Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol57.Issue4.Jan2001:

Soil & Tillage Research 57 (2001) 225±235

Total, particulate organic matter and structural stability of
a Calcixeroll soil under different wheat rotations and
tillage systems in a semiarid area of Morocco
Rachid Mrabeta,*, Najib Saberb, Azeddine El-Brahlia,
Sabah Lahloub, Fatima Bessamb
a

Institut National de la Recherche Agronomique (INRA), Aridoculture Center,
PO Box 589, Settat 26000, Morocco
b
Faculty of Sciences, PO Box 20, El-Jadida 24000, Morocco

Received 21 December 1999; received in revised form 28 July 2000; accepted 12 October 2000

Abstract
Wheat production in Morocco is constrained by both scarce climate and degraded soil quality. There is an urgent need to
revert production decline while restoring country's soils. Among conservation tillage systems known for their improvement in
yield, no-till technology was found to in¯uence soil quality as well. Soil quality indices are also affected by wheat rotations at
medium and long-terms. This paper discusses changes in selected properties of a Calcixeroll soil, including total and

particulate soil organic matter (SOM), pH, total N and aggregation, subjected, for 11 consecutive years, to various
conservation and conventional agricultural systems. Tillage systems included no-tillage (NT) and conventional tillage (CT).
Crop rotations were continuous wheat, fallow±wheat, fallow±wheat±corn, fallow±wheat±forage and fallow±wheat±lentils.
Higher aggregation, carbon sequestration, pH decline and particulate organic matter (POM) buildup are major changes
associated with shift from conventional- to NT system. Better stability of aggregates was demonstrated by a signi®cantly
greater mean weight diameter under NT (3.8 mm) than CT system (3.2 mm) at the soil surface. There was 13.6% SOC
increase in (0±200 mm) over the 11-year period under NT, while CT did not affect much this soil quality indicator. Another
valuable funding is the strati®cation of SOC and total nitrogen in NT surface horizon (0±25 mm) without their depletion at
deeper horizon compared to tillage treatments. Fallow±wheat system resulted in reduction of SOC compared to WW, but 3year wheat rotation tended to improve overall soil quality. Bene®ts from crop rotation in terms of organic carbon varied
between 2.6 and 11.7%, with fallow±wheat±forage exhibiting the maximum. Combined use of NT and 3-year fallow rotation
helped to improve soil quality in this experiment. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: No-tillage; Soil quality; Organic carbon; Structural stability; Sustainability; Crop rotation; Intensi®cation

1. Introduction
In Morocco, dryland wheat cropping systems are
erratic and time stability is necessary. Intensive
*

Corresponding author. Tel.: ‡212-1-430768;
fax: ‡212-3-403209.

E-mail address: mrabet1@altavista.net (R. Mrabet).

cropping and tillage systems have led to substantial
soil quality deterioration of much of the country's
farmlands. Soil fertility, structure and organic matter
are declining as a result of tillage and agricultural
practices (i.e. grazing, straw exportation) that neglect
to incorporate suf®cient organic material into the
soil. The persisting use of these soil management
practices played a signi®cant role in the continuation

0167-1987/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 1 8 0 - X

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

of unsustainable agriculture. The real contribution and
potential of conservation tillage towards an effective

and sustainable use of soils is indisputable (Allmaras
et al., 1991; Blevins and Frye, 1993), and can result in
greater food availability for the growing population of
Morocco and many other countries (i.e. Latin America, Euro-Asia, Africa) as declared by Derpsch (1998)
and GTZ (1998). A well-planned wheat rotation that
promotes grain production and insures good soil quality, play an important role for establishing sustainable
Mediterranean agriculture (Shroyer et al., 1990; Boisgontier, 1991; Lopez-Bellido et al., 1996).
In 1983, an interest to improve the long-term sustainability of Moroccan cropping systems was accompanied by a re-thinking of advantages and
disadvantages of tillage. Hence, efforts were directed
into research, development and adoption of conservation agricultural practices (including conservation
tillage and fallow-based rotations), in order to rehabilitate Moroccan agriculture. Research has shown
that increased wheat yields may result when shifting
from traditional and conventional to no-tillage (NT)
system (Bouzza, 1990; Kacemi, 1992; Mrabet, 1997).
These authors correlated increased wheat production
under NT system to enhanced water use ef®ciency
(Kacemi et al., 1995). However, those yield improvements could also be due to a change in soil quality.
With this renewal interest in soil quality and long-term
stability, attributes such as soil organic matter (SOM)
and structural stability have taken a new signi®cance

