Soil & Tillage Research 123 (2012) 61–70
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Soil & Tillage Research
journal homepage: www.elsevier.com/locate/still
Earthworm activity and soil structural changes under conservation
agriculture in central Mexico
A. Castellanos-Navarrete a,1,*, C. Rodrı́guez-Aragonés b, R.G.M. de Goede a,
M.J. Kooistra c, K.D. Sayre d, L. Brussaard a, M.M. Pulleman a,d
a
Department of Soil Quality, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands
Department of Animal and Human Biology, Faculty of Biology, Havana University, P.O Box 10 400, Havana, Cuba
Kooistra Micromorphological Services, Snijdersteeg 16, 3911 VP, Rhenen, The Netherlands
d
International Maize and Wheat Improvement Center (CIMMYT), P.O. Box 6-641, 06600 Mexico D.F., Mexico
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 21 May 2011
Received in revised form 21 February 2012
Accepted 27 March 2012
Crop residue mulching combined with zero tillage and crop rotation, known as conservation agriculture
(CA), is being promoted as an alternative system to revert soil degradation in maize-based farming in the
central highlands of Mexico. The goal of this paper was to determine the effects of CA vs. conventional
tillage systems on soil quality, with a special focus on the role of earthworms in affecting the soil
structure morphology, and on crop yield. For the conventional tillage system, the effect of crop residue
retention (CONV + RES) was also compared to the conventional farmers’ practice (residues removed;
CONV). CA resulted in four times higher earthworm abundance when compared to CONV. Residue
retention per se (CONV + RES) did not favor earthworm abundance. In all cases the earthworm
community was dominated by exotic species. CA increased total N and soil organic C concentrations
relative to CONV, but only at 0–5 cm soil depth. Nevertheless, the more pronounced vertical stratification
of soil organic carbon content under CA favored soil surface aggregation and aggregate stability as
expressed by the aggregate mean weight diameter after dry sieving (MWDds = 2.6 mm for CA and
1.6 mm for CONV) and wet sieving (MWDws = 0.9 mm and 0.6 mm, respectively). Also, CA improved
topsoil water stable macroaggregation (WSA = 415 mg g1) when compared to CONV (251 mg g1).
Residue retention within conventional tillage (CONV + RES) led to small increases in topsoil aggregate
stability (i.e. MWDds and WSA). Soil structural improvements were accompanied by a higher direct
surface water infiltration. Micromorphological analysis of thin sections indicated a loose and highly
biogenic soil microstructure in CA, whereas CONV was characterized by a physicogenic microstructure,
despite similar soil bulk densities (SBD). SBD is thus a poor indicator of soil physical quality when
comparing different tillage systems. Redundancy analysis illustrated that CA resulted in improvement in
most parameters related to soil quality, especially at the soil surface, but significant yield increases were
recorded only in 2004. CONV + RES lead to marginal improvements in soil quality with no yield increases.
ß 2012 Elsevier B.V. All rights reserved.
Keywords:
Central Mexico
Crop residue management
No-tillage
Thin sections
Zea mays
1. Introduction
Zero tillage coupled with crop residue mulching and crop
rotation, which encompasses the basic principles for conservation
agriculture (CA), has come forward as a sustainable management
system that could revert physical soil degradation in resource-poor
farms across very different agro-ecological conditions (FAO, 2008).
In fact, such management has been a traditional practice in many
* Corresponding author. Tel.: +31 317 482151; fax: +31 317 485616.
E-mail address: [email protected] (A. Castellanos-Navarrete).
1
Present address: Technology and Agrarian Development Group (TAD),
Wageningen University, Hollandseweg 1, 6706 KN Wageningen, The Netherlands.
Tel.: +31 317 482776; fax: +31 317 485616.
0167-1987/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.still.2012.03.011
parts of Latin America, including Mexico, where the coa or espeque
(a wooden stick of about 50 cm) is used to plant, and residues are
generally left on the surface (Hernández-Xolocotzi, 1985).
Although CA is claimed to have many potential benefits, especially
in semi-arid regions (Shaxson and Barber, 2003), there is a need to
systematically evaluate and document which of these benefits can
be realistically obtained by implementation of CA-based crop
management practices under specific agro-ecological conditions
(Giller et al., 2009).
In the semi-arid highlands of Central Mexico, conventional
smallholder farming (CONV) is characterized by rainfed monoculture of maize (Zea mays L.), and to a lesser extent of wheat (Triticum
aestivum L.), using mechanical tillage in combination with very
limited returns of organic matter (i.e. crop residues are grazed by
communal livestock, removed for fodder and/or burned). Under
62
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
these conditions, bare soils exposed to intense and erratic summer
rain showers and winds, as well as high evapotranspiration levels
result in slaking and crust formation, whereas intensive tillage
causes the gradual loss of stable soil aggregates leading to soil
erosion and compaction on the long term (Fischer et al., 2002;
Govaerts et al., 2006). As an alternative, CA has been promoted to
improve crop water availability through the presence (year-round)
of a protective soil cover as well as by soil structural improvements
(Govaerts et al., 2006; Six et al., 2004) achieved by increases in
topsoil organic carbon (SOC) and reduced soil disturbance
(Franzluebbers, 2002; Six et al., 2004; Tisdall and Oades, 1982).
It is not clear to what extent residue retention under conventional
tillage (CONV + RES), as practiced by some farmers in these semiarid conditions, could lead to significant improvements in soil
structure irrespective of soil tillage, as found in other regions
(Zeleke et al., 2004).
