Pore system changes of damaged Brazilian oxisols and nitosols induced by wet-dry cycles as seen in 2-D micromorphologic image analysis

Soil pore structure characterization using 2-D image analysis constitutes a simple method to obtain essential information related to soil porosity and pore size distribution (PSD). Such information is important to infer on soil quality, which is related to soil structure and transport processes inside the soil. Most of the time soils are submitted to wetting and drying cycles (W-D), which can cause important changes in soils with damaged structures. This report uses 2-D image analysis to evaluate possible modifications induced by W-D cycles on the structure of damaged soil samples. Samples of three tropical soils (Geric Ferralsol, GF; Eutric Nitosol, EN; and Rhodic Ferralsol, RF) were submitted to three treatments: 0WD, the control treatment in which samples were not submitted to any W-D cycle; 3WD and 9WD with samples submitted to 3 and 9 consecutive W-D cycles, respectively. It was observed that W-D cycles produced significant changes in large irregular pores of the GF and RF soils, and in rounded pores of the EN soil. Nevertheless, important changes in smaller pores (35, 75, and 150 μm) were also observed for all soils. As an overall consideration, it can be said that the use of image analysis helped to explain important changes in soil pore systems (shape, number, and size distribution) as consequence of W-D cycles.


INTRODUCTION
The soil pore system (SPS) is directly related to the shape, size and spatial arrangement of individual soil particles and aggregates.This arrangement of soil constituents originates different soil pore distributions and porosities, which is crucial, for instance, for good soil aeration, water infiltration, and root penetration (Pagliai 1987).Modifications on soil pore size distribution (PSD), pore shape, and pore arrangement can be used to characterize possible SPS changes due to W-D cycles (Hussein and Adey 1998).
Compaction is one of the most important causes of SPS changes (Pires et al. 2004).These SPS alterations can raise questions regarding the representativeness of soil hydraulic characterizations performed in the laboratory.Soil compaction can be a result of compression processes which can produce strong modifications in SPS.LUIZ F. PIRES et al.As a result, soil bulk density increases followed by soil pore volume decreases can in general be observed.
Wetting and drying (W-D) cycles can also act as a mechanism of SPS changes (Pagliai et al. 1987, Hussein and Adey 1998, Bresson and Moran 2003).During a W-D cycle the soil pore system can be modified, mainly due to changes in soil particle orientations.Some authors have shown that sequences of W-D cycles can cause, for instance, aggregate formation in non-aggregated soils.It can in some circumstances regenerate structurally damaged soils and can induce, by increasing soil porosity, significant changes in SPS (Pagliai 1987, Sartori et al. 1985, Newman and Thomasson 1979, Telfair et al. 1957).
The 2-D image analysis of resin impregnated soil blocks has been used to evaluate SPS changes at meso and microscopic scales.It allows evaluating variations of the type and size distribution of pores in damaged soil samples after W-D cycles (Pillai and McGarry 1999).It can give useful information about the number, area, perimeter, diameter, shape, arrangement, and PSD.
The image analysis technique to quantify and characterize the porosity of impregnated soil blocks was firstly introduced by Jongerius et al. (1972) and Murphy et al. (1977a, b).Several other contributions describing its use were presented afterwards (Cooper et al. 2005, Horgan 1998, Thompson et al. 1992, Ringrose-Voase 1990, Bouma et al. 1979, Bullock and Thomasson 1979).
The objective of this study was to evaluate the ability of the 2-D micromorphologic image analysis in measuring the behavior of damaged SPS of weathered soils (Oxisols and Nitosols) with low clay activity from Brazil, through evaluations of PSD and pore shape changes, when these soils are submitted to sequences of W-D cycles.
Twenty-seven samples, nine from each soil, were collected at the soil surface layer (3-7 cm depth) with aluminum cylinders (h = 3.0 cm, D = 4.8 cm, V = 55 cm 3 ).According to the classical sampling procedure, the cylinders were introduced into the soil by the weight of a rubber mass falling from a given height.The extracted cylinders were completely filled with the soil and the excessive soil was carefully trimmed off each volumetric ring so that the volume of the ring was completely filled with the soil and the top and bottom surfaces of the sample were flat assuring that the soil volume could be taken as the cylinder volume.
To impose W-D cycles on the collected samples the capillary rise method was chosen to saturate them (Klute 1986).Twenty-four hours was found to be the necessary time interval to saturate the samples and to avoid entrapped air bubbles inside them.After saturated the samples were dried by submitting them to a pressure of 400 kPa in a pressure chamber (Klute 1986).These two steps consisted in a W-D cycle.
Three different treatments were investigated: 0WD, the control treatment, in which samples were not submitted to any W-D cycle; 3WD, samples submitted to 3 W-D cycles, and 9WD, samples submitted to 9 W-D cycles.
