow et al. 2005; Fuentes et al.
2009; Jantalia et al. 2007; Six et al. 2002). When considering the whole soil profile, results have been less
consistent (Govaerts et al. 2009a). Some research
showed that soils with zero or reduced tillage had greater SOC stocks than soils with CT in the first 30 cm soil
layer (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 the
opposite (Black and Tanaka 1997; Blanco-Canqui and
Lal 2008) and some research found no significant differences (Angers et al. 1997; Dolan et al. 2006). Crop
rotation can affect SOC because the SOC content
depends 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 precursors of the SOM pool, returning more crop residues
to the soil is associated with an increase in SOC concentration (Govaerts et al. 2009a).
The SOC distribution in soil aggregates largely determines the sequestration or release of C within a given
agricultural system (Blanco-Canqui and Lal 2007; Six et
al. 2000). Six et al. (2002) stated that a greater accumulation of SOC in ZT than in CT can be related to a lack of
soil disturbance and a better preservation of aggregates in
ZT compared to CT. Soil with ZT and crop residue
retention has a greater proportion of macro-aggregates
(>250 μ) with a higher organic C content than CT
without residues (Tantely et al. 2008). Tantely et al.
(2008) also reported that the macro-aggregates contained
more organic C than the micro-aggregates (<250 μ). The
differences in C concentration within the fraction of
microaggregates occluded in macroaggregates between
management systems can be linked to differences in the
amount, stability and turnover of the macroaggregateoccluded-microaggregates (Denef et al. 2007). The
macroaggregate-occluded-microaggregates have a
slower turnover due to the protective environment of
the macroaggregates. This slower turnover allows greater protection of the coarse particulate organic matter
(cPOM) and a greater stabilization of mineral-bound C
decomposition products in the macroaggregateoccluded-microaggregates (Denef et al. 2007). Macroaggregates have a higher particulate organic matter
(POM) content than other soil aggregates. Since POM
is more susceptible to degradation than the other organic
matter fractions, the organic C content in the macroaggregates is an indicator of the stability of the aggregates and the retention or loss of C as affected by
different management practices.
The SOC content and its distribution in the soil
profile as a result of management practices affect
emissions of CO2 (Osozawa and Hasegawa 1995).
Different effects of soil management on emissions of
CO2 have been reported. Some studies showed that
soil with CT emitted more CO2 than the same soils
with ZT systems (Chatskikh et al. 2008; Hutchinson et
al. 2007; Oorts et al. 2007; Schlesinger 1999; Ussiri
and Lal 2009). However, Hendrix et al. (1988) found
the opposite. Other authors reported no significant
differences in emissions of CO2 between soils with
ZT and CT (Elder and Lal 2008; Nouchi and
Yonemura 2008).
Plant Soil
Determining the effect of management practices on
SOC content, C distribution in the aggregate fractions
and the emissions of CO2 from the soil will help to
understand the processes that affect C sequestration.
Therefore, the objectives of this research were to determine the effect of 16 y of ZT compared to CT
combined with different crop rotations (monoculture
and rotation) and crop residue management (with and
without residues) on the retention and emission of C of
the soil considering: i) aggregate size distribution and
stability, ii) the contribution of C within different
aggregate fractions of the top-soil and iii) the emissions of CO2 from the soil.
Materials and methods
Experimental site
The study was conducted at CIMMYT’s experimental
station in El Batán, situated in the semi-arid, subtropical 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 Staff
2003) with a particle size distribution of 380, 370 and
250 gkg-1 of clay, silt and sand respectively. The
station has an average temperature of 14°C with
600 mm y-1 rainfall, with about 520 mm falling between May and October. The rainy season has typically
short, intense rain showers followed by dry spells and
evapotranspiration 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 of
7.5×22 m. The slope was 0.3% (north to west). Thirty
two treatments were applied in a randomized complete
block design with two repetitions (blocks). In this
research, only the sixteen core treatments were included (so thirty two plots were sampled). These treatments have not been changed since the start of the
experiment and consist of combinations tillage practice (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 (Triticum
aestivum L.) [W] or rotation of both crops [R]). Each
phase of the rotation was present each year.