Along with other soil macroinvertebrates (i.e. termites and
ants), earthworms have been defined as ‘ecosystem engineers’ due
to their role in soil structure formation and maintenance through
the creation of continuous macropores (Blanchart et al., 2004;
Edwards and Shipitalo, 1998), stable macroaggregates (Blanchart
et al., 2004; Six et al., 2004) and organo-mineral complexes
(Marinissen and Dexter, 1990; Shipitalo and Protz, 1989; Six et al.,
2004). Earthworm activity can be stimulated by reduced soil
disturbance and/or crop residue retention and as such be an
important determinant of soil structural characteristics under
different crop management systems (Pulleman et al., 2003). Soil
structural characteristics, and the specific role of different groups
of soil fauna in soil structure formation, can be assessed through
soil micromorphology which is based on identification of biogenic
features in thin sections prepared from undisturbed blocks of soil
(Kooistra and Brussaard, 1995; Kooistra and Pulleman, 2010). In
the absence of other relevant soil macrofauna, we hypothesized
that earthworms potentially play a key role in these semi-arid
agro-ecosystems by enhancing physical soil functions (e.g. rainfall
water infiltration) for crop production and should be considered as
an important indicator in integrated soil quality assessments.
Given the climatic conditions (i.e. erratic and intense rainfalls),
main production constraints (i.e. low rainfall use efficiency;
susceptibility to soil erosion) and goals (i.e. sustained agricultural
productivity for maize and wheat) (Govaerts et al., 2005; Masera
et al., 1999), soil quality was defined in this specific case as the
fitness of the soil to maximize rainfall productivity while
minimizing negative impacts such as those related to soil erosion
and soil C loss. Integrated evaluation of soil quality changes across
management systems has been done using ordination techniques
like factor analysis (Govaerts et al., 2006; Sena et al., 2002),
although those studies have not included important biological
parameters indicating the presence or activity of soil ecosystem
engineers. CA management, as compared to the conventional
farmers’ practice, was hypothesized to lead to increased soil
organic C contents, soil structural stability and soil physical
properties in the top soil layer. These improvements in soil quality
parameters are often accompanied by larger numbers and biomass
of earthworms, which have a distinct effect on soil structure
morphology, and lead to higher and more stable crop yields.
Retention of maize residues under the conventional farmers’
practices was expected to lead to intermediate improvement in
soil quality parameters, earthworm abundance and crop yields.
In order to test these hypotheses we aimed: (i) to determine the
effects of conservation agriculture (CA) vs. conventional tillage
systems (CONV), as well as the effects of contrasting residue
management (CONV vs. CONV + RES) within conventional farmers’
practices, on different soil quality parameters, including earthworm communities, (ii) to visually assess the impact of earthworms on soil structure through soil micromorphological analysis;
and, (iii) to provide an integrated evaluation of soil quality changes
due to CA vs. CONV management, and relate this to crop yields.
2. Materials and methods
2.1. Site characteristics and sampling
The study was conducted at the CIMMYT field station ‘‘El
Batán’’, situated in the semi-arid subtropical highlands of Central
Mexico (198310 N 988500 W; 2.240 m asl). The average annual
rainfall is 626 mm, with 87% of total rainfall falling between May
and October, and the potential evapotranspiration is
1900 mm yr1 (Govaerts et al., 2005). One crop per year can be
produced during summer, followed by a long dry winter, often
with frost during the night. The research station is located in the
area of the former lake of Texcoco. The soil is a fine, mixed, thermic,
Cumulic Haplustoll according to the USDA soil taxonomy system
(Soil Survey Staff, 1998) with a clayey loam texture (Govaerts et al.,
2005) and a slope of <0.3% (Govaerts et al., 2007).
Before the establishment of the field experiment, the area had
been under continuous maize cultivation with residue retention
and zero tillage for 10 years (1988–1998). A field experiment
comparing conventional and zero tillage treatments in three blocks
was set up in 1998 for continuous maize cultivation. In 2001 the
existing tillage treatments were split into two parts, resulting in a
split plot design with 3 10 m plots (n = 3). The zero tillage
treatments with residues left over the soil surface were converted
to a maize–wheat rotation to represent a conservation agriculture
(CA) system, with both phases of the rotation present each year.
The conventional tillage treatments under continuous maize
cultivation were split into two different residue treatments, with
maize residues retained (CONV + RES), and with maize stubble
removed from the soil before ploughing (CONV). This allowed for a
comparison between the most common farmers’ practice in the
region, maize monoculture with conventional tillage after maize
residues have been removed (CONV), and the same practices but
with residue retained (CONV + RES). The three CA plots that were
in the wheat phase of the rotation at the time this study was
carried out were only considered for yield analysis. Conventional
tillage was done by a tractor-drawn disc plough to a depth of
approximately 15 cm. Fertilization consisted of 120 kg ureaN ha1 applied at the first node stage for wheat and at the 4–6
leave stage for maize for all treatments. Maize (hybrid) and wheat
varieties were selected each year based on the best available
variety and were identical for all treatments during the same year.
Weeding and pest management operations were timely performed.
Soil and earthworm sampling took place in September 2005 and
January 2006 (for time-to-pond measurements) in three replicate
plots per treatment. Only CA plots that were under maize at the
time of sampling were included. Undisturbed samples for soil
micromorphological study were taken from two replicate plots
only, as these analyses were intended to be qualitative. All
samplings excluded a 1 m wide border area for each plot. Yield data
were obtained from both maize and wheat plots according to
Govaerts et al. (2005).
2.2. Earthworm abundance, biomass and species composition
Earthworm sampling was based on the TSBF methodology
(Anderson and Ingram, 1993): two monoliths (30 30 wide and
30 cm deep) were extracted per replicate plot. The monoliths were
then subdivided in two depth layers (0–15 and 15–30 cm). The soil
was hand sorted to collect the earthworms. Fresh earthworms
were weighed using a micro-balance, killed in 70% ethanol and
preserved in diluted formaldehyde (10%). After 48 h, earthworms
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
63
were transferred to flasks containing 50% ethanol to ensure their
conservation. Taxonomic determination to species level was done
for a representative subsample of the earthworms collected in each
plot, including both adults and juveniles.
weighed before calculating the bulk density (Blake and Hartge,
1986).
2.3. Chemical soil characteristics and C stocks
Time-to-pond (TTP) was determined as a measure of direct
surface water infiltration vs. susceptibility to runoff using a metal
ring placed on the ground to demarcate a circular area (296 cm2)
without impeding water flow out of this area. The area was
watered using a watering pot with outflow of 0.15 l s1 from a
height of 75 cm, and the time and water volume added until the
water began to run out of the circular area were recorded (Govaerts
et al., 2006). Time-to-pond was measured twice, once during the
crop cycle (TTPdc) at the time of soil and earthworm sampling
(early September), and once after tillage (TTPat) (mid-January), and
at six different locations within each replicate plot.