For image analysis nine samples of each soil were used and for each treatment three samples were selected for impregnation with resin.From the impregnated samples two vertical slices (4.6 × 3.0 cm) were cut (Murphy 1986) and a small area (2.8 × 1.1 cm 2 ), located next to the border of the samples, was selected for 2-D image analysis procedures (Fig. 1).A color CCD camera of 1024 × 768 pixel resolution (area of 100 μm 2 pixel -1 ) coupled to a petrographic microscope (10 times magnification) was used to obtain the digital images.The images were stored on a PC and processed by the Noesis-Visilog 5.4 computer software.Pore area (φ) of the selected soil samples was evaluated dividing the total void space area of the selected cross-section by the total area of the field under analysis, expressed as a percentage.Measured values of φ in general underestimate the volumetric total porosity.During the analysis pores were separated according to their shape and size.Soil pores were divided into three shape classes (rounded, elongated, and irregular) characterized using two indexes and thresholds (Lima et al. 2006, Cooper et al. 2005).Pores of each shape group were subdivided into three different size classes (20-50; 50-500; and > 500 μm).
The SAS software (SAS Institute 1996) was used for data processing.Normality tests were performed on the data prior to their statistical processing.An analysis of variance (ANOVA) and the Duncan test (α = 0.05) were performed to analyze statistical differences and to discriminate means.

RESULTS AND DISCUSSION
Pore shape presented important changes due to W-D cycles and they induced changes in SPS (Figs. 2A to 2C).For the GF soil (Fig. 2A), the W-D cycles promoted a decrease in the number of rounded pores, a slight increase in the elongated ones, and a significant increase in the irregular pores.For the control samples (0WD), the φ composition was: rounded pores 60%, irregular pores 31%, and elongated ones 9%, indicating that the 3WD and 9WD treatments, showed remarkable differences in relation to 0WD.Nevertheless, 3WD and 9WD did not presented substantial differences regarding to their pore shape.As an example of the mentioned changes, irregular pores, that were initially responsible for 31% of φ for 0WD, after 9W-D cycles represented 62% of φ.
For the EN soil (Fig. 2B), with exception to irregular pores, the 3WD presented the most significant modifications, mainly in the rounded pores.A possible explanation for this result can be the small number of replicates for 3WD (only 2 replicates) as compared to the other treatments (6 replicates).This small number of replicates for the 3WD treatment was due to problems during soil impregnation with resin.Comparing 0WD samples to those submitted to 9 W-D cycles, the increase factor for rounded, elongated, and irregular pores was 2.2, 3.2, and 3.6, respectively.Although representing only 15% of φ after 9 W-D cycles, the high increase factor observed for elongated pores (3.2) shows that W-D cycles improved the EN soil from the agronomic point of view, as also observed by Sartori et al. (1985).
The RF soil showed similar results as those found for the GF soil (Fig. 2C).A slight decrease of rounded pores after 9 W-D cycles was observed.Initially rounded pores represented 42% of φ (0WD), a percentage that decreased to 24% after 9 W-D cycles.In relation to elongated pores there were minimum differences among  An Acad Bras Cienc ( 2009) 81 (1) treatments.The percentage in φ due to irregular pores changed from 42% to 66% after 9 W-D cycles.For the RF soil, both rounded and irregular pores are the main pores present in the impregnated analyzed blocks for 0WD.Nevertheless, it was observed that the number of the irregular pores increased enormously after the sequences of W-D cycles for this soil.
The results for the GF and RF soils show that few sequences of W-D cycles can cause strong modifications on SPS of damaged (compacted) samples.On the other hand, W-D cycles did not promote any great change in any particular class of pore of EN samples.A constant increase in φ due to the W-D cycles was only observed for this soil.Pillai-McGarry and Collis George (1990) working with puddled and non-puddled soils found that a minimum of 3 W-D cycles caused significant changes in soil structure; however, in another study, Pillai-Mc-Garry (1991) showed that more than 3 W-D cycles were needed to regenerate the original structure of the topsoil layer of a Vertisol.Rajaram and Erbach (1999) indicated that only one cycle of W-D changed several properties (cone penetration resistance, soil cohesion, aggregate size) of a clay loam soil.The results presented by these authors indicated that there is no detailed information about the number of cycles needed to cause any significant change in SPS.Pillai-McGarry (1991) investigating Vertisols suggested that W-D cycles represent an important process in soil structuring.Sarmah et al. (1996) found an increase in water infiltration and a reduction in shear strength due to low to moderate changes in soil structure of samples after repeated W-D cycles.Pardini et al. (1996) also indicated possible variations of surface roughness, porosity, and pore size distribution during W-D cycles.They found after three W-D cycles an increase in soil porosity induced by the formation of big cracks and fissures.The presence of cracks was not observed in our soil samples, although W-D cycles could promote the formation of microfissures.However, our studied soils are not composed by swelling clays that present large content of smectite clay minerals (Hussein and Adey 1998).Generally this kind of behavior occurs in some clayey soils in which it is common to observe soil cracks or fissures (Chertkov and Ravina 2000).