The soil preparation in CT consisted of harrowing at
20 cm depth, with a disc harrow starting some days after
harvest and repeated when needed for weed control (at
least once) during the dry season. To prepare the seed
bed a spike tooth harrow was used once. The ZT plots
were sown directly with maize or wheat using an
Almaco® seeder and an Aitcheson® machine respectively, both using disc openers for seed placement.
Sowing was done in May and the harvest in October
for wheat and November for maize. In the treatments
with residue retention (+r) all the residues of the former
crop were kept on the field; in CT plots the residues
were incorporated by tillage and in ZT they were left on
the soil surface. In the –r treatments (residues removed),
most of the aerial residues were removed simulating
farmers’ practice. Both crops were fertilized at the rate
of 120 kg de N ha−1 using urea, with all N applied to
wheat at the first node growth stage (broadcast) and to
maize at the 5–6 leaf stage (surface-banded).
Soil sampling and analysis
Undisturbed samples were collected at four locations of
each plot from the 0–5 cm and 5–10 cm layer in
September 2007. The C content in the aggregates was
measured with a C autoanalyzer (TOC-5050A –Total
Organic Carbon, Shimadzu©). Inorganic C in the soil
was negligible so total C was considered as the organic C.
Soil samples (0.1 to 0.8 g) grounded and sieved through a
0.15 mm mesh were subjected to a dry combustion at
900°C for 3 to 4 min. The emitted CO2 was registered by
means of an infrared sensor and considered as organic C.
Wet aggregate stability was determined on a 20 g
dried 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 a
mesh opening of 4.75, 2.00, 1.00, 0.50, 0.25 and
0.05 mm, submerged in a cylinder of distilled water
and driven up and down at 60 cycles per minute. The
fractions held in the sieves were collected and dried at
105°C for 18 h. The proportion of aggregates of different sizes and the mean weighted diameter (MWD) per
sample were determined. The amount of organic C was
determined in all fractions obtained with a C autoanalyzer. The C contribution of each fraction to the total C
of the sample was calculated by grouping the fractions
by size as follows: (a) <0.25 mm (micro-aggregates), (b)
Plant Soil
0.25 to 1 mm (small macro-aggregates) and (c) 1 to
8 mm (large macro-aggregates).
In situ emission of CO2, soil water content, air
temperature and precipitation
The emissions of CO2 and soil water content were determined monthly, while additional measurements were
done when soil was fertilized, tilled or planted. Eight
samples were collected along two virtual lines drawn in
the central part of the plot (at 2.5 m from each border) at
intervals of 4 m. The CO2 emission was measured with a
portable non-dispersive infrared gas analyzer (EGM-4
CO2) and a soil respiration chamber which contains an
air suction pump. The portable chambers were placed on
the bare soil, i.e. no plants. An internal solenoid (built
into the CFX-1 Soil Respiration Chamber) switches the
gas stream from reference and analysis for 30 s intervals.
The increase in the concentration of CO2 in the air above
the soil was calculated giving the concentration of CO2 in
mg kg−1 and rates of CO2 in g m−2 h−1. Soil water content
was determined gravimetrically at 0–20 cm.
Statistical analysis
The experiment was a randomized complete block
(RCB) design with two replicates. The effect of treatment on SOC content of the total sample and of the
aggregates and its distribution were analyzed statistically with the General Linear Model (GLM) procedure
for analysis of variance with significance set at 5%
(SAS Institute 1994). The following class factors were
considered: rotation, tillage type, residue management
and repetition. The CO2 and water data of the experiment were analyzed using a linear mixed model
where the repeated measurements of each treatment
in every month were modeled with an unstructured
variance-covariance matrix using PROC MIXED
(SAS Institute 1994).