Composite samples consisting of five random soil cores (diam.
20 mm) per plot were taken from 0 to 5, 5 to 15 and 15 to 30 cm
depth. Samples were air-dried and crushed to pass through a 2 mm
sieve. Organic particles >2 mm were discarded. Analysis for
general soil characteristics including soil texture, pH (Salinity
Laboratory Staff, 1945), total N (Bremner, 1960), soil organic
carbon (SOC) (Walkley, 1947), extractable P (Bray and Kurtz, 1945),
micronutrients (Viets and Lindsay, 1973) and cation exchange
capacity (CEC) (Schollenberger and Simon, 1945) were done at the
CIMMYT Soil and Plant Analysis Laboratory according standard
procedures.
2.4. Soil aggregate distribution and stability
During hand sorting of monoliths, the soil material was gently
broken up along natural planes of weakness into aggregates. A
representative subsample was then passed through an 8 mm sieve,
air-dried at room temperature and thoroughly mixed. Coarse plant
residues, roots and few stones >8 mm were removed. The two
internal replicate monoliths were bulked to form one composite
sample per depth layer that was used for dry and wet sieving. To
determine dry aggregate size distribution, a sub-sample of 500 g
was separated into six size fractions by mechanical sieving through
a stack of sieves with openings of 6.3, 4, 2, 1 mm and 500 and
250 mm. They were shaken for 5 min at a frequency of 210 cycles
min1. Soil remaining on each sieve was collected and weighed
(Govaerts et al., 2006; Kemper and Rosenau, 1986). To determine
the water-stable aggregate size distribution, 50 g of air-dried soil
was fractionated by wet-sieving. Samples were submerged in
demineralized water prior to wet sieving for 5 min to allow slaking
(Le Bissonnais, 1996) and then manually sieved over a 2 mm mesh
placed in demineralized water according to the method of Elliott
(1986). The sieves were gently moved up and down 50 times for
2 min. The suspension with the soil material that passed the 2 mm
sieve was transferred to a basin holding the 250 mm sieve and the
procedure was repeated. Similarly, the procedure was repeated for
the 53 mm sieves. All size fractions were oven dried at 75 8C and
weighed. The <53 mm fraction was calculated by difference.
The aggregate size distribution after dry and wet sieving was
expressed as the mean weight diameter (MWDds and MWDws,
respectively) calculated as the sum of the proportion of each size
fraction’s weight multiplied by the mean diameter of that fraction
(van Bavel, 1949):
MWD ¼
n
X
di wi
i¼1
where d is the mean diameter of each size fraction i, w is the
proportion of total sample weight occurring in the size fraction i
and n is the number of size fractions. Water stable aggregation
(WSA) was presented as the percentage of total macroaggregates
(>250 mm) after wet sieving.
2.5. Soil bulk density
For soil bulk density (SBD), two samples per replicate plot were
collected at 5, 15 and 25 cm soil depth using a metal ring of known
volume (i.e. 100 cm3). Soil was oven-dried for 48 h at 105 8C and
2.6. Time-to-pond
2.7. Soil structure morphology
Undisturbed soil samples from two replicate plots per
treatment were collected from the wall of the 30 cm deep pits
that remained after monolith extraction. Per depth profile, three
8 6 cm metal tins were cut vertically into the soil at 0–8, 9–17
and 18–26 cm soil depths. The sample location was 15 cm from
the stem of the maize plant (row spacing = 75 cm). The samples
were kept refrigerated and were fumigated before transport to the
Netherlands for thin section preparation to avoid post-sampling
disturbance of the samples by soil biota. At destination the
samples were air-dried at stable room temperature and impregnated with a colorless unsaturated polyester resin. After
evaporation of the largest part of the acetone from this solution,
samples were hardened by gamma radiation. The thin sections,
with a thickness of 25 mm, were made from the core of the
hardened block following the procedure developed by Jongerius
and Heintzberger (1975). Qualitative description of soil structural
characteristics was done through microscopic observations and
following the terminology of Bullock et al. (1985) with two classes
of groundmass: physicogenic (angular blocky peds further
subdivided into pure, slaked material and organic particles) and
biogenic (granular to subangular blocky macroaggregation,
further subdivided into macrofauna and mesofauna-worked
groundmass); and, with three classes of voids: physicogenic
(pure or modified), biogenic (macro- and mesovoids) and
infillings (biogenic void infilled by soil material or shapeless
excreta) (Kooistra and Brussaard, 1995; Kooistra and Pulleman,
2010).
2.8. Data analysis
Data were analyzed using the SPSS 17.0 package and divided
into two different datasets to test separately for the effect of
management system (CA vs. CONV) and for the effect of residue
management under conventional tillage (CONV vs. CONV + RES).
Two separate ANOVAs were performed given the trial was set up in
1998 to compare zero tillage and conventional tillage systems.
Residue management was only established as a split plot
treatment within CONV in 2001. Data distributions were tested
through both the Kolmogorov–Smirnov and Shapiro–Wilk tests.
Transformation attempts of non-normal data were unsuccessful.
Non-parametric data were: total nitrogen (mg kg1), SOC
(mg kg1), earthworm total abundance (individuals m2) and
time-to-pond (s). Mean comparisons were performed with oneway ANOVA for parametric data, and Mann–Whitney and Kruskal–
Wallis tests for non-parametric data. Significance of difference
between treatment means was examined by a LSD range test
procedure at *P < 0.05.
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
64
Earthworm abundance (ind. m-2)
200
CONV
CONV+RES
180
*
CONV
CA
160
140
120
100
80
*
60
40
20
0
0-15
15-30
Depth (cm)
0-15
15-30
Fig. 1. Earthworm abundance per depth layer and treatment. Treatments compared were: conventional tillage (CONV) vs. conventional tillage with residue (CONV + RES)
(left); and, conventional tillage (CONV) vs. conservation agriculture (CA) (right). Mean values followed by an asterisk are significantly different between the two treatments
compared (*P < 0.05). Error bars indicate the standard error.
