Crop Science 42:773-780 (2002)
© 2002 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Spatial Distribution of Roots and Water Uptake of Maize (Zea mays L.) as Affected by Soil Structure
Mariana Amato*,a and
Joe T. Ritchieb
a Dipartimento di Produzione Vegetale, Univ. della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
b Dep. Crops and Soil Science, Michigan State Univ., East Lansing, MI 48824
* Corresponding author (amato{at}unibas.it)
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ABSTRACT
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The incomplete soil exploration by roots is a possible cause of reduced water uptake and the occurrence of strong gradients in water potential between root clusters and the bulk soil. Therefore, the hydraulic or root signal-elicited behaviors of plants might correspond to a lower water content than that of the bulk soil. Evidence supporting this hypothesis is scarce and often circumstantial due to the lack of appropriate techniques. This work studies the relations of root clustering caused by localized soil compaction and the spatial patterns of water uptake in plants grown on stored water. Five soil structure treatments were compared in 100-cm high containers: clay-loam soil (C), sandy-clay soil (S), sandy clay with large clay-loam peds (S + LA), sandy clay with small clay-loam peds (S + SA), and compacted clay-loam soil (CC). Root length density (RLD) and volumetric water content (
v) (time-domain reflectometry) were measured in bulk soil and across peds in 2-cm increments. In the S treatment, plant growth was rapid initially, but green leaf area declined to zero when water was exhausted. Root length density was quite uniform in each layer. In the CC treatment, root density and water uptake were limited and leaf area remained stable throughout the experiment, suggesting that incomplete water extraction was a consequence rather than a cause of reduced plant growth. In the other treatments, when plants had lost all leaves, the water content of peds ranged from 0.23 cm3 cm-3 in the outer 2 cm, where roots were present, to more than 0.30 cm3 cm-3 in the center, scarcely penetrated by roots. Thus, there was incomplete water extraction in the C and S + LA, and to a lesser extent in S + SA treatments. This study shows that large gradients in soil water content occur in cases of root clustering caused by localized soil compaction. These gradients are associated with a reduction in water uptake.
Abbreviations: v, volumetric water content in the soil, cm3 cm-3 • BDO, the moist bulk density at which there is no inhibition of rooting • BDX, the moist bulk density at which rooting is severely impaired • C, clay-loam soil • CC, compacted clay-loam soil • RLD, root length density, the length of roots found in a unit soil volume (cm cm-3) • S, sandy-clay soil • S + LA, sandy clay with large clay-loam peds • S + SA, sandy clay with small clay-loam peds • TDR, time-domain reflectometry
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INTRODUCTION
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THE ROLE THAT SOIL STRUCTURAL STATUS PLAYS in modulating plant water uptake has been studied mainly in relation to the influence of tillage on plant water relations. Soil compaction is believed to cause a reduction in water uptake due to limited penetration of roots (Hamblin, 1985). Also, the effect of soil compaction on root clustering has been proposed to explain limited water extraction within layers colonized by roots (Tardieu, 1987). This effect is not considered in classical plant water uptake models that are based on average RLD in each soil layer. Such models assume that roots are parallel and uniformly distributed in a given soil region. Each root therefore exclusively draws water from a cylindrical region the diameter of which is the mean distance between roots (Gardner, 1960; Newman, 1969). On the basis of this assumption, the distance water has to travel from the bulk soil to the root is often on the order of millimeters (Tardieu and Manichon, 1986).
Such models are inappropriate in cases where root distribution deviates from regular or random patterns (Passioura, 1985; Tardieu and Manichon, 1986; Tardieu et al., 1992). In such cases, the distance between the root and the bulk soil is greater, and can limit water movement within a time-frame useful for the plant to overcome water-deficit stress. Also, the soil around roots will be drier than the average soil, therefore hydraulic or root signal-elicited behaviours of plants will correspond to a lower water content than that measured on the bulk soil. In order to better describe the soil–root–water system's dynamics, models of plant water uptake have been proposed that take root spatial distribution into account (Passioura, 1985; Lafolie et al., 1991) and predict a slower rate of water uptake in case of root clustering. Data to validate these models is needed. Studies in which both root distribution and soil water content were measured showed less uptake where root clustering occurs (Tardieu, 1987, 1988b; Pardo et al., 2000). The distribution of water around roots, however, was not reported due to the lack of adequate experimental techniques.