Results
monoculture of wheat (10.5%), monoculture of maize
(7.5%) and crop rotation (11.7%). The proportion of
small 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 greatest
percentage of micro-aggregates was found in CTM-r
(36.0%) and the lowest in ZTR+r and ZTM+r (both
19%). 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 highly
significant (P<0.01) effect on the percentage of large
macro-aggregates in soil under maize and maize-wheat
rotation and on the percentage of micro-aggregates in
soil under monoculture of maize. Tillage had a highly
significant (P<0.01) effect on the percentage of large
macro-aggregates for all rotations and on the percentage
of small and micro-aggregates in soil under monoculture
of maize. However, there was no significant interaction
effect of residues and tillage on the proportion of different aggregates size (Table 1).
In the 5–10 cm layer, the ZTW+r, CTW+r and
ZTW-r treatments had the highest proportion of large
macro-aggregates (38%, 35% and 34% respectively).
Conventional tillage without residues under monoculture of maize and maize-wheat rotation and ZTM-r had
the lowest proportion of large macro-aggregates (11.5%,
12.5% and 13.7% respectively). All the treatments with
monoculture of maize (regardless residues management
or type of tillage) had the greatest proportion of small
macro-aggregates (average 62%) (Fig. 1b). The greatest
MWD was found in ZTW + r (1.53 mm), CTW + r
(1.40 mm) and ZTW-r (1.47 mm) and the lowest in
CTR-r (0.70 mm), CTM-r (0.71 mm) and ZTM-r
(0.72 mm) (Fig. 1b). Residue management had a significant (P<0.05) effect on large macro-aggregates for all
crop rotations and on small macro-aggregates in soil
with monoculture of wheat and rotation. Tillage significantly (P<0.05) affected the percentage of large macroaggregates in soil with wheat monoculture and maizewheat rotation. The interaction effect of residue management and tillage on the proportion of different aggregates size was not significant (Table 1).
Aggregate distribution and stability
In the 0–5 cm soil layer, the greatest proportion of large
macro-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
C distribution in the aggregate fractions
and contribution to total C
In the 0–5 cm layer of soil with residue retention
(regardless of crop rotation or tillage), the organic C
Plant Soil
a) 0-5 cm soil layer
80
2.5
Large macroaggregates
Small macroaggregates
Microaggregates
MWD
70
2.0
Aggregate distribution (%)
60
50
1.5
40
1.0
30
20
0.5
10
0
Mean weight diameter (mm)
Fig. 1 Aggregate distribution and mean weighted
diameter (MWD) in a) the
0–5 cm and b) 5–10 cm
layer. Soil with zero tillage
(ZT) or conventional tillage
(CT), maize monoculture
(M), wheat monoculture
(W) and rotation (R), with
residues (+r) or without
residues (−r) at CIMMYT’s
long-term tillage sustainability trial at El Batán
(Mexico) for 16 y. Bars are
plus one STD
0.0
R-r
ZT
r
R+
R-r
CT
ZT
r
R+
CT
r
W-
ZT
r
W+
r
WCT
ZT
r
W+
CT
r
-r
M-
ZT
M
CT
r
+r
M+
ZT
M
CT
b) 5-10 cm soil layer
80
2.5
70
2.0
50
1.5
40
1.0
30
20
0.5
10
Mean weight diameter (mm)
Aggregate distribution (%)
60
0.0
0
R-r
ZT
r
R-r
CT
r
r
R+
R+
ZT
CT
r
r
W-
W-
ZT
CT
r
W+
r
W+
ZT
CT
r
r
M-
M-
ZT
CT
r
M+
M+
ZT
CT
content in the large macro-aggregates was greater
(average 2.4%) than in soil where residue was
removed (average 1.3%), except in the CTR + r
treatment where it was similar (1.5%) to treatments
without residues and in the ZTW-r (2.1%) where it
was similar to treatments with residues (Table 2).