Relations between soil parameters and management were
analyzed through a direct multivariate ordination analysis in
which dependent variables (soil parameters: calcium (Ca2+), cation
exchange capacity (CEC), earthworm abundance (WRM), extractable phosphorus (P), magnesium (Mg2+), mean weight diameter of
dry-sieved samples (MWDds), mean weight diameter of wet-sieved
samples (MWDws), pH; potassium (K), sodium (Na+), soil bulk
density (SBD), soil organic carbon (SOC), time-to-pond during the
crop cycle (TTPdc), time-to-pond after harvest and tillage of the
conventionally tilled treatments (TTPat), total nitrogen (N) and
water stable aggregation (WSA)) were arranged on axes linearly
constrained by the independent variables (management parameters: CONV, CONV + RES, and CA) using CANOCO 4.5 software
package (ter Braak and Smilauer, 2002). Detrended correspondence analysis (DCA) using detrending-by-segments indicated that
the gradient length (expressed on standard deviations units of
dependent variables turnover) was <4 which allows the use of
both principal component analysis (PCA) and redundancy analysis
(RDA) due to predominantly linear responses along the ordination
gradient. While unconstrained (indirect) ordination methods such
as PCA (e.g. Govaerts et al., 2006; Sena et al., 2002) provide an
indication of the variation in the dependent variables, a
constrained (direct) ordination method such as RDA was preferred
given it allows to specifically analyze the variation in the
dependent variables in relation to the environmental variables,
in this case management treatments (Jongmans et al., 1995; ter
Braak and Smilauer, 2002). Soil chemical, biological and physical
parameters were centered and standardized (to zero mean and
unit variance) and significance of the first and all ordination axes
was calculated by a Monte Carlo significance test (200 permutations). Only key parameters (i.e. loadings > 0.20) were included in
the RDA interpretation, which was illustrated through a biplot.
Finally, Pearson’s (parametric) and Spearman’s correlations (nonparametric) were used to evaluate relationships between selected
parameters.
3. Results
3.1. Effects of management on biological, chemical and
physical soil parameters
3.1.1. Earthworm abundance and species composition
Earthworm abundance was significantly higher under CA
(194 ind. m2) compared to CONV (Fig. 1). No significant
differences in earthworm abundance were found between tilled
plots with residue (CONV + RES; 52 ind. m2) and without residue
retention (CONV; 46 ind. m2). The same trend was found for
biomass with the following results: 117.6 (CA), 34.0 (CONV + RES)
and 29.1 (CONV) g m2 (data not shown). Earthworm communities
comprised the species Aporrectodea longa (Ude, 1896), Aporrectodea tuberculata (Eisen, 1874) and Bimastos parvus (Eisen, 1874),
with only few Phoenicodrilus taste (Eisen, 1895) specimens under
CONV. No differences in earthworm species composition were
observed between the management treatments. On average, A.
longa was the predominant species (47%), followed by A.
tuberculata (32%) and B. parvus (19%) (data not shown).
3.1.2. Chemical soil parameters
CA plots had significantly (*P < 0.05) higher total N and SOC
concentrations when compared to the CONV treatment, but only in
the top 5 cm (1.53 g N kg1 versus 1.04 g N kg1 and
15.43 g C kg1 versus 9.40 g C kg1 for CA and CONV, respectively)
(Table 1). Other chemical soil parameters such as pH, P-Bray, K+,
Mg2+, CEC, Ca2+, Na+ as well as micronutrients (i.e. Fe, Mn, Zn and
Cu; data not shown) did not differ between tillage systems. No
differences in any soil chemical characteristic were found between
CONV and CONV + RES.
3.1.3. Physical soil parameters
Both alternatives to the CONV treatment resulted in higher
mean weight diameter for dry-sieved soil aggregates (MWDds) at
0–15 cm depth (Table 2). But significant differences in aggregate
stability, as expressed in MWD after wet-sieving (MWDws), were
only found between CA and CONV. At 0–15 cm depth CA resulted in
a significantly (**P < 0.01) greater MWDws when compared to
CONV (0.9 mm vs. 0.6 mm, respectively). The top 15 cm of the soil
under CA contained 415 mg g1 of water stable macroaggregates
(>250 mm; WSA), a significantly higher amount than in the CONV
treatment (251 mg g1). Residue retention in tilled plots also
increased WSA (333 mg g1) when compared to plots where
residues were harvested (251 mg g1).
Differences in time-to-pond between management systems
were only found during the crop cycle (TTPdc). Direct surface water
infiltration was particularly high for CA (15.8 s). Residue incorporation under conventional tillage (CONV + RES) also led to a
significant increase in TTPdc (7.4 s) when compared to CONV (6.1 s)
(Fig. 2). After harvest and tillage of the conventional tillage
treatments, TTP values were similar across treatments. Soil bulk
density (SBD) did not differ significantly according to management
system or residue treatment and average values across treatments
ranged from 1.37 g cm3 at 0–5 cm to 1.45 g cm3 at 5–15 cm and
1.35 g cm3 at 15–30 cm depth (cf. Table 1).
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
65
Table 1
Chemical and physical soil properties for three soil layer depths (0–5, 5–15 and 15–30 cm). Soil bulk density values correspond to 5, 15, and 25 cm depths. Mean values
followed by different letters are significantly different between the two treatments compared (*P < 0.05). The standard error is given between parentheses. Treatments
compared were: conventional tillage (CONV) vs. conventional tillage with residue incorporation (CONV + RES); and conventional tillage (CONV) vs. conservation agriculture
(CA).
