Short communication
Effects of tillage and management practices on soil crust
morphology under a Mediterranean environment
AsuncioÂn UsoÂn, Rosa Maria Poch
*Departament de Medi Ambient i CieÁncies del SoÁl, Universitat de Lleida, Av. Rovira Roure 177, 25198 Lleida, Catalonia, Spain
Received 31 July 1998; received in revised form 11 March 1999; accepted 23 July 1999
Abstract
Soil crust formation can be affected by soil tillage. Alternative soil conservation practices consisting of reduced tillage were tested against traditional tillage, which involves mechanical weeding by frequent ploughing in rainfed vineyard soils in Catalonia, Spain. After 2 years of the experiment (1994±1996), thin sections of the surface crusts were studied to evaluate the effects of the soil management treatments on crust morphology and genesis, using micromorphological observations and pore characterisation with image analysis. Reduced tillage caused thicker and more complex crusts consisting of layers with different degrees of sorting and pore types, compared to traditional tillage. Total porosity of crusts did not differ from that of non-crusted areas, but pores in crusts were less interconnected, more horizontally distributed and more elongated than in the underlying non-crusted material. The soil type, especially structure and texture, affected crust morphology and played an important role in the process of crusting. The results show that reduced tillage may be limited as an alternative management practice when used to reduce crust formation in Mediterranean conditions, due to the dif®culty to establish an effective
groundcover.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Soil crusting; Conservation tillage; Conventional tillage; Mediterranean soils; Micromorphology; Porosity; Image analysis
1. Introduction
In Mediterranean and semi-arid lands, weed control in crops with low ground cover is traditionally done by frequent ploughing, not only to restrict water con-sumption but also to increase surface roughness and water in®ltration. Nevertheless, this type of tillage diminishes soil structural stability and leaves the soil surface unprotected, and therefore prone to soil
seal-ing and crustseal-ing. Crust formation is strongly affected by soil tillage. Those soil management practices that leave soil residues at the surface may reduce or eliminate surface crusts (Cassel et al., 1995). In other cases, Jones et al. (1994) concluded that in dryland conditions the low residue production under no tillage was not enough to prevent crust formation.
The objective of this study was to characterise the porosity of soil crusts formed under different manage-ment practices in vineyard soils under a Mediterranean environment, using micromorphological techniques, as a ®rst stage to evaluate the importance of tillage systems.
*Corresponding author. Tel.: 34-973-702621; fax:
34-973-702613.
E-mail address: [email protected] (R.M. Poch)
2. Materials and methods
2.1. Location and experimental set-up
The study area is located in the Anoia-PenedeÁs region in Catalonia (NE Spain), where high quality wine is produced under rainfed conditions. The annual rainfall is between 500 and 600 mm, and the average annual evapotranspiration is 1050 mm. Alternative management practices were tested during 2 years (1994±1996) against the traditional tillage which implies frequent ploughing down to 15 cm (6±10 times/year) to maintain the soil free of weeds. The treatments were performed on three representative soil types in the area (Table 1). The experimental design was a random block with three replicates. The size of
each plot was 36 m2. The treatments included cover
crops (Trifolium subterraneum L.,Festuca elatior L.,
Vicia sativa L. and Lolium multi¯orum Lam.) or reduced tillage (natural cover), until the beginning of the dry season (April). At this time, cover was incorporated into the soil by ploughing to increase organic matter in the soil. All plots were sown and/or ploughed in October, and again at the end of April to incorporate the soil cover. In addition, those under conventional tillage were ploughed two times during the same period (February/March and April). The total amount of rainfall from October to April was 436 mm, distributed over 53 days.
2.2. Micromorphological study
After 2 years from the initiation of the experiment,
undisturbed blocks (15 cm side10 cm deep) from
one to three replicate plots of all treatments were taken from the soil surface before ploughing in April. Two
vertical thin sections (125 cm) were made of each
block. Thin section study and description followed the guidelines of Bullock et al. (1985). For the interpreta-tion of the descripinterpreta-tions a morphosynthetic approach was followed (Stoops, 1994), to allow their relation to crust genesis and behaviour.
2.3. Image processing and quanti®cation
From each uncovered thin section, two areas were selected: one within the crust and another in the part of the section not affected by crusting. Images of the
porosity pattern of the selected areas were obtained
with scanning electron microscope (SEM) as
3.73.7 mm back-scattered electron scanning
images (BESI). On these images the pores between 50 and 1000mm in equivalent diameter (diameter of a circular pore having the same area) were processed with a Quantimet 720 Image Analyzer (Jongerius and Bisdom, 1981). Average values for both ®eld and object parameters were obtained. Field parameters were porosity, equivalent pore number (pore count corrected by size according to Ringrose-Voase, 1991) and anisotropy. Anisotropy values lower or higher than one show horizontal or vertical orientation of the pores, respectively. Object parameters were pore area, perimeter, equivalent diameter, orientation, digi-tation and elongation. The effect of the treatments, presence of crust and pore size was evaluated within each location by ANOVA. For those factors that were signi®cantly different with a 95% con®dence the Duncan test for mean separation was applied.