The objective of the present work is to compare plant growth and spatial patterns of root and water distribution of maize (Zea mays L.) grown on different types of structured soil materials in order to test the hypothesis that, under water-limited conditions, root clustering resulting from localized soil compaction causes reduced water extraction and the occurrence of gradients in water content between root clusters and the bulk soil.
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MATERIALS AND METHODS
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A greenhouse experiment was conducted at Potenza, Italy, in which maize ‘Dekalb Vitrex 200L’ was grown in cylindrical plastic tubes. The tubes, 100 cm high and 25 cm in diameter, were split in half longitudinally and then reassembled with tape so that they could be taken apart for soil sampling at the end of the experiment. Plastic fabric was attached to the bottom and 3 cm of fritted clay were packed on the bottom to allow drainage. Five sets of six containers were placed in one plastic tub each and filled with the soil media. Plastic tubs were 100 cm wide x 70 cm long x 90 cm high, had one drainage hole, and were used to create temporary water tables for soil preparation and irrigation. Five soil media were tested to provide contrast in structural status. They were obtained starting from two field soils (C and S): C (sand 36.5%, silt 24.1%, clay 39.4%), taken from a vertic-ustortents soil at Corleto Perticara in four layers of 25-cm depth. The four layers did not differ significantly in texture, but the organic matter content decreased from 
1.5% in the top layer to 0.3% in the bottom layer. Undisturbed peds from the C soil, taken from the 25- to 50-cm layer in the field at a soil water content of 0.07 g g-1, equal 0.12 cm3 cm-3 on average. At that water content, soil peds were easy to distinguish and remove from the soil; and S soil (sand 52.3%; silt 10.6%; clay 37.1%) taken from Gaudiano di Lavello in four layers of 25-cm depth. The organic matter content changed from 1.3% in the top layer to 0.2% in the bottom one. The two soils differed in their clay mineralogy, since clay in the C soil was rich in montmorillonite as contrasted with the clay in the S soil. The soil layering in the tubes corresponded to the four 25-cm layers taken from a given base soil (C or S). Soil structure treatments were as follows:
Clay-loam soil.
The C soil was packed in the tube in 25-cm layers by pouring the sieved air-dried soil in 5-cm increments, and shaking the container for 3 min. After the placement of each layer the soil was allowed to settle by two wetting-drying cycles to an average bulk density of 1.2 g cm-3.
Compacted clay-loam soil.
The C soil was wet-compacted to a bulk density of 1.5 g cm-3 by the following procedure: the amount of water held by the sieved soil at -0.03 MPa was determined with a pressure plate. Sieved, air-dried C soil was weighed and laid on the greenhouse bench, its moisture content was determined, and an amount of water sufficient to hydrate the soil to -0.03 MPa was added. The soil was then covered and left 1 wk to equilibrate. The moist soil was poured in the container in 5-cm increments, and compacted with a wooden rod of 5 cm in diameter and 110 cm of length, which was raised 5 cm above the soil surface and let fall in concentric circles from the container walls to the center. About 50 taps per soil layer were used. Any water squeezed out from the soil under compaction was absorbed with a sponge.
Sandy-clay soil.
The S soil was sieved and placed in containers in 25-cm layers. Each layer was allowed to settle by two wetting-drying cycles to a bulk density of 1.0 g cm-3.
Sandy-clay soil containing large peds of the clay-loam soil collected from the field.
Three subprismatic peds per container, 14 to 18 cm in dimension were placed with their approximate center laying at the height of 25, 50, and 75 cm from the soil surface, and equidistant from the container walls. The average bulk density of the final mixture was 1.1 g cm-3. Peds represented 16 to 18% of soil volume and 25% of the total soil weight.
Sandy-clay soil containing small peds of the clay-loam soil collected from the field.
Eight subprismatic peds, 6 to 10 cm in dimension were used per tube, regularly distributed in the container. The average bulk density of the final mixture was 1.1 g cm-3. Peds represented 16 to 17% of soil volume and 25% of the total soil weight.
