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Soil compaction around Eucalyptus grandis roots: a micromorphological study.

Introduction

Plant growth and development depend on many environmental factors including soil compaction. The harmful effect of compaction encompasses decreases in soil porosity, structure, and permeability to air and water, with negative consequences to root growth and plant yield (Lipiec and Hatanob 2003).

There are many studies on the effect of compaction on the growth of various crops, but little is known about the effects of soil compaction around the roots in planted forests, especially Eucalyptus, caused by either their own growth or by natural conditions, and their influence on water and nutrient uptake mechanisms. The few studies report considerable soil variability around the roots of forest trees (Brown 1977; Ryan and McGarity 1983).

As the root expands, the pressure on the adjacent soil decreases the proportion of macro- to micro-pores, increasing soil bulk density. Soil bulk density is a much-used reference tool to characterise compaction and/or the soil porosity (Langmaack et al. 1999). However, due to small-scale spatial variability, soil bulk density is not suitable for localised compaction studies. Fernandes (1996) assessed the influence of chemical, physical, and mineralogical characteristics during compaction in 30xisols and concluded that the soil bulk density did not reflect the intensity of the topsoil compaction observed in the field, although increasing compaction with ploughing was verified.

Regarding organic matter, the susceptibility of the soil to compaction depends largely on the quantity of organic matter (Greacen and Sands 1980). There is a general consensus in the literature that the addition of organic matter improves structure and reduces compaction (Resende et al. 1995).

Soil compaction also depends on the size, shape, and quantity of aggregated particles, and generally mixtures of different size particles lead to greater compaction than a set of particles of approximately the same size, because rearrangement of smaller particles occupies the spaces left by the larger particles (Costa 1985; Bennie and Burger 1988).

The reduction in porosity caused by compaction could limit or halt plant development by water or oxygen deficit in the root environment (Brown et al. 1992). With decreasing diameter and connectivity of pores, the hydraulic conductivity and water infiltration rate decrease in the profile, causing less flow to the roots. Another factor that leads to reduced root systems is the accumulation of CO2 and low oxygen diffusion (Nadian et al. 1996; Borges et al. 1997).

The structure of the compacted soil, when analysed at a more detailed scale, shows reduced macropore size and number, and changes in the shape and continuity of the pores. On a less magnified scale, alterations such as coalescence of aggregates can be observed affecting characteristics such as total porosity and soil density. The alteration in these characteristics can determine changes in the internal conductivity, permeability, and water/air diffusion through the pore system (Young 1998).

The balance of external and internal forces accounts for the root growth, and roots can exert pressures from 9 to 13 MPa on the soil pore walls (Taylor 1974). What is of interest, however, is not the maximum pressure that the roots can exert but rather the resistance that the soil environment imposes that will considerably reduce root elongation (Camargo 1983). Root radial and axial expansion causes compaction in the soil and can create fractures in the soil to prevent excessive energetic expenditure during root elongation (Young 1998).

Dexter (1987) created a simplified model to illustrate the compaction effect by the roots, based on the theory of cylindrical body expansion in a plastic frictional media, to evaluate the soil compression around the roots. An exponential model was obtained that shows that with an increase in the root volume, pore space is lost in the soil around the roots. There is a minimum porosity in the soil which does not allow greater compression. The density decreases exponentially with distance from the root surface as a function of the root diameter.

Taylor (1974) studied the influence of compaction on root development, observing that the root needs to force its passage through the porous space, exerting pressure on the pore space. If the maximum pressure that the root can exert is less than the soil resistance, the root system will decrease, and consequently plant growth and development will be harmed. Although some roots can grow in compacted soil, most grow through the fracture planes. A larger root that grows between fracture planes presses the soil around it, so that the lateral root ramifications cannot penetrate it (Camargo 1983).

The present study was carried out to assess the effects of root growth of Eucalyptus grandis on adjacent soil, using computerised image analysis and micromorphological techniques complemented by infiltration tests in intact clods.

Material and methods

Undisturbed soil samples were collected in a Eucalyptus grandis woodland with trees of 12m height and 3 by 2m spacing, located in Vicosa, Minas Gerais, Brasil (20[degrees]45'S, 42[degrees]52'W) at an average altitude of 650m. The climate is Cwa type (Koppen) with mean annual temperature of 19.5 21.8[degrees]C and mean annual precipitation of 1100-1400mm.

