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 Mrabet
a,*, Najib Saber
b, Azeddine El-Brahli
a,
Sabah Lahlou
b, Fatima Bessam
baInstitut National de la Recherche Agronomique (INRA), Aridoculture Center,
PO Box 589, Settat 26000, Morocco
bFaculty 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 3-year 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
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 *Corresponding author. Tel.:212-1-430768;
fax:212-3-403209.
E-mail address: mrabet1@altavista.net (R. Mrabet).
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 Amer-ica, Euro-Asia, Africa) as declared by Derpsch (1998) and GTZ (1998). A well-planned wheat rotation that promotes grain production and insures good soil qual-ity, play an important role for establishing sustainable Mediterranean agriculture (Shroyer et al., 1990; Bois-gontier, 1991; Lopez-Bellido et al., 1996).
In 1983, an interest to improve the long-term sus-tainability of Moroccan cropping systems was accom-panied by a re-thinking of advantages and disadvantages of tillage. Hence, efforts were directed into research, development and adoption of conserva-tion agricultural practices (including conservaconserva-tion tillage and fallow-based rotations), in order to reha-bilitate 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 improve-ments 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 dry-land 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 conven-tionally 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 manage-ment 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 con-sists 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 decom-posable 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 til-lage 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 differ-ences between no- and conventional-tillage systems in the chemical nature of SOM.
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. Conse-quently, they re¯ected the need to search on problems induced by pH decline such as phosphorus ®xation, herbicide ef®cacy, aluminum toxicity and root bio-mass reduction.
Long-term experiments are the primary source of information to determine the effects of cropping sys-tems 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 aggrega-tion of a Calcixeroll soil after 11 years of experimen-tation.
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 is230 m above mean sea level. The soil is a clay soil referred to as vertic Calcixeroll. The soil has a shallow 50 mm self-mulching 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 devel-oped. 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
(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 tem-peratures 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.
Table 1
Typical properties of Sidi El Aydi soil in 1987 (information from Mrabet (1997))
Property Description (0±200 mm) Soil type Vertic Calcixeroll
Slope Less than 1%
Clay (g kgÿ1) 530
Silt (g kgÿ1) 255
Sand (g kgÿ1) 215
Gravel Less than 1%
Texture Clay
pH (1:1 soil:water) 8.25 Organic carbon (g kgÿ1) 13
Calcium carbonate (g kgÿ1) 200
Cation exchange capacity (cmol (Na
) kgÿ1) 55
Average dry bulk density (Mg mÿ3) 1.23
Soil moisture at 0.3 bar (mÿ3mÿ3) 0.38
Soil moisture at 15 bars (mÿ3mÿ3) 0.19
Table 2
Monthly average precipitation and air temperatures (1967±1998), Sidi El Aydi, Settat, Morocco
Month Temperature (8C) Rainfall (mm) Minimum Maximum
January 6.0 20.0 59 February 7.2 21.3 56
March 8.7 23.7 46
April 10.3 25.3 40
May 12.7 27.4 14
June 15.9 30.6 3
July 18.0 34.4 1
August 20.2 31.8 0
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 con-tinuous 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 630 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 pre-paration 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.
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 Ach-tar 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 mid-February 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: ammo-nium nitrate, at a rate of 75 kg of material per hectare, and triple superphosphate, at a rate of 50 kg of
mate-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 insecti-cide/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
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 con-sidered. 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 200mm. 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
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 pro-cedure 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 mate-rial (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 dupli-cates.
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 mea-sured using Cambardella and Elliot (1992) method. Dispersing the soil in 5 g lÿ1
sodium hexametapho-sphate and passing the dispersed soil samples through a 53mm sieve isolated the POM fraction. Organic C and total N in POM were determined as described in previous paragraphs. Soil dry bulk density was deter-mined 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 (plevel 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 aggrega-tion 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 com-pared 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 25±70 70±200 Rotation
WW 72 A 57 AB 50 A 3.02 B 4.37 B 2.56 C WF 48 C 42 C 51 A 3.61 A 5.04 A 3.31 B WFgF 57 B 60 A 44 B 3.74 A 3.65 C 3.79 AB WCF 58 B 49 BC 42 B 3.37 A 4.32 B 3.40 B WLF 60 B 48 BC 51 A 3.74 A 2.69 C 4.40 A Tillage
NT 59 A 54 A 51 A 3.78 A 4.52 A 2.85 B CT 58 A 48 B 44 B 3.21 B 3.51 B 4.11 A Average 59 51 48 3.49 4.01 3.48
aIn each column, values followed by same letter are not
signi®cantly different atp0:05 using LSD test.
bPercent of water aggregate stability (stability of 1±2 mm
aggregates).
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 sta-bility. In the lower horizon, exceptionally WLF has the highest MWD, while continuous wheat exhibited lower MWD value. The other rotations were inter-mediate. 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.
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
water stable aggregation of third horizon. This helps to hypothesize that quantity and quality of residue incorporated or retained on the surface has an impor-tant impact on WAS. The WAS index was not different among tillage systems (sweep and no-till) when aver-aged 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.