The containers were filled with water by raising the water table in the tubs containing pots to 15 cm from the soil surface for 5 h each day for 7 d, and then drained in order to avoid anaerobic conditions. The containers were randomly placed in the greenhouse, covered with plastic and allowed to drain for 4 wk. In each tube, 2.3-g NH4NO3 was applied with a light irrigation from the top of the pots, and one maize plant was transplanted in each container at the 4-leaf stage by placing the root with attached soil in the top 6 cm of soil of each container, which did not contain any aggregates. Water applied at transplanting was 50 cm3, corresponding to <1% of the field capacity. Plants were grown on stored water. Each treatment was replicated six times.
The daily average air temperature in the greenhouse for the period of the experiment ranged between 18 and 28°C.
On the green leaves of each plant, leaf length from tip to ligule or leaf base was measured weekly starting at 13 d after transplanting. For 10 plants grown out of the experiment, the area of each leaf and the length from the leaf tip to the ligule was measured, and a logarithmic equation was chosen, based on R2 maximization, to express the allometric relation between the two leaf characters:
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On the basis of this relationship, green leaf area was calculated from leaf length for each leaf and sampling date. The experiment was terminated when the green leaf area of the plants was reduced to <20 cm2 plant-1 for Treatments C, S, S + LA, and S + SA. Treatment CC was ended on Day 47 from transplanting, after leaf area reached a stable value, although no signs of wilting were shown by the plants.
For three replications (tubes) per treatment, two 100-cm3 soil cores were taken at the end of the experiment in each of the layers: 0 to 25, 25 to 50, 50 to 75, and 75 to 100 cm. The soil in the cores was dried at 110°C and weighed to obtain the bulk density.The RLD (cm cm-3) in each core was then determined using the technique of Newman (1966). The bulk density of the peds was also determined for Treatment S + LA on 3 peds per tube, and for Treatment S + SA on eight peds per tube, by water displacement after coating the peds with liquid paraffin. For Treatments C and S, the number of 100-cm3 cores sampled was four per layer. Root length density also was determined for the peds. For the S + SA treatment, there were approximately two peds for each of the 25-cm soil layers. For the S + LA treatment, there were only three large peds per container. Each of them was sliced along its central horizontal plane, which laid at the edge of two adjacent layers as described above, and the upper part of the ped was used for RLD determinations attributed to the upper soil layer, and the lower portion of the aggregate was used for RLD determinations attributed to the lower 25-cm soil layer.
According to this sampling scheme, therefore, results of large-scale RLD values are reported in four soil layers for each treatment as are data for bulk density of the soil matrix. The bulk density of peds is reported as an average of the whole container.
The total water uptake across the period of the experiment was determined by weight difference. Since the actual soil volume in the tubes changed during the course of the experiment due to shrinkage, mostly in the C soil, the amounts of final soil water content reported are corrected for the volume variations.
Small-scale characterization of root and water spatial variation was made on the remaining three replicated containers per treatment. Measurements of v and RLD were made according to the following sampling scheme:
Sandy-clay soil treatment.
Measurements were made on three 10-cm horizontal transects selected at random for each of the 0- to 25-, 25- to 50-, 50- to 75-, and 75- to 100-cm layers. Volumetric water content was measured with time-domain reflectometry (TDR) every 2 cm, using small probes 20-mm long, and with a distance between rods of 14 mm (Amato and Ritchie, 1995). During the measurements, the soil was covered with plastic film to minimize evaporation. TDR readings were converted to v based on calibration made on the two soils used in this experiment and reported by Amato and Ritchie (1995). After TDR readings, the transect soil was sampled with a steel sampler designed to collect five contiguous 2 by 2- by 2-cm soil cubes, on which RLD was determined according to Newman (1966).
Other treatments.