The Eucalyptus grandis woodland is 27 years old, growing on a clayey Kandiudox (58% clay in the B horizon). According to field observation, this Oxisol shows intergrade features between the upslope Red Yellow Latosol (Oxisol) and the down slope Red Yellow Podzolic (Ultisol). The structure is composed of moderately developed sub-angular blocks, and well-developed small micropeds, with a very homogeneous constitution. The mineralogy is kaolinitic, with the presence of gocthite as The main Fe-oxide. The soil is deep, reaching >0.20m at the B/C horizon boundary, being dystrophic and acid.

Pits 2 by 2 by 2 m were dug around 2 Eucalyptus grandis trees, 2 m apart, to collect undisturbed samples from the vicinity of roots (Fig. 1). These soil cores measured approximately 0.6 by 0.6m, consisting of a root in the middle of each sample. This was done with careful sampling in special aluminium boxes for smaller sizes, or by hand for larger roots. Additional soil clods of different root diameters were obtained for imaging at binoculars or scanning electron microscopy (SEM).

[FIGURE 1 OMITTED]

Two methods were used to quantify the degree of compaction caused by the roots. The first was a micromorphological method applied to undisturbed samples obtained up to 6cm from the soil root contact. The second method was the localised infiltration test, where intact blocks were used that had been compacted by roots of different diameters.

Soil porosity in the root vicinity was assessed by micromorphological analysis. Samples were impregnated with polyester resin (POLYLITE T-208), diluted in styrene. A mixture of 60% resin, 40% styrene, and 18 drops of catalyser, was used for impregnate. Polymerisation was performed in a ventilated room for 30 days, replacing the resin as the soil blocks absorbed the mixture, until the samples were completely saturated. After polymerisation, a thin slice was cut with a diamond saw from the soil root contact between 0.5 and 1.0 cm thick, transverse to the root soil contact surface. The samples were mounted on glass slides (2 by 6cm) using araldite glue and ground to a thickness of 30 [micro]m. All slides were prepared in the same direction as the cuts, to a distance of 4 cm from the root.

Slides were prepared of samples collected from roots of 0.3, 0.9, 1.3. 2.8, 3.5, 6.4, 8.0, 9.0, and 10.2 cm diameter. The analyses were carried at in 2 distinct areas: one directly influenced by the root up to 1 cm from the root soil contact surface, and the area less influenced by the root, at 3-4 cm from the contact surface.

The slides were observed under a petrographic microscope and binocular magnifying glass. Under the binocular, the photographs were taken at 10 x magnification, with 6 photographs for each slide. Under the petrographic microscope, the photomicrographs were taken with 40- and 100-fold magnification to observe the microstructure of the area under direct root influence, compared to the area less influenced by the roots. A total area of 12 [cm.sup.2] was studied, which represents a good estimate of soil porosity and structure in highly weathered soils (Schaefer et al. 2001). The photomicrographs were digitalised in a HP Scanjet 4C, with 200 dpi resolution. The sequential set of images (mosaic) was handled by ADOBE-Photoshop 5.5 program, enabling a continuous digital mosaic of photographs to be produced. After this procedure, the QUANTIPORO program was used to process a quantitative evaluation of porosity (Viana and Fernandes Filho 2001). This program allowed the determination of the spatial distribution of the porosity, both in the area under direct root influence and in the reference area less influenced by the roots, assessing the effect of the root on the soil structure for each root diameter. The pore space, as an image component, was determined by unambiguous colour discrimination of soil matrix and pores, according to recommendations of Moran (1994).

Pore orientation was determined by the QUANTIPORO program, and all pores of diameter >0.6cm were excluded, due to insufficient resolution at optical microscopy scale. The roundness was between 0 and 1, and the greater the value, the greater the degree of roundness. The roundness value was calculated from the formula (4 x [pi] x area/[perimeter.sup.2]). Value closer to I indicate that the object approaches the perfect circumference; for values <0.6 the object presents varying shapes from ovals to flattened.

Based on general observations, we set (i) the distance of 0-1 cm from the root surface to distinguish the area under direst root influence and (ii) the distance of 3-4 cm to represent the area under less influence.