In the small macro-aggregates, the greatest organic
C was found for ZT+r regardless of crop rotations
(average 2.32%), while the lowest organic C was
found in soil without residues cultivated with
maize and wheat-maize in rotation (average 1.3%)
(Table 2). No differences between treatments were
found for the micro-aggregates. Residue management
had a significant (P<0.05) effect on C content in large
and small macro-aggregates for soil cultivated with
maize. Tillage had no clear effect on the C in the large
and small macro-aggregates. The interaction effect of
residue and tillage was only significant for the organic C
in the small macro-aggregates in soil cultivated with
maize (Table 1).
In the 5–10 cm soil layer, the organic C in the small
macro-aggregates was significantly greater in plots
when residue was retained (average 1.6%) than when
it was removed (average 1.3%) (Tables 1 and 2). No
such effect, however, was found in the microaggregates and large macro-aggregates, except in the
CT treatment with rotation where the C content in the
large macro-aggregates was higher when residue was
retained than when it was removed. Tillage had no
effect on the C in the large macro-aggregates and it
only affected significantly (P<0.05) the C content in
small macro-aggregates and micro-aggregates in soil
cultivated with wheat. The interaction between residue
and tillage significantly (P<0.05) affected C in large
and small macro-aggregates in treatments with monoculture of wheat (Table 1).
Plant Soil
Table 1 The effect of residue, tillage and their interaction on
the percentage of large macro-, small macro- and microaggregates, the C in large macro-, small macro- and microaggregates
and the contribution of the C in large macro-, small macro- and
microaggregates to the overall C content soils cultivated with
maize or wheat as monoculture, or a rotation of wheat and maize
in CIMMYT’s long-term tillage sustainability trial, El Batán
(Mexico) for 16 years
0–5 cm soil layer
Maize
Treatment
F value
5–10 cm soil layer
Wheat
P value
F value
Rotation
P value
Maize
Wheat
Rotation
F value
P value
F value
P value
F value
P value
F value
P value
0.0053
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
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
0.94
0.6549
0.89
0.3629
0.03
0.8644
0.01
0.9116
1.80
0.2049
2.28
0.1423
0.0186
R*T
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
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
0.5945
2.53
0.1230
2.00
0.1828
0.60
0.4520
0.12
0.7303
Percentage microaggregates
Residue (R)
15.54
0.0020
0.30
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
0.93
0.3460
0.01
0.9043
1.70
0.1994
0.27
0.6117
5.39
0.0309
0.79
0.3779
0.0001
R*T
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
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
0.5619
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
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
0.0293
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
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
0.0048
0.48
0.5275
0.82
0.4166
1.08
0.3185
Contribution of the C in the small macroaggragetes
Residue (R)
0.05
0.8326
2.56
0.1846
11.87
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
Plant Soil
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, or
a rotation of wheat and maize at CIMMYT’s long-term tillage sustainability trial El Batán (Mexico) for 16 years
Maize monoculture
CT a
ZT
b
Wheat monoculture
LSD
c
F value P value CT
Maize-Wheat crop rotation
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
0.0163
0.3756 0.2378
0.0025
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,
residue removed
b
ZT: zero tillage,
c
LSD: Least significant difference (P<0.05), d +R: crop residue retained, e −R: crop
In both the 0–5 cm and the 5–10 cm layer, the C in
the small macro-aggregates contributed most to the
total organic C for all treatments (average 55%), except for ZTW+r where the major contribution came
from the large macro-aggregates (average 50%)
(Fig. 2). In maize monoculture and residue removal
(ZTM-r and CTM-r) and CTR-r, the micro-aggregates
contributed more C to the total organic C than in the
other treatments (average of 0 to 10 cm CTM-r 32.5%,
ZTM-r 29.5% and CTR-r 27.5% respectively) (Fig. 2).