among scientists (Karlen et al., 1997).
In semiarid conditions where decomposition near
the soil surface may be constrained by dryness, the
adoption of conservation tillage practices may reduce
water vapor loss and increase crop yield, thereby
favoring organic matter accumulation from the
higher inputs of residue (Campbell and Janzen,
1995). However, conventional tillage (CT) for weed
control and seedbed preparation may enhance organic
matter loss by rendering the soil more susceptible to
oxidation and erosion (Havlin et al., 1990; Wood et al.,
1990).
In Morocco, the low organic matter levels of dryland soils are due to widespread use of tillage, summer
clean fallow and overgrazing of stubble (Mrabet et al.,
1993). It is of paramount importance to improve the
SOM levels by crop residue retention. SOM can be
important in increasing water stable aggregation by

slowing water entry into aggregates and reducing
unstable aggregates which break down when rapidly

wetted (Tisdall and Oades, 1982). Tillage is normally
performed to create a good soil structure of the
seedbed; ruefully most authors disagree on persistence
of adequate soil physical conditions under conventionally tilled systems. Soil aggregation was found to
relate to SOM quality as well (Six et al., 1998) and that
SOM quality is more prone to changes in soil management strategies than total SOM (Janzen et al., 1992;
Biederbeck et al., 1994).
Particulate organic matter (POM) was de®ned as
this fraction associated with sand-sized particles, and
separated from the soil by sieving. Generally, it consists mainly of ®ne root fragments and other organic
debris (Cambardella and Elliot, 1992). The POM can
account for well over 10% of the soil C (Carter et al.,
1994; Gregorich et al., 1994). This pool is signi®cant
to SOM turnover because it serves as a readily decomposable substrate for soil microorganisms and as
short-term reservoir for plant nutrients. This fraction
has been suggested as a sensitive indicator of changes
of SOM because of its responsiveness to management
practices (Gregorich and Carter, 1997). Reduced tillage and no-till help to sequester more POM in the soil
than cultivated systems (Angers et al., 1993). However
for Capriel et al. (1992), twofold difference in SOM

among contrasting soil management systems did not
lead to changes in it chemistry (in terms of humic and
fulvic acids). Arshad et al. (1990) found minor differences between no- and conventional-tillage systems in
the chemical nature of SOM.
Lamb et al. (1985) reported that most differences
in N content among tillage systems were in the
0±100 mm topsoil because of greater disturbance with
tillage implement. They recommended reduction in
soil stirring for saving nitrogen to crops. Lately, Power
and Peterson (1998) showed that under no-till fallow
nitrogen balance was positive, however, for plowed
fallow some N loss occurred. They related these
results to slower oxidation of organic N in the cooler,
more aggregated, moister conditions found under NT.
Grant and Lafond (1994) recorded higher concentration of nitrogen and carbon under reduced tillage
systems, including NT under three horizons (0±50,
50±100 and 100±150 mm) than CT. In spite of this, 4
years were not enough to show N differences among
cropping sequences.

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

Tillage and crop rotation also affect soil pH, which
consequently in¯uence nutrient availability. Edwards
et al. (1992) reported a pH decline of 0.2 unit between
NT and CT. However, they had greater pH difference
among rotations with continuous corn having the
lowest pH and continuous soybeans the highest. These
pH variations have affected P, Ca, and Mg availability
to crops and liming was recommended. Bowman and
Halvorson (1998) were worried about reduction in pH
due to buildup of SOM and abundant use of N
fertilizers under no-till cropping systems. Consequently, they re¯ected the need to search on problems
induced by pH decline such as phosphorus ®xation,
herbicide ef®cacy, aluminum toxicity and root biomass reduction.
Long-term experiments are the primary source of
information to determine the effects of cropping systems and soil management on soil productivity. In
Morocco, several long-term tillage experiments were

carried out since 1983; still, the combined effects of
tillage and crop rotation on SOM, integrity and other
soil properties were not investigated yet. Hence, the
speci®c objective of this study was to quantify the
effect of contrasting tillage and cropping systems on
soil organic C, total N, and POM, as well as aggregation of a Calcixeroll soil after 11 years of experimentation.