Despite numerous studies about the effect of W-D cycles in soil structure of swelling soils like Vertisols, there are only a few reports about the influence of these cycles on the structure of tropical not swelling soils.Viana et al. (2004) conducted an experiment to investigate the structural modifications engendered by W-D cycles in Brazilian Latosols (Oxisols) with different mineralogical properties.They observed that after W-D cycles important alterations occurred in both soil shape and structural pattern, which were attributed to a re-organization of soil particles during the plasma shrinkage after drying process.Oliveira et al. ( 2005) also reported the influence of W-D cycles on the structure of Brazilian Latosols and they reported that there is a close interdependence among mineralogical composition, aggregate stability, and water-dispersible clay influenced by the cycles.Soils of reduced aggregate stability with kaolinitic mineralogy are more susceptible to the action of W-D on the water-dispersible clay.Soil mineralogy has a substantial effect on aggregate stability and dispersion, being the smectitic soils the most dispersive and kaolinitic ones the least.The dispersivity of illitic soils is intermediate, but may sometimes exceed that of smectitic soils.In soils dominated by 2:1 clays (Vertisols), the aggregate stability is affected mainly by polyvalent metal-organic matter complexes that form bridges between the negatively charged clay platelets.In 1:1 claydominated soils, the stability is attributed to the binding capacity of the minerals themselves (Wakindiki and Ben-Hur 2002).Unfortunately as we did not carry out mineralogical composition analysis in our work it is impossible to compare our results with those obtained by Viana et al. (2004) andOliveira et al. (2005) for the GF soil and the RF soil (both Latosols).In the world it is also possible to find some scientific literature about the effect of W-D cycles on the structure of not swelling soils in which the authors describe that important changes in SPS occurs after sequences of soil hydration and dehydration (Augeard et al. 2008, Bresson and Moran 2004, Li et al. 2004, Attou and Bruand 1998, Bresson and Moran 1995).
In relation to TNP for the GF soil (Fig. 2A) there were just slight differences among treatments in relation to changes in the elongated and irregular pores.This means that the increase in φ among treatments occurred due to an increase in the area of soil macropores (Pires et al. 2005).For the 3WD treatment, only the few irregular large pores contributed substantially to the observed changes in φ.W-D cycles caused changes in TNP mainly due to decreases in the number of small pores (35, 75, and 150 μm) (Fig. 3A).However, TNP was not substantially modified in relation to large pores (from 250 to 1000 μm) This confirms that the observed increase in φ among treatments probably occurred due to an increase in the area of soil macropores.Pardini et al. (1996) also reported increases in soil porosity of clayey soils due to the raising of the number of pores and also to the formation of cracks and fissures in the structure.The reduction of the contribution of rounded pores to φ can be explained by the reduction of TNP among treatments.
For the EN soil (Fig. 2B), the W-D cycles raised the number of rounded and elongated pores.This caused a constant growth in TNP and also increased the contribution of these pores to φ among treatments.In relation to small pores (35, 75, and 150 μm) the EN soil presented an opposite behavior in relation to those noticed for the GF soil (Fig. 3B).Nevertheless, the application of W-D cycles did not result in substantial differences related to larger pores (250 to 1000 μm).The constant mentioned increase in TNP for different treatments is confirmed by analyzing the areas below the curve (A BC ) of the plot shown in Figure 3B.
For the RF soil (Fig. 2C), a decrease in the total number of rounded pores was observed between 0WD and 9WD treatments and also the maintenance of the number of elongated and irregular pores (except for 3WD).Probably the observed changes in φ were due to the increase of the area of large macropores for 9WD (Pires et al. 2005).Figure 3C shows that this soil presented an increase in TNP for smaller pores (35, 75, and 150 μm) for 3WD and a decrease for 9WD in relation to 0WD.As compared to the other soils, the TNP of large pores (from 250 to 1000 μm) for this soil was not substantially affected by the W-D cycles.This confirms that the changes in φ are related to the increase of the area of this type of pores after the application of W-D cycles.The W-D cycles produced important changes in PSD for the GF soil samples (Fig. 4A).There was a great variation in TNP of irregular pores larger than 500 μm and rounded pores with radii between 50 and 500 μm.Contributions to φ due to rounded pores with radii between 20 and 1000 μm decreased from 59% (0WD) to 30% (9WD), and due to irregular ones passed from 31% (0WD) to 62% (9WD).Several authors have shown modifications in SPS as a consequence of soil structure modifications resulting of natural soil processes or anthropogenic actions (Zund et al. 1997, Pillai and McGarry 1999, Bresson and Moran 2003, Kutílek et al. 2006).