Plant Soil
a) 0-5 cm soil layer
100
Large macroaggregates
Small macroaggregates
Microaggregates
80
SOC contribution to total C (%)
Fig. 2 The contribution of
the C in the different aggregates to total organic carbon
of the soil sample in a) the
0–5 cm and b) the 5–10 cm
layer. Soil with zero tillage
(ZT) or conventional tillage
(CT), maize monoculture
(M) wheat monoculture (W)
and rotation (R), with residues (+r) or without residues
(−r) at CIMMYT’s longterm tillage sustainability
trial at El Batán (Mexico)
for 16 y. Bars are plus one
STD
60
40
20
0
ZT
r
R-
ZT
CT
r
r
RCT
ZT
R+
ZT
CT
r
W-
r
r
r
R+
CT
ZT
W-
W+
CT
ZT
ZT
r
+r
r
MCT
W
CT
M+
M+
M-
r
MCT
ZT
r
ZT
+r
M
CT
b) 5-10 cm soil layer
100
SOC contribution to total C (%)
80
60
40
20
0
r
r
R-
R-
r
R+
r
R+
r
r
r
r
W-
W-
ZT
CT
W+
ZT
r
W+
M-
r
r
M+
The emission of CO2 from soil cultivated with
maize and wheat increased during the rainy season, i.e. from June to September and after harvest
(Fig. 3). In February and May in treatments cultivated with maize, the CO2 emission was the highest from the CT+r treatments with maize-wheat
rotation (0.39 gC m−2 h−1 and 0.45 gC m−2 h−1
for February and May, respectively) and monoculture (0.36 gC m−2 h−1 and 0.42 gC m−2 h−1 for
February and May, respectively), and twice as high
as from soil with ZT (the highest in both months
was 0.17 g C m−2 h−1). At the second measurement in November, the highest CO2 emissions were
from the CTR + r and CTR-r treatments (0.44 g
C m−2 h−1) while in the ZT treatment the CO2 emission
was 0.04 gC m−2 h−1. At the first measurement in
CT
ZT
CT
Monthly CO2 fluxes and soil water content
December, the highest CO2 emission values were
found in the CTM+r (0.17 gC m−2 h−1) and ZTM
+r (0.19 gC m−2 h−1) treatments. At the second
measurement of December, all the CT treatments
emitted more CO2 (average 0.065 g C m−2 h−1)
than the ZT treatments (average 0.05 gC m−2 h−1). For
soil cultivated with maize, the CT treatments had
significantly higher CO2 emissions than the ZT
treatments. For soil cultivated with wheat, CTR-r
had significantly higher CO2 emissions than all
other treatments.
For the treatments with maize, the ZT+r treatment showed the greatest soil water contents from
February to May and August to December. In July,
the greatest soil water content was detected in the
CTR+r and ZTR+r treatments (Fig. 4). The soil
water content was affected by tillage and residue
management from February to April and August to
Plant Soil
Fig. 3 The emission of CO2
from soil subjected to zero
tillage and conventional
tillage, maize and wheat
monoculture and rotation,
with residues and without
residues in CIMMYT’s
long-term tillage sustainability trial at El Batán
(Mexico) for 16 y. Bars
are±one STD
3.0
Maize+residue
Maize-residue
Wheat+residue
Wheat-residue
Rotation+residue
Rotation-residue
2.4
a) Conventional tillage
1.8
1.2
0.6
(g C m-2 h-1)
0.0
b) Zero tillage
3.0
2.4
1.8
1.2
0.6
0.0
January
February March April
December. For the treatments with wheat, the largest
water contents were found in the ZT+r treatments
(Fig. 4). The soil water content was affected by residue
management and tillage from February to May and
August to November.