Table 1
Typical properties of Sidi El Aydi soil in 1987 (information from
Mrabet (1997))
Property

Description
(0±200 mm)

Soil type
Slope
Clay (g kgÿ1)
Silt (g kgÿ1)
Sand (g kgÿ1)

Gravel
Texture
pH (1:1 soil:water)
Organic carbon (g kgÿ1)
Calcium carbonate (g kgÿ1)
Cation exchange capacity (cmol (Na‡) kgÿ1)
Average dry bulk density (Mg mÿ3)
Soil moisture at 0.3 bar (mÿ3 mÿ3)
Soil moisture at 15 bars (mÿ3 mÿ3)

Vertic Calcixeroll
Less than 1%
530
255
215
Less than 1%
Clay
8.25
13
200

55
1.23
0.38
0.19

(31 years) is 358 mm with a maximum of 740 mm and
a minimum of 128 mm. The average rainfall from
1986 to 1998 is only 296 mm. Drought is frequent at
various crop stages (Yacoubi et al., 1998). Average
precipitation, and minimum and maximum air temperatures are given in Table 2.
In this site, a long-term experiment was conducted
to search and detect the effects of various rotation and
tillage systems on wheat production and soil quality.

2. Materials and methods
2.1. Research site and experiment set-up
The experimental site is located at the Institut
National de la Recherche Agronomique (INRA)
experiment station at Sidi El Aydi, 15 km North of
Settat, Morocco. Sidi El Aydi station is at 338000 N

latitude and 098220 W longitude, and is 230 m above
mean sea level. The soil is a clay soil referred to as
vertic Calcixeroll. The soil has a shallow 50 mm selfmulching surface horizon and cracks when dry (clay
minerals are mainly montmorillonite). The percent
clay increases, SOC decreases, while percent silt is
constant with depth. The structure is poorly developed. The slope is negligible. More characteristics of
the soil are given in Table 1.
Regional climate is semiarid with a winter rainfall
pattern. The long-term average annual precipitation

Table 2
Monthly average precipitation and air temperatures (1967±1998),
Sidi El Aydi, Settat, Morocco
Month

Temperature (8C)

Rainfall
(mm)

Minimum

Maximum

January
February
March
April
May
June
July
August
September
October
November
December

6.0
7.2
8.7
10.3
12.7
15.9
18.0
20.2
18.2
12.8
10.1
8.4

20.0
21.3
23.7
25.3
27.4
30.6
34.4
31.8
31.6
28.7
24.1
21.4

59
56
46
40
14
3
1
0
5
27
49
58

Total or average

12.4

26.7

358

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

The experiment started in 1987±1988 (Kacemi, 1992;
Kacemi et al., 1995; El-Brahli et al., 1997). The
experimental design used randomized blocks with
split-plots and three replications. Factors investigated
were rotation, assigned to the whole plots, tillage
applied to the subplots. Rotations studied were continuous wheat (WW), wheat±fallow (WF), fallow±
wheat±corn (WCF), fallow±wheat±forage (WFgF)
and fallow±wheat±lentils (WLF). Each phase of a
rotation was present every year and each treatment
was cycled on its assigned plot. The subplots were
6  30 m2 . The tillage treatments were NT and
reduced tillage with V-Blade Sweep (1987±1993)
and changed to CT with offset disk harrow (1993 to
present). The CT system consisted of one or two
passes with an offset disk harrow for seedbed preparation and several passes for weed control in fallow
phases. Depth of offset disk tillage was in the range
100±150 mm, depending upon the conditions of the
soil at the time of tillage. The only soil disturbance in
NT occurred during seeding and fertilizer banding
operations.

rial per hectare, were placed in the seed row as starter
fertilizers. Additional urea fertilizer was surface
broadcast at tillering stage of wheat (50 kg of material
per hectare). For corn, ammonium nitrate, at
100 kg haÿ1, and triple superphosphate, at 50 kg of
material per hectare, were applied at planting. Soil
tests at Sidi El Aydi are high in K and therefore K
fertilizer was not necessary. These application rates
ensured that nutrients (N, P and K) were not limiting
production since no de®ciency symptoms occurred.
An application of glyphosate at a rate of 3±4 l haÿ1
was made to control any standing vegetation prior to
planting of crops and in fallow. Before seeding, all
wheat, forage and fallow plots were sprayed with
chlorosulfuron at a rate of 10 g haÿ1. Corn and lentils
were sprayed at seeding with simazine at rate of 1.5
and 1 l haÿ1, respectively. The use of these herbicides
provided good weed control throughout the crop
growing seasons and in fallow. Carbofuran insecticide/nematicide was used on all crops to avoid insect
damage, mainly Hessian ¯y in wheat (at 25 kg haÿ1).
2.3. Soil sampling and preparation for analysis