Undisturbed samples from the root soil contact surface were also carefully selected and mounted on aluminum discs for observation under scanning electron microscope. Aspects of the surface compaction by the root were observed at high magnification in gold-coated specimens, such as particle orientation on the surface, fractures caused by root growth pressure, the state of particle aggregation inside the clod, and different phases of organic matter decomposition left by the root.

The water infiltration velocity was measured in situ, by which the infiltration time was measured to assess the speed at which of one drop of water, dispensed by a pipette, infiltrated the compaction surface of a clod, pressed by roots of different diameters, compared to the time taken in the non compacted part of the same clod as reference.

For comparison, the infiltration speed was determined using ethanol (C[H.sub.3][C[H.sub.2]OH) and acetonitrile (C[H.sub.3]CN), which are solvents less polar than water.

Results and discussion

Micromorphological aspects

The porosity in the area under direct root influence (up to 1 cm from the root-soil contact surface) was always less than the porosity in the area less influenced by the root (at approximately 3-4 cm from the contact surface) for 0.3-3.5 cm diameters (Fig. 2).

[FIGURE 2 OMITTED]

The porosity differed little between the 2 areas for diameters 3.5-6.4 cm. The area under direct root influence generally showed less porosity in all cases studied. For root diameters >6.4cm, there was increased compaction and porosity loss up to a distance of 4.0cm along the perpendicular axis of the soil root contact surface in all the study area, indicating that these diameters cause compaction at greater distances.

Porosity values measured by optical microscopy in both zones, (i) areas under direct root influence and (ii) areas less influenced by the roots, refer approximately to the soil macroporosity, and therefore do not include microporosity values, as observed in studies by Faria et al. (1998) and Schaefer et al. (2001). Taking this into account, the number of pores in the 2 studied areas could be compared on all the slides by the QUANTIPORO program analysis, grouping them in 3 size classes: pores with area <0.01 [cm.sup.2], area 0.01-0.05 [cm.sup.2], and area >0.05 [cm.sup.2]. The number of pores counted in an area of 12 [cm.sup.2] followed the same tendency as the total porosity; as the root diameter increased the number of pores decreased (Fig. 3). There were many pores both in the area under direct root influence and in the area less influenced by the root up to 2.8cm diameter, where a greater number of pores in the large size class was detected. The number of pores decreased with diameter >3.5 cm and the pore size classes were more evenly distributed.

[FIGURE 3 OMITTED]

In samples with smaller root diameter, there were more pores in the size class with area >0.5 [cm.sup.2], due the greater presence of long thin pores vertical to the root-soil contact surface. These characteristic pores were observed on all slides except on the slide of the smallest diameter roots. They were grouped in the greater size class. The observation that these pores were not on the slide with the smallest root diameter sample (0.3 cm) was attributed to the fact that roots in this diameter class did not exert sufficient pressure to deform the pores. The type of pore in the area under direct root influence is similar to that in the area with less influence. These pores do not stand out compared to the others in root samples greater than 6.4 cm diameter.

Effects of root pressure on the pore spatial shape

The physical impact of root growth on adjacent soil was shown by the presence of parallel fractures forming angles of approximately 45[degrees] to the root-soil contact surface. These fractures indicate chiseling forces, resulting from pressure, associated with tangential traction forces (Lima et al. 1992). Chiseling fractures were observed with 1.3 cm root diameter and larger sizes, indicating that there was cell division in the cortex as the roots lengthened and expanded; the internal part of the cells presents a centripetal growth and the external a centrifugal or irregular growth, so a series of cells was directed to the root periphery (Raven et al. 1978). At the same time many secondary walls of the tracheal elements, first formed from the protoxylem, are deposited in the form of rings. Localised pressures, either ringed or spiral, allow the traqueal elements to be distended or stretched during the organ-lengthening process (Fahn 1995), and thus may explain the helicoidal root development. This helicoidal growth, increasing root diameter, can exert pressure that reduces the pore space, as well as traction, giving rise to chiseling fractures.

The presence of these fractures may be an important mechanism to increase water and oxygen diffusion from regions distant from the root to the rhizosphere environment. Therefore, chiseling may represent a compensating effect for the porosity lost by the expansion and pressure of the root in the soil.