For the EN soil (Fig. 4B), the percentage of φ related to rounded pores with radii varying from 20 to 500 μm was reduced from 68% (0WD) to 44% (9WD), a different behavior as compared to the GF soil.The main contribution to φ for the 3WD treatment was due to the irregular pores larger than 500 μm (51%).For 9WD, 17% of φ is related to rounded pores larger than 500 μm.Another important result for this soil was the increase factor of about 3.2 in the contribution of elongated transmission pores (50-300 μm) to φ between 0WD and 9WD treatments.Changes in all classes and types of pores among treatments indicate some soil structure remediation with the sequences of W-D cycles.
The results for the RF soil (Fig. 4C) indicate that contributions of irregular pores larger than 500 μm to φ varied from 41% (0WD) to 65% after 9 W-D cycles.A slight decrease from 16 to 10% in the contribution of elongated transmission pores (50-400 μm) to φ was observed.Also, after 9 W-D cycles a decrease of rounded pores (from 42% to 24%) was observed for RF soil samples.An increase in φ due to the raising of the number of pores larger than 500 μm confirms that W-D cycle repetitions reduced the damaged areas for this type of soil.
Figure 5 illustrates the changes in pore morphology for all soils submitted to W-D cycles.The objective of showing these 2-D micromorphologic images is to give an insight on the effects of W-D cycles on damaged SPS.
In conclusion it can be said that there were remarkable differences between compacted SPS of 0WD and those submitted to sequences of W-D cycles, for all soils.For all soil samples an evolution from massive structures (Figs. 5A,5D and 5G) to structures with a great number of large and connected pores after W-D cycles could be noticed.A progressive increase in φ as the soil structure changed from massive, in plowed (disturbed) soils, to complex crumb, block, and platy after repeated W-D cycles was also reported by Hussein and Adey (1998).Pagliai et al. (1987) and Sartori et al. (1985), also demonstrating important modifications on the structure for different soil textures after sequences of W-D cycles.Viana et al. (2004) working with Brazilian Latosols showed similar results like those obtained by us for the GF soil and the RF soil after the application of W-D cycles.The hydration and dehydration produces cycles of contraction and retraction of the clay particles and aggregates in a soil affecting its pore distributions.Aggregation by contraction promoted by gravitational forces produces large, irregularly distributed pores which in later stages of development are separated by connected pores as observed by Li et al. (2004).
During wetting and subsequent drying some soil physical properties may change by mechanical factors affecting the total porosity, stress state and energy state (Baumgartl 1998).The wetting and drying processes, in general, result in small alterations in total volume of the core sample (Pires et al. 2005), caused by stresses due to water/air interfaces originated from capillary forces, which increase with soil drainage.As a consequence, after each re-wetting process the soil structure will undergo modifications achieving a new state of energy, which most of the time promotes definitive changes in soil structure like the formation of connected soil pores (Viana et al. 2004).
To conclude it is important to recognize that the hydration of the soil samples used in this study was obtained through the capillary rise method, which sometimes can cause slaking of the soil aggregates leading to significant changes in soil structure.According to Baver et al. (1972) and Klute (1986) when a dry soil aggregate is suddenly surrounded by liquid water, capillary and adsorptive water forces drive the water into the aggregate, compressing the entrapped gas in the intraaggregate pores.In this case an aggregate instability will take place due to an internal collapse caused by the escape of compressed air trapped in the micropores.The procedure of dehydration by using 400 kPa of pressure also can induced some changes in soil structure due to pressure and temperature effects during the time needed to obtain the thermodynamic equilibrium (Moraes et al. 1993).

Fig. 2 -
Fig. 2 -Effects of wetting and drying (W-D) cycles on the pore shape of damaged samples for the three soils.Round = rounded; Elon = elongated; Irr = irregular; φ = total porosity by image; TNP = total number of pores; 0WD (samples not submitted to W-D cycles), 3WD (samples submitted to 3 W-D cycles), and 9WD (samples submitted to 9 W-D cycles) treatments; Porosity values followed by the same letters are not significantly different according to the Duncan test (α = 0.05).

Fig. 3 -Fig. 4 -
Fig. 3 -Changes in total number of pores (TNP) for different pore radii with repetitions of wetting and drying (W-D) cycles.A BC represents the calculated area below the curve r (T N P)dr = A BC .

Fig. 5 -
Fig. 5 -Binary images (pores appear in white and the soil matrix in black) representing the variations on the soil pore system (SPS) of damaged samples with repetitions of wetting and drying (W-D) cycles.Lines at the right corner of the images represent the scale (1000 μm).