For the other treatments, the sampling scheme was modified according to the soil structure: in the S + SA treatment, water content by TDR and RLD were determined in the bulk soil (sandy-clay) on three replicates for each of the four soil layers described above. In each layer, two small clay-loam peds were present, and they were extracted from the soil and sliced along the central horizontal plane. The surface obtained was covered with plastic film and divided into concentric rings by tracing lines parallel to the ped surface every 2 cm on the film. For each ring, three TDR readings were made. The ped was then sliced again 2 cm below the surface used for measurement, and this 2-cm thick section was cut into concentric rings following the lines previously drawn on the plastic. The soil from each ring was divided into three subsamples for RLD determination. Thus, v and RLD were measured as a function of distance from the surface of the ped. Internal ped layers, however, were often too small for triplicate measurements. In this case, the number of samples was reduced to two or one.
Sandy-clay with large clay-loam peds treatment.
Sampled in a similar fashion, but there were only three large peds per container. Each was extracted from the soil and sliced along its central horizontal plane, and TDR and RLD measurements were made as described above. Therefore, for small-scale measurements only, results are attributed to three rather than four soil layers: 0 to 33, 33 to 66, and 66 to 100 cm.
Clay-loam soil treatment.
Shrinkage cracks had formed due to soil drying in the 0- to 25- and 25- to 50-cm layers. Structural units (peds) therefore were distinguishable. For these layers, RLD and TDR water content were measured on three naturally occurring peds per layer as described above.
Compacted clay-loam soil.
Small-scale measurements were not made in the CC treatment due to problems in soil sampling and TDR probe insertion in the compacted soil.
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RESULTS
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Water Use
Figure 1
reports the initial and final v of the soil for the five treatments, and the water extracted by plants and evaporated from the soil surface. The initial v that was held in the tubes against gravity ranged from 0.25 cm3 cm-3 in the S treatment, to 0.43 cm3 cm-3 in the CC treatment. At the end of the experiment, most of the initial water was still present in the CC treatment, while the water content of the S treatment was reduced to 0.15 cm3 cm-3, which corresponds to a laboratory-determined soil tension of -1.5 MPa. The total water extracted from the S treatment was 0.11 cm3 cm-3. In the S + LA and S + SA treatments, the contribution of the clay-loam peds was responsible for an initial water content higher than that of the S treatments. Total water extracted, though, was not statistically different from that observed in S. For the C treatment, the total amount of water extracted was even smaller, and at the end of the experiment the v was 0.28 cm3 cm-3, corresponding with a laboratory-determined tension of -0.5 MPa (Comegna et al., 1990), which is well above the value to which roots dried the soil in the S treatment.
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Fig. 1. Initial and final water content (v) in the containers and water extracted from planting to termination of the experiment for treatments C (clay-loam soil), S (sandy-clay soil), CC (compacted clay-loam), S + LA (Sandy-clay soil + large peds), S + SA (S soil + small peds). Vertical bars represent twice the standard deviation. Where vertical bars are not present, the standard deviation is <0.005 cm3 cm-3.
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Leaf Area
Figure 2
reports the time-course of leaf area for plants growing in the five soil structure treatments. Initial values were quite uniform, 220 cm2, and then increased rapidly in the S treatment, reaching a maximum at 20 d after transplanting. Thereafter, LA decreased quite rapidly, falling to <20 cm2 at 34 d. In the S + LA and S + SA treatments, values reached a maximum at 27 d. The greatest values were found in S + SA. The C soil showed lower values, and a slower decrease after Day 32 of transplanting. Leaf area in the CC treatment showed small variations reaching stable values after 27 d.
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Fig. 2. Green leaf area per plant. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are smaller than symbols.
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Soil Bulk Density
Bulk density values are reported in Table 1. In all soils, the bulk density at the end of the experiment increased with soil depth. Values measured in the CC treatment were quite high, increasing slightly with depth. The high CV (ranging from 5.8 to 11.2%) were due to problems in sampling tube insertion in the compacted soil. Densities for the bulk (sandy-clay) soil were quite similar between treatments S, S + SA, and S + LA. The bulk density of the peds was higher than that of the bulk soil, and the values were greater in the small peds compared with the large ones.
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Table 1. Bulk density and coefficient of variation from 100 cm3 cores for the soil matrix as a function of soil depth and for soil peds. Treatment C = clay-loam soil, S = sandy-clay soil, CC = compacted clay-loam, S + LA = sandy-clay soil + large peds, and S + SA = sandy-clay soil + small peds.