2.2. Crop management
A no-till drill equipped with coulters, double-disk
openers and single-press wheels was used to plant
wheat, lentils and vetch±oat (a mixture of 80 kg haÿ1
of oat (variety Soualem) and 40 kg haÿ1 of vetch
(variety Guich) as forage crops). Wheat (variety Achtar or Tillila) was drilled at a rate of 120 kg haÿ1 in
rows spaced 200 mm apart for all plots. The same drill
was used for lentils and vetch±oat. Lentils (variety
Bakria) was planted 400 mm apart at a rate of
60 kg haÿ1 and vetch±oat at 120 kg haÿ1. Wheat,
lentils and vetch±oat were seeded on mid-November
of each year. Corn (variety Mabchoura or Doukkalia)
was planted either using a commercial 4-row no-till
planter or manually. Corn was planted in rows spaced
600 mm apart and thinned to 60±65 thousands plants
per hectare. Corn planting date ranged from midFebruary to mid-March depending on possibilities
to access the ®eld. All cultivars are adapted to the
environment of Sidi El Aydi.
Fertilizer applications, based upon soil tests, were
as follows. For wheat, vetch±oat and lentils: ammonium nitrate, at a rate of 75 kg of material per hectare,
and triple superphosphate, at a rate of 50 kg of mate-

The samples were collected from the non-traf®c
areas between the crop rows before cultivation and
sowing in July 1998 in fallow phases and in wheat±
wheat (WW). Samples of the topsoil zones at depth of
surface (0±25 mm), near surface (25±70 mm), and
subsurface (70±200 mm) were collected. Two random
cores from each depth in fallow and four random cores
in WW were sampled. All three replicates were considered. The bulk soil sample was divided in two
subsamples, one for aggregation and the other for
chemical and biochemical measurements. The second
subsample (50±100 g of soil) was air-dried and sieved
to pass 2 mm screens. For SOM, POM and total
nitrogen, the soil was ground and sieved to 200 mm.
For pH measurements, 2 mm sieved samples were
used. At the time of sampling, the CT fallow plots
have not been plowed since winter 1998 (150 days
before sampling).
2.4. Measurements and methods
Soil measurements taken were dry aggregate size
distribution, wet aggregate stability (WAS), SOM,
total nitrogen, carbon and nitrogen in POM and pH.

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

Dry aggregate stability was determined by dry
sieving. In this later, a soil from each ®eld sample
(100±150 g) was air-dried and dry-sieved through a set
of sieves of screen with 10±8±6±5±4±3±2±1 and
0.250 mm openings. The sieving was done with a
mechanical shaker at 1440 vibrations per minute for
5 min. The dry aggregate stability was expressed as
mean weight diameter (MWD) of the soil aggregates
(Youker and McGuinness, 1956). For WAS, the procedure of Kemper and Rosenau (1986) was used. Four
grams of 1±2 mm air-dried aggregates were placed
into sieves and wetted with suf®cient distilled water to
cover the soil when the sieve is at the bottom of its
stroke. The sieves were allowed to raise and lower
1.3 cm, 35 times/min for 3 min. The remained material (stable aggregates) in the sieve was dispersed with
2 g lÿ1 sodium hexametaphosphate. The sieving was
carried out until only sand particles are left in the
sieve. There was not pretreatment of aggregates before
sieving. The wet aggregation was calculated as the
ratio of stable aggregate weight to total sample weight
corrected for sand. All analyses were done in duplicates.
Soil organic carbon was appreciated using the wet
oxidation method of Walkley and Black (Nelson and
Sommers, 1982). Total N was determined using the
semi-micro Kjeldahl digestion method as described by
McGill and Figueiredo (1993). The POM was measured using Cambardella and Elliot (1992) method.
Dispersing the soil in 5 g lÿ1 sodium hexametaphosphate and passing the dispersed soil samples through
a 53 mm sieve isolated the POM fraction. Organic C
and total N in POM were determined as described in
previous paragraphs. Soil dry bulk density was determined using core method for each depth (Lahlou,
1999), in order to convert soil carbon and nitrogen
from percent to Mg haÿ1 (Ellert and Bettany, 1995).
Soil pH was measured in a 1:2 soil/distilled water
suspension using a pre-calibrated glass electrode
(McLean, 1982).
2.5. Statistical analysis
The data were statistically analyzed as a split-plot
design for each depth using the GLM procedure of the
statistical analysis systems (SAS Institute, 1990).
Analysis of variance was utilized to ®nd signi®cance
of effects of tillage and rotation on soil quality attri-

butes and least signi®cance difference test (LSD) was
used to seek differences among treatments (p level of
5%) (Snedecor and Cochran, 1980).