3. Results and discussion
3.1. Crust morphology
The crusts were described and classi®ed according to Valentin and Bresson (1992). Crusting and sealing modi®ed the porosity, microstructure and pedofea-tures of the original Ap material on all three soils. Under reduced tillage with natural cover, complex crusts were found, which consisted of an erosion crust over a runoff crust with variable thickness, which alternated laterally with still crusts. All these were underlain by a coalescing or slaking crust. The process of formation corresponds to that suggested by Valentin and Bresson (1992): the ®rst stage would be the formation of a structural crust, which would reduce in®ltration capacity and increase runoff. Upslope eroded materials would accumulate over these struc-tural crusts forming the runoff crusts. Finally, other events could erode part of the latter. A striking feature was the thickness and uniformity of the runoff crusts in Sant SadurnõÂ, made of partly collapsed microag-gregates that create a packing porosity. These materi-als had a low sealing index (Ramos and Nacci, 1997), which means that they were rather resistant against raindrop impact, and that the prevailing sealing
General characteristics of the Ap horizons of the studied soils and experimental set-up for three locations in the Anoia-PenedeÁs region of Catalonia
Location Subirats Sant SadurnõÂ Masquefa
Soil type (FAO, 1998) Calcaric Fluvisol Haplic Calcisol Calcaric Regosol CaCO3(g kg
ÿ1) 230 400 360
Organic matter (g kgÿ1) 12 6 7
Texture Loam Loam Silt loam
EC1:5(dS m
ÿ1258C) 0.14 0.13 0.14
pH (1:5 soil±water suspension) 8.1 8.4 8.5
Treatments and soil cover 1 Ð Conventional tillage 1 Ð Conventional tillage 1 Ð Conventional tillage 2 Ð Reduced tillage, natural cover 2 Ð Reduced tillage, natural cover 2 Ð Reduced tillage, natural cover 3 Ð Reduced tillage, cover crops
Surface cover at the moment of sampling 1 0% 1 0% 1 0% (first numbers refer to the treatment) 2 15±30% 2 15±30% 2 15±30%
3 65%
Number of thin sections 1 4 1 2 1 6
(first numbers refer to the treatment) 2 12 2 6 2 14 3 4
Uso
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Resear
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mechanism was due to coalescence under humid conditions. The studied runoff crusts were formed by accumulation of these relatively stable microag-gregates transported by runoff events when the hydraulic conductivity of the underlying material was reduced by pore isolation. In Masquefa, the sealing index was higher (Ramos and Nacci, 1997), which means that the crust was produced by raindrop impact, and therefore a protecting groundcover would be more effective against its formation.
3.2. Image analyses of the porosity
Table 2 displays the parameters that showed sig-ni®cant differences according to the source of varia-bility. Regarding the ®eld parameters (Table 3), there
were signi®cant differences between treatments only in Masquefa, where for the same pore count the size of the pores was larger under reduced tillage, probably due to the presence of faunal pores. The pores in Masquefa were also smaller and more numerous. This means that the type of material had a stronger in¯u-ence in the crust morphology than the treatments.
Table 3 also showed that the lowest values of anisotropy were found in crusts, which is easily explained by their parallel distribution referred to the soil surface. There were signi®cant differences in all locations due to the presence of crust, with lower values in the runoff crusts. In Subirats and Sant SadurnõÂ there were also differences depending on the interaction between treatment and crust. In Sub-irats reduced tillage and cover crops results in the
Table 2
Parameters obtained from image analysis and locations where signi®cant effects on these parameters were observed by ANOVA (95% con®dence)
Treatments Crusting Pore size Interactions Field parameters Porosity Masquefa Not applicable Not applicable
Equivalent diameter Masquefa Not applicable Not applicable
Equivalent number All locations Treatmentpore size in Masquefa Anisotropy Subirats All locations Treatmentcrust in Subirats and
Sant SadurnõÂ
Object parameters Digitation Sant SadurnõÂ All locations Treatmentpore size in Subirats Elongation Subirats Subirats Treatmentcrust in Subirats
Sant SadurnõÂ
Table 3
Field parameters of analysis of pore images (porosity between 50 and 1000mm)a
Location Treatmentb Porosity (%) Mean equivalent diameter (mm)
Pore number (count per mm2)
Anisotropy
Crust No crust Mean Subirats 1 11.3 120.4 7.2 0.948 a 0.995 a 0.969 r
2 13.0 113.5 7.3 0.935 a 0.948 a 0.941 r 3 11.6 131.4 5.7 0.848 b 0.953 a 0.900 s Mean 12.0 119.4 6.9 0.921 y 0.958 z
Sant SadurnõÂ 1 14.2 120.5 6.9 0.920 b 0.960 ab 0.940 2 14.8 117.1 8.1 0.867 c 0.975 a 0.921 Mean 14.5 118.0 7.8 0.880 y 0.971 z
Masquefa 1 10.3 b 92.8 b 10.1 a 0.928 0.972 0.950 2 15.6 a 109.9 a 10.2 a 0.906 0.947 0.925 Mean 13.0 106.0 10.2 0.911 y 0.953 z
aThe values followed by the same letter are not signi®cantly different within one location. b1: Conventional tillage; 2: reduced tillage, natural groundcover; 3 reduced tillage, cover crops.
strongest anisotropy. This can be due to the presence of a crust before the establishment of the vegetation, which has been preserved after the growing season.