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Root Growth
A summary of the results for the RLD averaged across measurements on 100 cm3 cores and on peds for each soil layer is shown in Table 2. The RLD was greatest in the sandy clay soil, and decreased slightly with depth. The pattern was quite similar in the S + LA and S + SA treatments, and lowest in the CC soil, in which it declined markedly below 50 cm. No roots were found in the CC treatment below 75 cm. Variability was quite high in all treatments, especially so in the C, S + LA, and S + SA tubes, if all samples were pooled to calculate mean values for each layer. If samples taken from the bulk soil and from the peds were considered separately for the S + LA and S + SA tubes (Fig. 3) , the distribution became bimodal, with much less variability within these soil structures. In the 0- to 25-cm layer of the S + LA treatment, for instance, the coefficient of variability was reduced from 79% for the pooled samples to 40% for roots growing in peds only.
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Table 2. Average root length density in each soil layer. Treatment C = clay-loam soil, S = sandy-clay soil, CC = compacted clay-loam, S + LA = sandy-clay soil + large peds, S + SA = sandy-clay soil + small peds.
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Fig. 3. Root length density (RLD) in peds and fine soil of Treatment S + SA (Sandy-clay soil + small peds) and S + LA (sandy-clay soil + large peds) at different soil depths at the end of the experiment. Vertical bars represent the standard deviation.
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Small Scale Root and Water Characterization
Figure 4
reports the spatial variation for RLD and TDR-determined v in the four soil structure treatments. Values of RLD in the 25- to 50-cm layer, across a 10-cm transect in the S treatment ranged from 0.94 to 1.43 cm3 cm-3, and v ranged from 0.126 to 0.133 cm3 cm-3. No clear trend in space was detected for either RLD or v.
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Fig. 4. Small scale measurement of volumetric water content (v) determined by TDR and root length density for four soil structural treatments. S = across a 10-cm soil transects in the 25- to 50-cm layer of Treatment S (sandy-clay soil). S + LA = in the bulk soil and across peds in the 33- to 66-cm layer of Treatment S + LA (sandy-clay soil + large peds). C = across peds in the 25- to 50-cm layer of treatment C (clay-loam soil). S + SA = in the bulk soil and across peds in the 25- to 50-cm layer of Treatment S + SA (sandy-clay soil + small peds). Vertical bars represent twice the standard deviation.
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The RLD and v measured outside and across peds in the 33- to 66-cm layer of the S + LA treatment are also shown in Fig. 4. Root length density in the bulk soil averaged 1.58 cm cm-3. Roots were found in the 0- to 2-cm layer (ped surface) in the ped, where the average RLD was lower than in the bulk soil (0.74 vs. 1.5 cm cm-3), whereas in the internal layers of the peds roots were found only occasionally, probably growing in cracks or biopores. Earthworm pores were found across aggregates collected from the same soil in the field, a few of them with roots, scarcely ramified, growing along the pore walls. The corresponding v values showed a water content gradient within peds, with values ranging from 0.246 cm3 cm-3 in the external layer (0–2 cm) to 0.350 cm3 cm-3 and higher in the layers beyond 4 cm from the ped surface. Values for v of the S soil outside the peds were lower than those of the ped outer layer, but in this case at least the discrepancy is probably due to the different texture and density of the two soils. The soil water potential values were likely much closer than the values of v.
The corresponding figure for the 25- to 50-cm layer of the C treatment (Fig. 4) shows very similar trends to that in the Treatment S + LA, although the actual values for RLD at the ped surface are higher, probably as a result of some compensatory growth since in the C treatment roots could not grow in fine soil. A large proportion of the roots measured in the 0 to 2 cm were in fact located on the surface of the peds. The root spatial distribution across peds was similar for the S + SA treatment (Fig. 4), for which few if any roots were found beyond the surface layer of the peds. For both C and S + SA treatments, the measured gradient in v across peds was smaller than that found in the S + LA treatment.