Discussion
Aggregate distribution and stability, and C distribution
in the aggregate fractions
Aggregate stability depends on various factors, i.e.
texture, clay mineralogy, cation content, aluminum
May June
July August Octobre November
December
January
and iron oxides and soil organic matter (Bronick
and Lal 2005; Le Bissonnais 1996). Several studies in different soils and climates showed a positive correlation between soil organic matter and
the structural stability of both macro and microaggregates (Mohanty et al. 2007; Shukla et al. 2006;
Wander and Bollero 1999). Organic matter stabilizes
aggregates by at least two different mechanisms: (1)
by increasing the inter-particle cohesion within aggregates thereby decreasing their breakdown and (2) by
increasing their hydrophobicity and thus decreasing
their breakdown by slaking (Eynard et al. 2006; Le
Bissonnais 1996). Malhi and Lemke (2007) showed in
an 8 year study that residue retention lead to a lower
Plant Soil
Fig. 4 Water content of soil
subjected to zero tillage and
conventional tillage, maize
and wheat monoculture and
rotation, with residues and
without residues in CIMMYT’s long-term tillage
sustainability trial at El
Batán (Mexico) for 16 y.
Bars are±one STD
Maize+residue
Maize-residue
Wheat+residue
Wheat-residue
Rotation+residue
Rotation-residue
35
30
25
a) Conventional tillage
20
15
10
5
(%)
0
b) Zero tillage
35
30
25
20
15
10
5
0
January
February March April
proportion of fine (<0.83 mm diameter) and a greater
proportion of large (>38.0 mm) dry aggregates, as
well as a larger MWD compared to treatments
without residue retention regardless of the type of
tillage. They concluded that there was no beneficial effect on soil aggregation when tillage was
stopped. In contrast, our results confirmed earlier
observations at this site that ZT+r increased aggregate stability compared to CT (Fuentes et al.
2009; Govaerts et al. 2009b). Soils with CT+r had
a low MWD compared to plots with ZT+r, which
indicates that despite the incorporation of residues
there was a negative effect on soil stability with
tillage. The commonly used practice in the study
May June
July August Octobre November
December
January
area of CT-r with monoculture of maize resulted in
poor structural stability.
The increased aggregate stability in ZT+r compared to ZT-r and CT practices resulted in increased
infiltration (Govaerts et al. 2009b) and soil water
content, especially during drought periods (Verhulst
et al. 2011). Since water is an important limiting factor
for crop growth in the study area, the increased water
content in ZT+r ensures high and stable yields for this
management practice compared to practices involving
tillage or ZT-r (Govaerts et al. 2005; Verhulst et al.
2011).
Fuentes et al. (2009) evaluated chemical and physical soil quality in 2003 and 2004 in the same
Plant Soil
experiment. They reported that ZT+r treatments had a
higher MWD than the ZT−r and CT treatments. In this
paper, a detailed study of aggregation is made and
linked to soil C stocks and CO2 emissions. Six et al.
(1999) reported that CT causes disruption of soil
aggregates, especially macro-aggregates (>0.25 mm).
Our results showed that small macro-aggregates
(0.25–1 mm) were more abundant in soils with CT
and with or without residue than large macroaggregates and micro-aggregates (Fig. 1). It can be
hypothesized that the negative effect of tillage was
greater for large macro-aggregates than small macroaggregates. The abundance of small macro-aggregates
might be the result of the breaking up of large macroaggregates and/or the physical and chemical characteristics of small macro-aggregates makes them more
resistant to break up by tillage.
Six et al. (2002) found a greater accumulation of
organic C in the top-soil of systems with ZT compared
to CT due to a better preservation of aggregates in ZT.
The C not exposed is longer retained in the soil (Six et
al. 2004a). It has been reported that the stability of a
soil can be related to the proportion of large macroaggregates, normally containing most of the C in the
soil (Six et al. 2004b). In our study, ZT and residue
retention increased the proportion of large macroaggregates in most of the treatments.