Equivalent pore number, corrected by pore size (Ringrose-Voase, 1991) is shown in Fig. 1 for Masquefa. The overall analysis did not show differences between
crusted and non-crusted areas. Under reduced tillage
there were more pores between 100 and 500mm, in
agreement with the larger pore area between 200 and
500mm. Small pores were also more numerous,
prob-ably related to the ®ner texture which favours the for-mation of smaller pores (vesicles) in the micromass.
Fig. 1. Equivalent pore number in Masquefa, classi®ed according to pore size. Columns with different letters within the sets abcd, rs and yz are signi®cantly different with a 95% con®dence.
Table 4
Digitation and elongation values in Subirats for the different pore classesa
Treatmentb Crust 50±100mm 100±200mm 200±500mm 500±1000mm Mean Digitation
1 Yes 1.40 1.26 1.59 2.06 1.55
No 1.09 1.25 1.57 2.10 1.50
Mean 1.25 de 1.26 de 1.58 cd 2.08 b 1.52
2 Yes 0.97 1.30 1.59 2.44 1.54
No 1.08 1.23 1.57 3.19 1.64
Mean 1.03 e 1.27 de 1.58 cd 2.77 a 1.59
3 Yes 1.14 1.23 1.50 1.62 1.34
No 1.60 1.28 1.60 1.87 1.57
Mean 1.37 cde 1.25 de 1.55 cd 1.77 bc 1.46
Mean 1.14 z 1.26 z 1.58 y 2.44 x
Elongation
1 Yes 1.89 2.06 1.81 1.89 1.92 b
No 1.85 1.85 1.83 1.82 1.84 b
Mean 1.87 1.96 1.82 1.85 1.88 x
2 Yes 1.85 2.00 2.01 1.96 1.95 b
No 1.80 2.00 1.96 1.94 1.92 b
Mean 1.82 2.00 1.99 1.95 1.94 y
3 1 2.07 2.35 2.69 2.90 2.44 a
2 2.38 1.97 1.98 1.53 2.00 b
Mean 2.22 2.16 2.34 2.08 2.21 z
123 Yes 2.04 r
No 1.92 s
In Masquefa, although the pore number was the same in crusted and non-crusted areas, large pores within crusts were less frequent, more digitated and distributed in bands. This corresponds to the formation process of a coalescing crust, from a structural layer where large compound packing pores become vughs, which means a less connected pore system. This is in agreement with the results of Norton (1987) where the crusts had a lower percentage of planar pores, but not necessarily a lower total porosity.
In all locations and treatments digitation increased with pore size, which is easily explained by their nature (packing pores and vughs). In Subirats (Table 4) there were also signi®cant differences depending on the interaction between pore size and treatment. This is due to the fact that almost all the largest pores under reduced tillage with natural groundcover are vughs due to the collapse of the structure.
Although no differences in elongation were found considering all the data together, in Subirats (Table 4) pores under conventional tillage were longer than those under natural vegetation and these were longer than those under the cover crop. Pores within the crusts were also longer than those in non-crusted areas, and the signi®cant interaction shows that pores within crusts under cover crops were longer than the rest. The observations with the polarising microscope con®rm that this parameter is closely related to runoff crusts. The absence of statistical differences in Masquefa suggested that the susceptibility for crust formation was lower in this location.
4. Conclusions
After 2 years of experiences in vineyards under a Mediterranean climate, reduced tillage did not reduce crust formation as compared with conventional tillage. Crusts under reduced tillage showed a sequence of events, producing structural crusts due to the slaking or coalescence of aggregates, followed by a series of deposition crusts with different degrees of sorting, sometimes eroded. Under conventional tillage the crusts were structural, discontinuous and due to a single crusting event.
Crust morphology and porosity also depended on the microstructure and texture of the soils. Granular
structures produced relatively porous runoff crusts formed by aggregates, which in humid conditions gave rise to a non-connected porosity consisting of vughs. Crusts contained horizontally distributed and more elongated pores than the underlying non-crusted groundmass. Neither total porosity nor other morpho-logical object parameters differed signi®cantly in crusted or non-crusted soils.
Reduced tillage can be questioned as a management technique to reduce crust formation in Mediterranean areas, which was probably related to the dif®culties in the quick establishment of an effective groundcover in a short term and to the low structural stability of the surface soils.
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