P-Bray
(mg kg1)
Ca2+
(mg kg1)
Na+
(mg kg1)
SOC
(g kg1)
48.7
58.5
48.7
55.9
(15.6)
(12.2)
(15.6)
(15.9)
1544
1625
1544
1497
(79)
(23)
(79)
(38)
604
626
604
608
(36)
(11)
(36)
(17)
9.40
15.43
9.40
10.53
(0.76)
(0.43)
(0.76)
(0.37)
(0.07)
(0.08)
(0.07)
(0.03)
43.1
48.9
43.1
37.9
(13.1)
(13.8)
(13.1)
(13.7)
1692
1916
1692
1772
(49)
(78)
(49)
(57)
646
703
646
692
(35)
(37)
(35)
(23)
8.53
10.17
8.53
9.43
(0.07)
(0.07)
(0.07)
(0.02)
34.6
38.3
34.6
30.7
(13.0)
(10.6)
(13.0)
(11.5)
1863
2226
1863
1926
(28)
(17)
(28)
(12)
708
818
708
749
(21)
(77)
(21)
(48)
8.50
8.90
8.50
8.63
Depth (cm)
Treatment
pH-H2O
N (g kg1)
0–5
CONV
CA
CONV
CONV + RES
6.6
6.3
6.6
6.5
(0.3)
(0.1)
(0.3)
(0.1)
1.04
1.53
1.04
1.19
(0.08)
(0.05)
(0.08)
(0.05)
5–15
CONV
CA
CONV
CONV + RES
6.9
6.8
6.9
6.9
(0.2)
(0.1)
(0.2)
(0.1)
1.02
1.07
1.02
1.08
15–30
CONV
CA
CONV
CONV + RES
7.1
7.1
7.1
7.2
(0.2)
(0.1)
(0.2)
(0.1)
0.96
0.93
0.96
0.98
A
B
CEC
(mequiv. 100 g1)
SBD
(g cm3)
13.7
14.9
13.7
13.7
(0.71)
(0.23)
(0.71)
(0.34)
1.39
1.40
1.39
1.33
(0.04)
(0.03)
(0.04)
(0.04)
(0.87)
(0.99)
(0.87)
(0.15)
14.8
16.7
14.8
15.7
(0.63)
(0.66)
(0.63)
(0.47)
1.43
1.46
1.43
1.46
(0.01)
(0.02)
(0.01)
(0.05)
(0.70)
(0.70)
(0.70)
(0.43)
16.2
19.0
16.2
16.9
(0.42)
(1.49)
(0.42)
(0.98)
1.31
1.38
1.31
1.36
(0.01)
(0.05)
(0.01)
(0.00)
A
B
increase in biological activity, particularly in CONV + RES. A
subangular blocky structure was found, although weaker and
looser in CONV + RES when compared to CONV, with many
earthworm channels of 1 mm in diameter along with cracks due to
shrinkage and swelling. In CA, the structure was highly biogenic (as
in the upper soil layer) with evidence of macrofauna activity, often
modified by mesofauna and infilled with loosely accumulated
small excrements.
3.1.4. Impact of management and earthworms on soil
structure morphology
Micromorphological analysis of undisturbed soil samples
confirmed the clayey loam texture. The groundmass contained
fine silt grains up to sand size mineral grains and very few rock
fragments (up to 100 mm in diameter), predominantly of volcanic
origin, including alkaline feldspars. The smallest grains were partly
weathered; the larger minerals were angular and fresh to slightly
altered. There was no sedimentary layering visible.
The occurrence of surface crusting depended on treatment
(Table 3). Soil crusts were virtually absent under CA and the soil
surface consisted of a layer of worm casts and elongated worm
channels (either open or filled with cast material) (Fig. 3). Plots
under CONV were characterized by greater presence of slaked
material with semi-horizontal pressure-induced cracks. In CONV + RES, surface crusts were discontinuous and the soil surface was
locally characterized by a loose and open structure. At 2–14 cm
depth, soils under conventional tillage had a subangular blocky
structure interrupted only by physicogenic voids, such as planar
voids, created by tillage (cf. Table 3). Within conventionally tilled
plots, residue removal resulted in a more massive soil structure
with less evidence of biological activity. In contrast, under CA the
soil microstructure was loose and highly biogenic with intact
earthworm casts ranging from 2 to 4 mm in diameter. At 15 cm soil
depth, a compacted soil layer was present across all treatments.
While this plough pan was clearly identifiable within the
conventionally tilled treatments, especially at CONV, 17 years of
no tillage (including four years of CA) left a remnant plough pan
extensively reworked by soil macrofauna. Soil structure changed
below the plough pan in conventional tillage plots as a result of an
3.2. Integrated analysis of soil quality parameters through RDA
Constrained (by management) multivariate regression
explained 40.2% of the variance of measured soil parameters
(F = 2.017, P = 0.004, n = 200). The resulting RDA biplot indicated
that RDA axis 1 was particularly important in explaining the
variation (i.e. 35.1%) in soil parameters related to soil quality, and
corresponded to the difference between CA and conventional
tillage treatments (Fig. 4). CA was associated with higher
earthworm abundance (0–15 cm and 15–30 cm), nitrogen concentration (0–5 cm), SOC (0–5 cm), MWDds (0–15 cm), MWDws (0–
15 cm and 15–30 cm), WSA (0–15 cm) and TTPdc, but a relatively
low Na+ concentration. Both earthworm abundance (0–15 cm) and
SOC (0–5 cm) were significantly correlated to MWDws (r = 0.68;
*P < 0.05 and r = 0.72, *P < 0.05, respectively) in the 0–15 cm soil
layer. Soil organic carbon at 5–15 cm depth was statistically
correlated to both WSA (r = 0.48, *P < 0.05) and MWDds (r = 0.76,
**P < 0.01) (data not shown). Compared to CA and CONV,
CONV + RES had lower WSA (15–30 cm), SBD (5 cm) and Ca2+
(5 cm). The conventionally tilled treatment had a lower TTP after
tillage than CA and CONV + RES. CA had higher maize yields in
Table 2
Mean weight diameter (MWD) after dry and wet sieving, and contents of total water stable aggregates (WSA; 250 mm–8 mm). Mean values followed by different letters are
significantly different between the two treatments compared (*P < 0.05) and with a star for highly significant differences (*P < 0.01). The standard error is given between
parentheses. See Table 1 for treatment abbreviations.