3. Results and discussion
3.1. Aggregation
Signi®cant differences were found in aggregate
stability indexes among wheat rotations and tillage
systems (Table 3). In general, water stable aggregation
tended to decrease with depth, however, dry aggregation is higher in intermediate horizon, as found by
Kacemi (1992) in 1991. The MWD ranged from 2.56
to 5.04 mm depending on soil depth and rotation,
while WAS ranged from 42 to 72%.
3.1.1. Dry aggregation
No-till system (NT) has favored dry aggregation in
both surface horizons (0±25 and 25±70 mm) as compared to CT system. In deeper horizon (70±200 mm),
dry aggregation is much higher under CT than NT
(Table 3). Higher MWD under NT as compared to CT
was also observed by Unger and Fulton (1990) and
Arshad et al. (1994). In the surface horizon, rotation
Table 3
Water stable- and dry-aggregation as affected by wheat rotation and
tillagea
Horizon depth (mm)
WASb

MWDc

0±25

25±70

70±200

0±25

Rotation
WW
WF
WFgF
WCF
WLF

72
48
57
58
60

57
42
60
49
48

50
51
44
42
51

3.02
3.61
3.74
3.37
3.74

Tillage
NT
CT

59 A
58 A

54 A
48 B

51 A
44 B

3.78 A
3.21 B

4.52 A
3.51 B

2.85 B
4.11 A

Average

59

51

48

3.49

4.01

3.48

a

A
C
B
B
B

AB
C
A
BC
BC

A
A
B
B
A

B
A
A
A
A

25±70

70±200

4.37
5.04
3.65
4.32
2.69

2.56
3.31
3.79
3.40
4.40

B
A
C
B
C

C
B
AB
B
A

In each column, values followed by same letter are not
signi®cantly different at p ˆ 0:05 using LSD test.
b
Percent of water aggregate stability (stability of 1±2 mm
aggregates).
c
Mean weight diameter (mm).

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

including fallow do not differ but have improved dry
aggregation as compared to continuous wheat. More
distinct effects were found in 25±70 mm horizon with
a tendency for WF to promote higher structural stability. In the lower horizon, exceptionally WLF has the
highest MWD, while continuous wheat exhibited
lower MWD value. The other rotations were intermediate. Hence, longer time is required for cropping
systems in improving dry aggregation (soil structure)
of this vertic Calcixeroll soil under pluvial regime of
semiarid Morocco. At the same experiment, Kacemi
(1992) did not ®nd signi®cant difference in MWD
among crop rotations after 4 years.

water stable aggregation of third horizon. This helps
to hypothesize that quantity and quality of residue
incorporated or retained on the surface has an important impact on WAS. The WAS index was not different
among tillage systems (sweep and no-till) when averaged over rotation at Sidi El Aydi site for the soil
surface (0±25 mm) as expressed by Kacemi et al.
(1995) in 1991. However, for horizons 25±50 and
50±200 mm, NT (2.55 mm) had higher MWD than
sweep (2.32 mm). Moreover, comparing results in
Table 3 and those obtained by Kacemi (1992), there
is a tendency for increased aggregation at the NT
plots.

3.1.2. Wet aggregation
Soil under NT showed greater overall improvement
of its water stable aggregation compared to tilled
system, as found for dry aggregation. This is re¯ected
in all three horizons.
Continuous wheat permitted highest WAS index
(72%), triennial rotations were intermediate (58%)
and WF was lowest (48%) in the surface horizon
(Table 3). In the intermediate horizon, WFgF has
comparable WAS as WW but improved it compared
to other rotations. For the deepest horizon, WFgF and
WCF had lower aggregation. Difference between WW
and WF sequences was not of much signi®cance.
Particularly, as for dry aggregation, WFL favored

3.2. Soil organic carbon
Total carbon content was higher in the 0±25 and
25±70 mm depths in NT than in CT. Elimination of
soil mixing in NT lead to a concentration of organic
matter at the soil surface. In other terms, low storage of
C under CT was probably due to high oxidation rates,
release of organic compounds to the soluble form, and
greater microbial activity.
Particularly, SOC is higher in the entire pro®le
(0±200 mm) under NT, which helped to conclude that
there is a strati®cation of SOC in surface horizons
without any depletion of it at deeper horizon compared
to treatments receiving tillage (Table 4). In fact, at the

Table 4
Soil organic carbon and total nitrogen contents as affected by wheat rotation and tillagea
Horizon depth (mm)
0±25