3.2. 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 70±200 0±200
SOCb TNc SOC TN SOC TN SOC TN
Rotation
WW 7.09 A 0.57 A 8.94 A 0.85 A 20.36 BC 2.14 A 36.39 A (10.9)d 3.56 A WF 5.29 BC 0.46 C 7.73 B 0.83 AB 21.78 AB 2.09 A 34.80 B (6.1) 3.38 B WFgF 5.85 B 0.48 BC 8.51 A 0.79 BC 22.29 A 2.05 A 36.65 A (11.7) 3.32 B WCF 5.96 B 0.50 B 7.87 B 0.75 C 19.85 C 2.07 A 33.68 C (2.6) 3.32 B WLF 5.25 C 0.47 BC 7.87 B 0.76 C 23.31 A 2.11 A 36.43 A (11.0) 3.34 B Tillage
NT 7.21 A 0.57 A 8.39 A 0.84 A 21.68 A 2.11 A 37.28 A (13.6) 3.52 A CT 4.48 B 0.42 B 8.06 B 0.75 B 21.38 A 2.11 A 33.92 B (3.3) 3.28 B Average 5.85 0.50 8.20 0.79 21.53 2.11 35.58 3.35
aIn each column, values followed by same letter are not signi®cantly different atp0
:05 using LSD test.
bSoil organic carbon (Mg haÿ1).
cTotal nitrogen content in soil (Mg N haÿ1).
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 charac-teristics 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 crop-ping 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
most soil disturbance has happened. In other ways, NT conserved more nitrogen in 0±70 mm than CT. How-ever, 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, how-ever, NT helped an accumulation of organic N mate-rials near soil surface. High TN values under NT than CT imply that N was incorporated in microbial bio-mass 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
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.
3.4.2. 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) appar-ently 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.
Nevertheless, research on these soil attributes need to be investigated for complete assessment of the changes.
3.5. Carbon to nitrogen ratios
Wheat rotations and tillage systems caused differ-ences 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 incor-porated residues.
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 below-ground 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 70±200 0±200
POM-Cb POM-Nc POM-C POM-N POM-C POM-N POM-C POM-N Rotation
WW 3.12 B 0.29 A 4.28 B 0.40 AB 8.28 C 0.87 C 15.64 C 1.55 B WF 2.58 B 0.20 C 3.94 B 0.36 BC 10.63 AB 1.05 B 17.15 B 1.61 B WFgF 3.81 A 0.28 A 5.23 A 0.44 A 12.11 A 1.25 A 21.15 A 1.97 A WCF 3.82 A 0.28 A 4.29 B 0.34 C 9.56 BC 0.86 C 17.67 B 1.48 CB WLF 3.21 AB 0.24 B 3.74 B 0.33 C 9.52 BC 0.79 C 16.47 BC 1.36 C Tillage
NT 4.07 A 0.31 A 4.58 A 0.38 A 9.68 A 0.91 B 18.33 A 1.60 A CT 2.55 B 0.21 B 4.01 B 0.37 A 10.35 A 1.01 A 16.91 B 1.59 A Average 3.31 0.26 4.30 0.37 10.02 0.96 17.63 1.60
aIn each column, values followed by same letter are not signi®cantly different atp0
:05 using LSD test.
(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 reduc-tion 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
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 asso-ciated 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 deple-tion at deeper horizon compared to treatments receiv-ing 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 accu-mulation and aggregation. WFgF, which is character-ized by high crop residue production, resulted in higher aggregation, SOC and POM. This hypothesis
Table 6
C/N ratio for total and POM as affected by wheat rotation and tillagea
Soil depth (mm)
0±25 25±70 70±200 0±200
C/Nb POM-C/Nc C/N POM-C/N C/N POM-C/N C/N POM-C/N Rotation
WW 12.4 A 10.8 B 10.5 A 10.7 A 9.5 A 9.5 A 10.2 A 10.1 A WF 11.5 A 12.9 A 9.3 A 10.9 A 10.4 A 10.1 A 10.3 A 10.6 A WFgF 12.2 A 13.6 A 10.8 A 11.9 A 10.9 A 9.7 A 11.0 A 10.7 A WCF 11.9 A 13.6 A 10.5 A 12.6 A 9.6 A 11.1 A 10.1 A 11.9 A WLF 11.2 A 13.3 A 10.4 A 11.3 A 11.0 A 12.0 A 10.9 A 12.1 A Tillage
NT 12.7 A 13.1 A 10.0 A 12.0 A 10.3 A 10.6 A 10.6 A 11.4 A CT 10.7 B 12.1 A 10.7 A 10.9 B 10.1 A 10.2 A 10.3 A 10.6 A Average 11.7 12.6 10.3 11.4 10.2 10.4 10.4 11.0
aIn each column, values followed by same letter are not signi®cantly different atp0
:05 using LSD test.
bC/N of the soil. cC/N of POM.
Table 7
Soil pH (water) as affected by wheat rotation and tillage systemsa
Horizon depth (mm)
0±25 25±70 70±200 Rotation
WW 7.6 B 7.9 B 8.1 B WF 7.7 B 8.0 B 8.1 B WFgF 8.0 A 8.1 A 8.3 A WCF 8.0 A 8.2 A 8.2 A WLF 8.0 A 8.1 A 8.2 A Tillage
NT 7.8 B 8.1 A 8.2 A CT 8.0 A 8.0 A 8.2 A Average 7.9 8.1 8.2
aIn each column, values followed by same letter are not
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 improve-ment. 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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