Depth (cm)
Treatment
WSA (mg g1)
MWD
Dry sieving (mm)
0–15
15–30
CONV
CA
CONV
CONV + RES
1.61
2.62
1.61
2.25
(0.19)
(0.15)
(0.19)
(0.08)
CONV
CA
CONV
CONV + RES
2.06
2.31
2.06
1.97
(0.40)
(0.11)
(0.40)
(0.08)
A
B
A
B
Wet sieving (mm)
0.59
0.86
0.59
0.69
(0.05)
(0.09)
(0.05)
(0.07)
0.64
0.80
0.64
0.50
(0.05)
(0.05)
(0.05)
(0.04)
A
B*
251
415
251
333
(13.3)
(24.1)
(13.3)
(22.2)
365
355
365
305
(11.8)
(37.0)
(11.8)
(30.2)
A
B
A
B
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
66
Table 3
Description of macrostructure morphology of soil thin sections. See Table 1 for treatment abbreviations. Ø stands for diameter.
Depth (cm)
Treatment
CONV
Description of macrostructure morphology
0–2
2–14
Slaked material, probably caused by traffic, reworked by soil fauna.
2–6 cm. Tilled groundmass consisting of mechanically reworked soil and with
remnants of compacted material (up to 10 mm Ø).
7–14 cm. Few visible organic fragments and soil material somewhat looser with
infillings containing worm casts. Porosity mainly based on irregular voids (tillage
and root voids).
Compact layer with pressure-induced cracks (plough pan), reworked by soil mesoand macrofauna.
Subangular blocky structure (from weakly to well developed, cracks and 2–3 cm Ø
peds). Macrofauna casts and fauna channels partly filled with mesofauna
excrements and used by roots.
15
18–26
CONV + RES
0–2
Surface crust, but including areas with a loose and open structure due to
decomposing roots and residues that attract soil biota.
Tilled groundmass constituted by mechanically reworked soil with fragments of
slaked material (up to 10 mm Ø). More worm channels than in CONV, but many
burrows disturbed by tillage.
Plough pan but less distinct that in CONV and modified by meso-, macrofauna and
roots.
Weak and loose subangular blocky structure with casts of 1–2 mm Ø. Many pores of
earthworms (circa 1 mm Ø).
Topsoil constituted mainly by excrements (worm casts as well as enchytraeid and
insect larvae excrements) containing fragmented organic residues, biological
channels and packing voids between the casts.
Loose and highly biogenic structure with worm casts of 2 to 4 mm. Big worm
channels sized 2–5 mm Ø partially filled with excrements and that constitute a
pore system with vertical and horizontal pores between 2 and 5 cm.
Slightly compacted layer with pressure-induced cracks, possibly remnant of a
plough pan, but less densely packed and discontinuous due to soil biological
activity (i.e. earthworms and roots). Presence of some earthworm channels covered
by a clay coating, and casts reingested by mesofauna.
Extremely biogenic structure with larger macrofauna activity. Casts of 1–2 mm Ø,
with low quantity of organic matter, and wide worm channels (up to 4 mm Ø) are
prevalent. Diapause and hatching chambers were also present (up to 2 cm wide).
Voids are commonly modified by mesofauna.
2–14
15
18–26
0–2
CA
2–14
15
18–26
2004 and 2005 when compared to CONV, but the yield difference
was only significant (*P < 0.05) in 2004 (Table 4). Within
conventional tillage, yields did not significantly differ between
the two residue management options.
4. Discussion
4.1. Management effects on biological, chemical and physical
soil parameters
Conservation agriculture (CA) resulted in an evident increase of
earthworm abundance when compared to conventional tillage
(CONV). Earthworm biomass under CA (117.6 g m2) was particularly high and even superior to values from some Mexican tropical
pastures (i.e. 73.2 g m2 on average; Fragoso et al., 1999). Increased
20
CONV
CONV+RES
Time-to-pond (s)
18
earthworm abundance and biomass under CA is in line with previous
research results (Chan, 2001; Kladivko, 2001). This is probably
explained by the absence of tillage which strongly reduced direct
physical damage to earthworms, as well as habitat disturbance
(Chan, 2001; Kladivko, 2001; Lavelle et al., 2001). Although
aboveground crop residues constitute a major food source (along
with soil organic matter and root residues) for most earthworm
species (Lavelle et al., 2001), residue retention per se did not favor
earthworm proliferation when incorporated into the soil by
conventional tillage (CONV + RES). Increased biomass returns to
the soil through crop rotations have also been considered beneficial
for earthworm proliferation, but evidence has been generally
inconclusive (e.g. Govaerts et al., 2006; Hubbard et al., 1999; Rovira
et al., 1987). Regarding species composition, earthworms found
belonged predominantly to the Lumbricidae family but included the
*
CONV
CA
16
14
12
10
8
*
6
4
2
0
During crop cycle
After tillage
During crop cycle
After tillage
Fig. 2. Time-to-pond measured during the crop cycle and after harvest and tillage of the conventionally tilled treatments. Treatments compared were: conventional tillage
(CONV) vs. conventional tillage with residue incorporation (CONV + RES) (left); conventional tillage (CONV) vs. conservation agriculture (CA) (right). Mean values followed by
an asterisk are significantly different between the two treatments compared (*P < 0.05). Error bars indicate the standard error.
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
67
Fig. 3. Soil structure morphology (0–30 cm depth) for conventional tillage (CONV), conventional tillage with residue incorporation (CONV + RES) and conservation agriculture
(CA). Images are scans of the thin sections without optical multiplication. Arrows indicate: (a): soil crust; (b) compaction; (c) reworked slaked material; (d) physicogenic
groundmass; (e) earthworm burrow; (f) bow-like infillings indicating earthworm-worked groundmass.
American Bimastos parvus. As reported for other agro-ecosystems
(Fragoso et al., 1999; Ortiz-Ceballos and Fragoso, 2004), the
earthworm community was mainly composed of exotic species
given their adaptability to intensive agricultural management
(Fragoso, 2001; Grosso et al., 2006). B. parvus presence under CONV
is particularly surprising. This is an epigeic species feeding at and
immediately below the soil surface and thus considered highly
sensitive to conventional tillage (Chan, 2001; Kladivko, 2001). Both
Aporrectodea tuberculata (endo-anecic) and Aporrectodea longa
(anecic) generally persist in tilled soils (Berry and Karlen, 1993;
Ernst and Emmerling, 2009), although they are new to maize and
wheat habitats in Mexico (E. Huerta, personal communication,
2011). In contrast to epigeic species, endo-anecic and anecic species
can have a large impact on soil porosity.