The ZT+r treatment had the highest proportion of
water stable large macro-aggregates, organic C and
MWD (Fig. 1). A possible explanation for the high
proportion of large macro-aggregates in treatments
with residue retention is that they are formed around
fresh organic matter, while micro-aggregates contain
older organic matter (Balesdent et al. 2000; Denef et
al. 2001a). In the majority of the treatments, the small
macro-aggregates contained less C than the large
macro-aggregates (Table 2). This behavior can be
explained by the concept of aggregate hierarchy
(Oades 1984; Tisdall and Oades 1982) which stated
that large macro aggregates tend to be richer in organic
matter compared to smaller aggregates because fresh
organic matter is the precursor in the formation of
macro-aggregates. Elliott (1986) showed that increasing organic C content is related to increasing aggregate
size. It is important to note that the organic matter in
macro-aggregates is labile while the one in smaller
aggregates is more stable. Therefore, tillage operations
generate a larger loss of organic matter in large macroaggregates than in small macro-aggregates.
The proportion of small macro-aggregates was
greater than that of the other two aggregate fractions. Consequently, the small macro-aggregates
contributed more C to the total C. However, in
the 0–5 cm layer of treatments with monoculture
of wheat with ZT+r, the greatest contribution of C
to the total C was from the large macro-aggregates
(Fig. 2). Tisdall and Oades (1982) explained that
roots are involved in stabilizing macro-aggregates.
Therefore, agricultural management practice partly
controls the formation of macro-aggregates because
it influences crop growth. Gregory (2006) showed
that wheat roots promoted aggregate formation to a
larger extent than maize roots, which was also
reflected in the results of this study.
The proportion of the micro-aggregates in all treatments was small and they had the lowest organic C
content. However, micro-aggregate formation (Gale et
al. 2000; Six et al. 1998) and micro-aggregates within
the macro-aggregates (Denef et al. 2001b; Six et al.
2000) can play an important role in C storage and
stabilization in the long term. The formation of
micro-aggregates occurs in advanced stages of organic
C decomposition, so the organic matter in the microaggregates is more stable or recalcitrant compared to
the organic C found in other aggregates, thereby favoring aggregate stability and C retention (Six et al.
2004a). Additionally, the organic C is physically protected in the micro-aggregates within macro-aggregates
(Denef et al. 2004).
Stewart et al. (2008) stated that the C sequestration
capacity of a soil is determined mainly by the protection of C in the aggregates. Soil C stocks change with
tillage and management practices (Govaerts et al.
2009a). Fuentes et al. (2010) reported for the same
experiment as this study that the SOC content in
the 0–10 cm layer was affected by tillage and residue management. The highest SOC content was
found in the 0–5 cm layer of the ZT+r compared
other treatments (Table 3). The SOC stock (calculated on equivalent soil mass basis) in the 0–10 cm
showed a similar tendency (Table 3). The soils with
ZT+r (both for monoculture and rotation), showed
higher percentages of SOC and SOC stock than
CT+r and CT−r (Table 3). Consequently, the combination of ZT with residue retention is what makes
aggregates more stable, protects C and thus increases
C sequestration and not zero tillage or residue retention separately.
Plant Soil
Table 3 The soil organic carbon (g kg−1 dry soil) in soils
cultivated with maize and wheat, subjected to: ZTM+r/−r0Zero
tillage monoculture with residues or without, ZTR+r/−r0Zero
tillage rotation with residues or without, CTM+r/−r0Conventional
tillage monoculture with residues or without and CTR+r/−r0Conventional tillage rotation with or without residues, in CIMMYT’s
long-term tillage sustainability trial, El Batán (Mexico) for 16 years
(Fuentes et al. 2010)
Soil organic carbon (g kg−1)
Maize
Soil organic carbon stock as
equivalent soil mass (g m−2)
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
a
2.0
1.7
2.7
2.0
257.8
306.5
LSD
a
P<0.05 level based on least square difference grouping (LSD)
CO2 fluxes and soil water
The CO2 emissions in-situ varied with time. An increase
in the emissions of CO2 was observed when fertilizer
was applied. However, it also coincided with crop
growth and the heaviest rainfall, which might have also
contributed to increased emissions. Therefore, it is difficult to pinpoint the exact reason why the emission of
CO2 increased right after fertilizer application (i.e. fertilizer, rainfall or plant growth) as no unfertilized plots
were included in the experiment. The CO2 emissions
increased after harvest. This has been reported and
attributed to an increased activity of heterotrophic
organisms associated with the decomposition of fresh
roots (Franzluebbers et al. 1995).