25±70

b

TN

A
BC
B
B
C

0.57
0.46
0.48
0.50
0.47

SOC

c

SOC

70±200
TN

SOC

0±200
TN

SOC

TN

Rotation
WW
WF
WFgF
WCF
WLF

7.09
5.29
5.85
5.96
5.25

Tillage
NT
CT

7.21 A
4.48 B

0.57 A
0.42 B

8.39 A
8.06 B

0.84 A
0.75 B

21.68 A
21.38 A

2.11 A
2.11 A

37.28 A (13.6)
33.92 B (3.3)

3.52 A
3.28 B

Average

5.85

0.50

8.20

0.79

21.53

2.11

35.58

3.35

a

A
C
BC
B
BC

8.94
7.73
8.51
7.87
7.87

A
B
A
B
B

0.85
0.83
0.79
0.75
0.76

A
AB
BC
C
C

20.36
21.78
22.29
19.85
23.31

BC
AB
A
C
A

2.14
2.09
2.05
2.07
2.11

A
A
A
A
A

36.39
34.80
36.65
33.68
36.43

In each column, values followed by same letter are not signi®cantly different at p ˆ 0:05 using LSD test.
Soil organic carbon (Mg haÿ1).
c
Total nitrogen content in soil (Mg N haÿ1).
d
Percent increase in SOC over the experiment period (11 years).
b

A (10.9)d
B (6.1)
A (11.7)
C (2.6)
A (11.0)

3.56
3.38
3.32
3.32
3.34

A
B
B
B
B

R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

lower horizon, SOC values were similar between the
two tillage treatments. Hassink (1997) explained that
clay and silt content affect soil capacity to store carbon
and hence clay soil can respond more to addition of
sources of C, such as plant residues and manure. The
high clay content of Sidi El Aydi soil justi®es this high
storage of carbon under NT.
Initially, in 1987 the original SOC was
32.82 Mg C haÿ1 using an average soil bulk density
of 1.23 Mg mÿ3 (Kacemi, 1992) for the 0±200 mm
horizon (Table 1). NT has increased the SOC by
13.6% during the 11-year period, however, CT has
increased it by only 3.3%. In other terms, the average
total increase in SOC for the (0±200 mm) under NT
and CT was 4.46 and 1.10 Mg C haÿ1, respectively.
The 3.3% positive change under CT can be explained
by recent (1993) shift from stubble mulch tillage with
sweep to CT and also by total incorporation of straw,
stubble and root residues (no exportation of residues
from the plots by animal grazing or other means).
Continuous wheat conserved high level of soil
organic carbon than the other rotations at 0±70 mm.
WF and WLF conserved the least amount of SOC.
WFgF and WCF were comparable and contained
intermediate SOC levels at 0±25 mm. In intermediate
depth, WFgF and WW helped to maintain more
organic carbon than other rotations. For the entire
pro®le (0±200 mm), WFgF sequestered more SOC
(increase of 11.7%), followed by WLF (11%) and
WW (10.9%). As shown in Table 4, WFgF seems to
improve SOM accumulation more than any other
rotation over time. Contribution of WLF and WCF
to SOC was random among soil depths, which could
be due to root pattern and morphology of these crops.
The differential effects of wheat rotation on SOC
re¯ect the importance of crop morphological characteristics and residue type in SOM buildup, as outlined
by Dinel and Gregorich (1995). These results agreed
with Wood et al. (1990) and Janzen et al. (1998), who
recommended reduction in fallow intensity in favor to
cropping intensi®cation for enhancing SOM.
3.3. Total nitrogen
Total nitrogen (TN) was in¯uenced by both cropping system and tillage in the two surface horizons
(Table 4). Most of the differences in TN among tillage
systems occurred in the seed zone (0±70 mm), where

231

most soil disturbance has happened. In other ways, NT
conserved more nitrogen in 0±70 mm than CT. However, these tillage systems had equal nitrogen content
at 70±200 mm. For 0±200 mm, signi®cantly more
nitrogen is found in NT than CT. In other words, like
total organic C, total N concentrations were higher in
NT than in CT in 0±25 and 25±70 mm. Hence, the CT
has accelerated breakdown of organic matter, however, NT helped an accumulation of organic N materials near soil surface. High TN values under NT than
CT imply that N was incorporated in microbial biomass near the soil surface and less is available for
mineralization or leaching. Nitrogen concentration
was higher under WW on all depths and for the whole
sampled pro®le. The other rotations did not differ
considerably of their nitrogen content (Table 4).
3.4. Particulate organic matter
3.4.1. Carbon content POM-C
Tillage impacts on SOC were consistent with those
on POM-C and hence POM would accumulate where
SOC aggregated. Both surface horizons (0±25 and 25±
70 mm), which represent seed zone for crops used in
this experiment, have the highest POM-C under NT
than under CT. Even though NT has higher POM-C
concentrated in surface horizons, there was no depletion of this SOM fraction in deeper horizon. For the
entire sampled pro®le (0±200 mm), differences
between NT and CT are minimal (Table 4). NT helped
to improve qualitatively and quantitatively SOM as
compared to CT. In 0±25 mm depth, 3-year rotation
permitted similar levels of POM-C, while WW and
WF resulted in lower POM-C. In other depths, WFgF
had highest POM-C content, as well as for the whole
pro®le (0±200 mm), than any other rotation. Despite
annual addition of organic residues under WW, WFgF
accumulated more total and particulate organic carbon
in the 0±200 mm pro®le. Magnitude of differences
between tillage systems is more important at 0±25 mm
depth than among rotations. This implies that tillage
and mainly residue retention had greater impact on
POM. WFgF is best conserving rotation of SOC, and
improved also the POM-C of the soil. Differences in
POM among rotations suggest differences in root
biomass, length and residue inputs from crops. Wander et al. (1998) found that NT practices increased
SOM-C and POM-C contents by 25 and 70%