Our study showed that 4 years of contrasting management was
not sufficient to lead to significant differences in soil C
sequestration relative to CONV, even after a history of no-tillage
(1988–1998). Calculation of SOC stocks for the whole 0–30 cm
layer, using bulk density data and soil organic C concentrations,
did not show significant differences (8.1 Mg ha1 in CA vs.
6.1 Mg ha1 in CONV). Some authors claim that differences in
soil C storage can take many years to develop (Gál et al., 2007; Six
et al., 2000), while a review by Govaerts et al. (2009) comparing
long-term soil C sequestration in tillage treatments found that
differences were not verified in 24% of cases reviewed. More
precise understanding of the conditions in which CA has potential
for soil C increases throughout the soil profile when compared to
conventional tillage is needed. In any case, CA led to a pattern of
SOC and total N stratification in which C concentrations were
significantly higher at 0–5 cm depth (cf. Table 1). Lower
macroaggregate turnover under CA has been found to favor
topsoil C accumulation as fine (53–250 mm) intra-aggregate
particulate organic carbon (iPOC C) into microaggregates (Angers
et al., 1997; Six et al., 2000). Topsoil SOC accumulation under CA
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
68
Fig. 4. Ordination diagram of the redundancy analysis (RDA) of the agricultural
management (triangles: CONV, CONV + RES, and CA) displaying 40.2% of the
variances in the soil parameters (dashed arrows). Only soil parameters of which
more than 20% of their variance is accounted for by the diagram are shown.
Eigenvalues of the two canonical axes are 0.351 (F = 3.251, P = 0.01, n = 200) and
0.051 (F = 2.017, P = 0.004, n = 200). Treatments were: conventional tillage (CONV),
conventional tillage with residue incorporation (CONV + RES) and conservation
agriculture (CA). Abbreviations indicate the following soil parameters: CA: calcium
concentration (mg kg1); CEC: cation exchange capacity (mequiv. 100 g1); NA:
sodium concentration (mg kg1); N: total nitrogen (g kg1); MWDds: mean weight
diameter of dry-sieved samples (mm); MWDws: mean weight diameter of wetsieved samples (mm); SBD: soil bulk density (g cm3); SOC: soil organic carbon
(g kg1); TTPdc: time-to-pond during the crop cycle (s); TTPah: time-to-pond after
harvest and tillage of the conventionally tilled treatments (s); WRM: earthworm
abundance (ind. m2); WSA: water stable macroaggregation (mg g1). Numbers
following parameters referred to sampling depth.
has been identified as a key in promoting progressive restoration
of physical soil functions through increased resistance to slaking
and to wind erosion (Franzluebbers, 2002; Govaerts et al., 2006;
Le Bissonnais, 1996). In the absence of tillage, carbon inputs into
the soil surface increase the formation of additional intermolecular bonds upon drying (Six et al., 2000) and reduce the wettability
of soil aggregates (Caron et al., 1996). This explains increased soil
surface aggregate formation and stabilization as reflected by
greater dry- and wet-sieved aggregate sizes when compared to
CONV at 0–15 cm depth (cf. Table 2). The role of crop rotation is
probably minimal on this regard given analogous wheat and
maize yields (see Section 3.2). In a very similar experiment,
Govaerts et al. (2006) did not find any effect on soil quality from
maize–wheat rotations (vs. maize monoculture) (Govaerts et al.,
2005). The aim of the present study was to compare management
systems as found in the region, rather than between individual
management components. While research needs to address if
proposed alternatives do provide sufficient medium-term improvement that justify farmers’ adoption, we would like to
emphasize that a full factorial evaluation of the role of individual
CA management components (residue, tillage, crop rotation)
would allow for a better mechanistic understanding and is
therefore an important additional research priority.
Improved soil structural stability, especially at the topsoil, has
been related to increased rainfall infiltration (Barthès and Roose,
2002) and reduced runoff (Kemper and Rosenau, 1986). This, along
with increased water infiltration through ponding as caused by
crop residue mulching (Gilley and Kottwitz, 1994), explains higher
time-to-pond values found in CA when compared to CONV during
the crop cycle. While similar direct surface water infiltration rates
are found in CONV short after tillage, this is only achieved
temporarily through mechanical made soil macroporosity (Carter,
1988). Low macroaggregate stability on CONV, as reflected in thin
sections by aggregate collapse into soil crusts and slaked layers,
during the crop cycle (i.e. a critical period for water use efficiency)
results in lower water infiltration (Carter, 1988; Kay, 1990).
Previous research has indicated how soil bulk density (SBD)
values might not differ when comparing different tillage systems
(Al-Kaisi et al., 2005; Logsdon and Karlen, 2004), especially in trials
less than 15 years old (Verhulst et al., 2010). However, in our study
similar SBD values across crop management systems corresponded
to very different structural morphologies as found through thin
section analysis. Under CONV, a SBD of 1.43 g cm3 corresponded
to the existence of a massive plough pan (cf. Fig. 3), while a SBD of
1.46 g cm3 under CA referred to a remnant plough pan with
denser biogenic structure, which does not likely imply reduced
hydraulic conductivity due to the presence of considerable
porosity (formed by soil macrofauna and decayed plant roots)
(Edwards and Shipitalo, 1998). In conventionally tilled plots
evidence of soil biological activity mainly consisted of small sized
macrofauna and soil mesofauna with less profound effects on soil
structure. Given different soil structural morphologies, SBD values
were a poor indicator of soil compaction.