In soil cultivated with maize, tillage increased the
emissions of CO2 compared to ZT. At ploughing, the
soil disturbance in the CT treatment increased the
emissions of CO2 as compared to the ZT treatments.
Ussiri and Lal (2009) suggested that mechanical tillage accelerated the decomposition of C. Our study
indicated that ZT increased the SOC pool probably
by slowing the decomposition of new maize and
wheat-derived C and protecting older more recalcitrant
C against decomposers.
Several experiments have shown that the top soil
with ZT and retention of residues had a larger SOC
content and lower decomposition rates of SOC than
the top soil with CT leading to a reduction in the
emissions of CO 2 for ZT (Diekow et al. 2005;
Fuentes et al. 2009; Jantalia et al. 2007; Six et al.
2002). The decomposition of plant residues is slower
in the ZT+r treatment due to reduced soil-residue
contact. Mechanical tillage stimulates decomposition
of organic material by aerating the soil, breaking up
aggregates and incorporating crop residues into the
soil, thereby increasing the contact between soil
microorganisms and crop residue (Ussiri and Lal
2009). In ZT, the contact between micro-organisms
and residues is delayed and organic material becomes
physically protected in aggregates that are not broken
up by tillage (Stewart et al. 2008). Additionally, soil
aggregates 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 aggregates against microbial decomposition than in less
stable aggregates as in CT. Better protected SOC
reduces C mineralization and thus CO2 emissions.
The soil in ZT+r had greater soil water contents
than in CT with or without residue retention
(Shaver et al. 2002; Ussiri and Lal 2009). Crop
residue on the soil surface forms a barrier against
evaporation thereby maintaining the water stored
in the plant root zone (Lichter et al. 2008). The
Plant Soil
differences in water content were due mainly to
the residue management while emissions of CO2
were mainly due to tillage. Lee et al. (2009) found
a linear relation between CO2 emission and soilwater content in plots with CT and cultivated with
maize, but no relationship was found for sunflower
and chickpea. They concluded that CO2 emission
is highly variable and depends on the crop, tillage
and season. Our study showed that the relation
between CO2 emissions and soil water-content varied in each production system. In CT + r with
maize or wheat both were highly correlated (P<
0.001, r2 0.70). In the ZT+r treatments, CO2 emission and soil water content were significantly correlated (P<0.05) with r2 of 0.50 similar to that of
CT-r and in ZT-r with r2 of 0.40.
Conclusion
Reduction in soil disturbance combined with residue
retention increased the C retained in the small and large
macro-aggregates of the top soil due to greater aggregate
stability and reduced the emissions of CO2 compared
with conventional tillage without residues retention and
maize monoculture (a cultivation system normally used
in the central highlands of Mexico). The retention of
residues increased the C in aggregates of the top-soil and
the reduction in soil disturbance resulted in a decrease in
emissions of CO2. A crop rotation of maize and wheat
reduced emissions of CO2 as compared to wheat monoculture. Zero tillage with residue retention and monoculture of maize or rotation with wheat was the most
attractive system to maximize C retention in the aggregates of the top-soil, under the experimental conditions.
Our results showed that the retention of C in the top-soil
depends mainly on the C content in the small and large
macro-aggregates of the 0–10 cm soil layer while aggregate stability depends primarily on the large macroaggregates.
Acknowledgments Mariela Fuentes received a PhD fellowship from CONACYT. Fieldwork was done in a long-term trial
established by Dr. R.A. Fisher at CIMMYT’s El Batán research
station. The research was supported by CIMMYT and its strategic donors and forms part of the strategic research network
developed in the frame of MasAgro (Modernización sustentable
de la agricultura tradicional), component ‘Desarrollo sustentable con el productor’. The authors thank M. Martinez, A.
Martinez, and H. González-Juárez for help with the field work.
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