232

R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

Nevertheless, research on these soil attributes need
to be investigated for complete assessment of the
changes.

compared with CT at the soil surface, however, lower
horizon (50±175 mm) lost its carbon content by 4 and
18%, respectively. Six et al. (1998) found that a
fraction of POM is lost due to tillage and is responsible
for micro-aggregate formation. This explains low
POM and WAS under CT in our experiment. Hence,
correlation needs to be in-depth investigated between
this SOM fraction and aggregation.

3.5. Carbon to nitrogen ratios
Wheat rotations and tillage systems caused differences in total nitrogen, organic carbon, POM-C and
POM-N, which however, did not in¯uence appreciably
C/N ratios (Table 6). Cambardella and Elliot (1992)
found that NT and native-sod had equal but have
higher C/N than stubble mulch and tilled fallow. High
C/N and POM-C/N under NT in the surface depth
explains that soil retained more C than N. This is due
to slow decomposition of surface residues than incorporated residues.

3.4.2. Nitrogen content POM-N
NT effect on POM-N was con®ned to 0±25 mm,
however, for other depths differences are slight among
tillage practices. Soil pro®le (0±200 mm) contains
equal POM-N under both tillage systems. In Table 5,
nitrogen content in POM is higher under WW and
WFgF for 0±70 mm horizons, while other rotations
cannot be differentiated. For deeper horizons, WFgF
retained more POM-N than WW and other rotations.
The 0±200 mm pro®le is much improved of POM-N
under WFgF than any other cropping system. WFgF
sequestered higher SOC, POM and nitrogen than other
rotations because of differences in crop residue inputs.
Retention of maximum levels of crop residues on the
soil surface and lack of soil disturbance (NT) apparently create a more favorable environment for POM
buildup. The high levels of SOC and POM under NT,
and particularly under WFgF, also express abundant
microbial biomass and available plant nutrients.

3.6. Soil pH
pH ranged from 7.9 to 8.2 at depths of 0±25 and 70±
200 mm, respectively, indicating the calcareous origin
of the soil. Under both tillage systems, the surface
retention or incorporation of crop residue and belowground biomass decreased pH of the soil (Table 1).
However, pH decline was more pronounced under NT
than CT at 0±25 mm (Table 7). Nevertheless, there
was no signi®cant effect of tillage on pH throughout
25±200 mm pro®le. A 0.2 unit pH difference in

Table 5
Carbon and nitrogen content of POM fraction as affected by wheat rotation and tillagea
Horizon depth (mm)
0±25

25±70
b

c

70±200

0±200

POM-C

POM-N

POM-C

POM-N

POM-C

POM-N

POM-C

POM-N

Rotation
WW
WF
WFgF
WCF
WLF

3.12
2.58
3.81
3.82
3.21

0.29
0.20
0.28
0.28
0.24

4.28
3.94
5.23
4.29
3.74

0.40
0.36
0.44
0.34
0.33

8.28
10.63
12.11
9.56
9.52

0.87
1.05
1.25
0.86
0.79

15.64
17.15
21.15
17.67
16.47

1.55
1.61
1.97
1.48
1.36

Tillage
NT
CT

4.07 A
2.55 B

0.31 A
0.21 B

4.58 A
4.01 B

0.38 A
0.37 A

9.68 A
10.35 A

0.91 B
1.01 A

18.33 A
16.91 B

1.60 A
1.59 A

Average

3.31

0.26

4.30

0.37

10.02

0.96

17.63

1.60

a

B
B
A
A
AB

A
C
A
A
B

B
B
A
B
B

AB
BC
A
C
C

C
AB
A
BC
BC

C
B
A
C
C

In each column, values followed by same letter are not signi®cantly different at p ˆ 0:05 using LSD test.
Carbon content in POM (Mg haÿ1).
c
Nitrogen content in POM (Mg N haÿ1).
b

C
B
A
B
BC

B
B
A
CB
C

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R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235
Table 6
C/N ratio for total and POM as affected by wheat rotation and tillagea
Soil depth (mm)
0±25