4.2. Soil quality changes
As hypothesized, earthworms played a key role in enhancing
physical soil functions. When conditions were conducible to their
proliferation, as in CA, earthworm activity was a major factor
affecting soil structural morphology as confirmed by micromorphological analysis and the positive correlation of earthworm
abundance to MWDws (r = 0.68; *P < 0.05). These organisms
significantly contributed to soil macroaggregation (>250 mm)
and biogenic macroporosity (1 mm) through their casting and
burrowing activities (Pulleman et al., 2005; Shipitalo and Protz,
1989). At the same time, the tight correlation between SOC and
both MWDds (r = 0.76, **P < 0.01) and MWDws (r = 0.72, *P < 0.05)
in the topsoil suggests that soil organic matter content is a major
factor explaining differences in stable aggregation in the soils
under study. These results are consistent with earlier studies in a
Table 4
Yield results from 2002 to 2005 for maize and wheat. Values followed by different letters are significantly different between the two treatments compared (*P < 0.05). The
standard error is given between parentheses. Wheat yields are not compared between treatments since only CA included a wheat phase as part of crop rotation. See Table 1 for
treatment abbreviations.
Treatment
Maize yields (kg ha1)
2002
CONV
CA
CONV
CONV + RES
4809
4414
4809
4927
2003
(183)
(827)
(183)
(389)
2862
3224
2862
3441
Wheat yields (kg ha1)
2004
(247)
(272)
(247)
(317)
4616
6220
4616
5291
2005
(279)
(281)
(279)
(42)
A
B
3363
5081
3363
4504
(1068)
(222)
(1068)
(344)
Average 2002–2005
2002
2003
2004
2005
3913
4735
3913
4541
–
6724 (81)
–
–
–
4438 (347)
–
–
–
4011 (274)
–
–
–
4229 (231)
–
–
(393)
(315)
(393)
(242)
A. Castellanos-Navarrete et al. / Soil & Tillage Research 123 (2012) 61–70
wide range of soil types (Tisdall and Oades, 1982; Six et al., 2004).
While earthworms are not always the major agent contributing to
soil structure formation and maintenance (Oades, 1993; Six et al.,
2004), in this study these organisms, along with sustained carbon
inputs and low macroaggregate turnover, can be considered key to
the formation of a loose and highly biogenic soil microstructure.
These comprehensive changes were well illustrated by the
multivariate analysis which showed how CA resulted in improved
overall soil quality with increases in all parameters related to soil
physical properties (i.e. topsoil MWDds, MWDws and WSA).
Enhanced dynamics between soil biota, carbon inputs and soil
aggregation as a result of minimum soil disturbance and sustained
crop residue returns under CA favored soil’s water infiltration
capacity while reducing susceptibility to losses through runoff and
wind erosion (Barthès and Roose, 2002; Govaerts et al., 2006; Six
et al., 2004). The conventional farmers’ practice (CONV) was
characterized by significant positive loadings only for Na2+. Even
after a decade of zero tillage (1988–1998), conventional tillage
without residue retention (CONV) led rapidly to a massive
physicogenic soil profile characterized by subangular blocky peds
with slight evidence of soil biological activity below the plough
pan. Changes brought to conventional tillage through crop residue
retention are well illustrated by the RDA biplot. Marginal soil
improvements in the form of increased WSA at 0–15 cm depth
when compared to CONV were sufficient to lead to sustained direct
surface water infiltration rates.
Although conservation agriculture did lead to significant
improvements in key soil parameters related to physical soil
quality, particularly important in semi-arid conditions, as compared to conventional management, these improvements were not
(immediately) translated into consistent yield differences (cf.
Table 4). However maize yields under CA were higher in 2004 and
2005, although not significantly in 2005, when compared to CONV
plots under equal N fertilization rates which may indicate a trend
in time as a transition period normally exist between changes in
tillage management and yield improvements (Govaerts et al.,
2005). Further research is needed to understand how soil quality
changes contribute to yield increases in both the short and longterm. Alternative management practices improving soil quality,
such as CA, would be hardly adopted by farmers if these are not
characterized by clear short-term comparative advantages (Giller
et al., 2009; Hellin and Schrader, 2003), especially under realistic
farm conditions (i.e. partial crop residue returns given competitive
uses) (Giller et al., 2009). Implementation and research in longterm trials considering farmers available resources seems thus
necessary to support the implementation of more sustainable crop
management systems.
5. Conclusions
Studied crop management systems lead in a relative short-time
span (i.e. 4 years) to distinct soil microstructure morphologies with
important implications in these semi-arid conditions. Conservation agriculture led to the formation of loose and highly biogenic
topsoils as a result of increased earthworm activity and sustained
carbon inputs into these soils. Enhanced dynamics between soil
biota, especially earthworms, carbon inputs and soil aggregation
contributed to higher direct surface water infiltration when
compared to conventional practices. Whereas tillage prevented
earthworm proliferation and topsoil organic carbon accumulation.
Poor aggregate stability under CONV resulted in soil crusts
formation and slaked soil layers which required continuous tillage
to improved water infiltration into these soils. While crop residue
retention under conventional tillage (CONV + RES) did not prevent
soil crusts formation, direct surface water infiltration was
significantly increased. Our results confirmed how soil quality
69
changes at the topsoil are specially key for soil functioning
(Franzluebbers, 2002). However, comprehensive soil quality
changes under CA were not translated into consistent yield
increases under the time-frame considered. While major changes
occurred at the soil surface, soil micromorphologies also diverged
in the rest of the sampled soil profile. Nevertheless, SBD values
were similar across treatments pointing to the inadequacy of this
indicator for comparisons between tillage systems.
Acknowledgments
We wish to thank Humberto González Juárez, Ing. Adrián
Martı́nez, Miguel Martı́nez and Ing. Lopez Cesati (SPAL) for their
help during sampling and laboratory analysis at CIMMYT research
station, to Dr. Mario Varela for his support in statistical analysis, to
Dr. Esperanza Huerta for her very helpful comments regarding
earthworm species distribution in Mexico and to Dr. Bram
Govaerts and Dr. Patrick Wall for sharing their expert advice.
We are also very thankful to Dick Schreiber and Obbe Boersma
(Wageningen University) for the thin section preparation, as well
as ISRIC – World Soil Information for the use of microscope
facilities.
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