The presence of these fractures showed that most porosity in the soil in contact with the root corresponds to chiseling fractures. The chiseling fractures are concentrated in the first millimetres of soil-root contact. These fractures decreased farther away from the contact surface, while normal pores increased. The fractured area increased with increasing diameter of the root that pressured the soil (Fig. 4).

[FIGURE 4 OMITTED]

The pores, including the chiseling fractures, presented similar orientation in the area under direct root influence and area with less root influence (Fig. 5) but the areas differed in the number of pores. Overall, the greater the number of straight lines in the diagrams, the greater the number of pores. The area with less root influence is less compacted than the area under direct root influence. Therefore it possesses a greater number of pores compared with the number of chiseling fractures.

[FIGURE 5 OMITTED]

There were more pores in the samples compacted by 0.9-1.3-cm-diameter roots at 0-120[degrees] to the horizontal plane of the root axis. The samples compacted by roots >1.3 cm in diameter had fewer pores and more chiseling fractures, and were oriented from 0-60[degrees], mostly in the 0-45[degrees] range. Although the sample compacted by the 0.3-cm-diameter root did not present chiseling, the pores with the same orientation as these fractures were due to the existence of many slits on the root soil contact surface.

The time taken for water infiltration on the surface compacted by roots increased with root diameter (Table 1). The time taken could not be measured on the non-compacted surface (without roots) because the drop was absorbed immediately after application. The same test was performed with ethanol and acelonitrile, less polar solvents than water, on the compacted surface and on the noncompacted surface, and no differences were noticed among the tested surfaces.

There was little difference in the water drop infiltration time up to 3.5 cm root diameter but there was a gradual increase in infiltration time above this root diameter. The surface compacted by the root showed hydrophobic characteristics that may have resulted from root exudates, or from the abundant fungal hyphae observed by SEM (Fig. 6). Thus, the result of the infiltration test gives the combined effect of porosity and hydrophobicity. However, if one considers that a single drop of water on a non-hydrophobic, dry soil surface would infiltrate promptly, regardless of porosity, then longer infiltration times with larger roots are due mainly to hydrophobicity, and not to reduced porosity.

[FIGURE 6 OMITTED]

The soil close to the root surfaces may be modified in many ways by the activity of the roots themselves or by microorganisms present in the rhizosphere. The number of microorganisms increases greatly close to the root surface, and this is related to organic substrate exudation (Foster 1981) and the hydrophobocity observed Wallis and Horne 1992).

Conclusions

1. Compaction and porosity reduction extend to distances greater than the greatest distance studied (4cm from soil-root contact), for root diameters >3.5cm: these effects are accompanied by reduced water infiltration on the root soil contact surface, partially attributed to enhanced hydrophobicity.

2. In a well-structured Oxisol (Kandiudox), root growth determined the formation of aligned, chiseling fractures, at angles usually <90[degrees] to the contact surface, which appeared to be a compensating effect for the loss of porosity at the root vicinity.

3. Micromorphological techniques and image analysis were useful for studying compaction caused by Eucalyptus grandis roots, showing clay oriented features, microfractures, superficial coating by fungi hyphae, and micro-slickenside effects on the root-soil contact surface.

4. These results should be taken with caution, in view of the impossibility of proceeding with replication of testing undisturbed soil clods. They may serve as a preliminary indication of soil compaction induced by tree-root pressure. Further studies are necessary to quantify the effects of root pressure at the soil-root microenvironment, before a more definite statement can be made.
Table 1. Infiltration time of one water drop on the root-soil contact
surface as variable of the root diameter

Root diameter (cm) Time of infiltration (s)

 1.6 11
 1.9 24
 2.2 107
 2.8 97
 3.2 48
 3.5 61
 3.8 217
 4.2 410
 4.8 1288
 5.0 1599
 6.6 1720
 7.3 2030
 9.0 2866
 9.6 2715


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Manuscript received 27 May 2004, accepted 12 November 2004

E. P. Clemente (A,B), C. E. G. R. Schaejer (A), R. F. Novais (A), J. H. Viana (A,) and N. F. Barros (A)

(A) Departamento de Solos, Universidade Federal de Vicosa, Vicosa, MG, Brasil.

(B) Corresponding author. Email: eliane depaula@yahoo.com.br
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Author:Clemente, E.P.; Schaefer, C.E.G.R.; Novais, R.F.; Viana, J.H.; Barros, N.F.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 1, 2005
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