25±70

C/Nb

POM-C/Nc

C/N

10.8
12.9
13.6
13.6
13.3

10.5
9.3
10.8
10.5
10.4

70±200
POM-C/N

C/N

10.7
10.9
11.9
12.6
11.3

9.5
10.4
10.9
9.6
11.0

0±200
POM-C/N

C/N

9.5
10.1
9.7
11.1
12.0

10.2
10.3
11.0
10.1
10.9

POM-C/N

Rotation
WW
WF
WFgF
WCF
WLF

12.4
11.5
12.2
11.9
11.2

Tillage
NT
CT

12.7 A
10.7 B

13.1 A
12.1 A

10.0 A
10.7 A

12.0 A
10.9 B

10.3 A
10.1 A

10.6 A
10.2 A

10.6 A
10.3 A

11.4 A
10.6 A

Average

11.7

12.6

10.3

11.4

10.2

10.4

10.4

11.0

A
A
A
A
A

B
A
A
A
A

A
A
A
A
A

A
A
A
A
A

A
A
A
A
A

A
A
A
A
A

A
A
A
A
A

10.1
10.6
10.7
11.9
12.1

A
A
A
A
A

a

In each column, values followed by same letter are not signi®cantly different at p ˆ 0:05 using LSD test.
C/N of the soil.
c
C/N of POM.
b

(0±25 mm) is very important for calcareous soils of
Morocco in terms of nutrient availability for crops,
especially P and N. Acidi®cation of soil horizons is
apparent under WW, WF as compared to WFgF, WCF
and WLF. Higher SOC may have induced pH reduction in these treatments. Blevins et al. (1985) reported
that lack of soil mixing due to NT increases the acidity
of surface soil, particularly if abundant fertilizers are
used. In their part, Karlen et al. (1994) noted a 0.4 pH
unit difference between NT and CT with chisel and
disc plow. These authors also found higher WAS and
Table 7
Soil pH (water) as affected by wheat rotation and tillage systemsa
Horizon depth (mm)
0±25

25±70

70±200

Rotation
WW
WF
WFgF
WCF
WLF

7.6
7.7
8.0
8.0
8.0

7.9
8.0
8.1
8.2
8.1

8.1
8.1
8.3
8.2
8.2

Tillage
NT
CT

7.8 B
8.0 A

8.1 A
8.0 A

8.2 A
8.2 A

Average

7.9

8.1

8.2

a

B
B
A
A
A

B
B
A
A
A

B
B
A
A
A

In each column, values followed by same letter are not
signi®cantly different at p ˆ 0:05 using LSD Test.

total carbon under NT after 12 years of continuous
corn. Though, Laryea and Unger (1995) and Grant and
Bailey (1994) did not ®nd a signi®cant effect of tillage
system on soil pH.

4. General discussion and conclusions
More than 11 years of NT cropping have altered
several soil properties at the site. Higher aggregation,
carbon sequestration, nitrogen conservation, pH
decline and POM buildup are major changes associated with shift from conventional- to NT system
mainly in the seed zone. Another important funding is
the strati®cation of SOM (in term of carbon, nitrogen
and quality) in no-till surface horizons without depletion at deeper horizon compared to treatments receiving tillage. NT stored 3.36 Mg haÿ1 of carbon more
than CT.
Cropping history affected both the quantity and
quality of SOM and aggregation of Sidi El Aydi soil.
Fallow system resulted in reduction of organic carbon
content, but 3-year rotation tended to improve soil
quality. In other terms, the type of crop in rotation
seemed to have affected the processes of SOM accumulation and aggregation. WFgF, which is characterized by high crop residue production, resulted in
higher aggregation, SOC and POM. This hypothesis

234

R. Mrabet et al. / Soil & Tillage Research 57 (2001) 225±235

was already investigated by Angers and Mehuys
(1988), and needs to be searched very carefully.
Future research should focus on understanding
fundamental mechanisms behind soil quality improvement. The carbon sequestration in soil under NT and
intensi®ed rotation correspond to mitigation of carbon
dioxide by these systems. The carbon storage by NT,
associated to an amelioration of soil cohesion, may
affect hydrodynamic characteristics of the soil. These
characteristics are under study at the same site.

Acknowledgements
The International Foundation for Science (IFS) is
acknowledged for its ®nancial support to ®rst author
(Dr. Rachid Mrabet) under a Grant No. C/2942-1.
Many people contributed to this long-term experiment
before and after its inception. We thank Drs. M.
Kacemi, A. Bouzza, K. Brengle, G.A. Peterson and
C.R. Fenster.

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