Values of TDR-determined v for the bulk soil and across peds for all layers are reported in Tables 3 and 4. Bulk soil v was quite low in some of the samples in the top layer of the tubes due to soil evaporation at the surface. Gradients across peds were found in all soil layers. In the C soil below 50 cm, shrinking cracks were not clearly detected; therefore, sampling across structural units was not possible. The values of standard deviation associated with v measurements were lower than 0.028 cm3 cm-3, except for the internal layers in the S + SA treatment, in which the standard deviation was greater. The reason for a greater variability in those layers is that peds in the S + SA treatment ranged from 8 to 10 cm in size. Therefore, the internal layer was actually at different distances from the ped surface in the different peds.
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Table 3. Time-domain reflectometry determined volumetric water content across peds and in bulk soil for treatment S + LA (sandy-clay soil + large peds), S + SA (sandy-clay soil + small peds), and C (clay-loam soil).
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Table 4. Time-domain reflectometry determined volumetric water content across 10-cm transects for Treatment S (sandy-clay soil).
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Values of RLD for the bulk soil and across peds for all layers is reported in Tables 5 and 6. Measured RLD in the bulk soil of treatments S + LA and S + SA was greater than in S, probably as a result of compensatory growth, since growth in peds was limited in treatments S + LA and S + SA. Roots were found beyond the outer 2 cm in peds only in biopores or secondary cracks.
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Table 5. Root length density (RLD) across peds and in bulk soil for treatment S + LA (sandy-clay soil + large peds), S + SA (sandy-clay soil + small peds), and C (clay-loam soil).
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In all clay-loam structural units, the v of the external layer ranged from 0.220 to 0.246 cm3 cm-3. According to Comegna et al. (1990), this amount corresponds to a soil matric potential of -0.8 to -1.0 MPa in this soil. This value is higher than that to which roots dried the soil in the S treatment, but since it is an average value for the whole 0 to 2 cm of the external layer, the actual value around roots may be lower. Dunham and Nye (1973), Hsieh et al. (1972) and MacFall et al. (1991) report large water content gradients across the first 3 to 5 mm from a root plane, and if measurements had been made with smaller spaced increments in this experiment, steeper gradients might have been found. A visual analysis of the samples suggested that roots were actually not uniformly distributed in the 0- to 2-cm layer, but density was greater close to the surface of peds.
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DISCUSSION AND CONCLUSIONS
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The overall behavior of plants in treatments C, S, S + LA, and S + SA indicates that water deficit limited plant growth and led to almost total loss of green leaf area. The faster initial growth and more rapid development of plant water deficit observed for the S treatment was presumably due to greater soil exploration, and consequently more rapid water depletion. The clay-loam contribution in the different growth media was to increase the amount of water held per unit soil volume, and to make it more slowly accessible due to incomplete soil exploration. For the CC treatment, the plants had not died, nor did they show signs of wilting at the end of the experiment, but their size and rate of growth were remarkably reduced. There is evidence in the literature (Masle and Passioura, 1987; Smit et al., 1989) suggesting that there is a direct effect of soil strength measured by soil density or penetration resistance on plant growth. Also, the air-filled porosity of the CC treatment was low, and poor aeration may have contributed to the reduced growth rate. In this treatment, little water was extracted by small but turgid plants, suggesting that factors other than water availability itself were limiting. If so, incomplete water extraction may be seen more correctly as a consequence rather than a cause of limited plant growth. Another possibility is that early water deficiency reduced growth and led to osmotic adjustments sufficient to maintain leaf turgor. In the other treatments where water extraction was less than expected (C, S + LA), little if any green leaf area was left after severe wilting. This indicates that water availability was a limiting factor, despite the average water content of the tubes corresponding with values of water potential above those to which plants dried the soil in uniform soil (S treatment).
The structural characterization of the growth media at the end of the experiment showed a greater bulk density in peds than in fine soil, and higher values in small peds than in large ones. Two main factors are likely to be the cause of the difference between the bulk density of the small and large peds. First, the bulk density of the peds was calculated on the volume as sampled at the end of the experiment. The water content of the inner layers of the large peds was higher than that of the outer layers at that time. The small peds were more uniformly dry, as shown by the small scale water measurements (Fig. 4). Therefore, while the density of the small peds corresponded with a lower water content, the density of large peds was an average from layers of increasing water content, and therefore corresponds with a wetter density than that of small peds. Second, large peds are more likely to enclose structural cracks or biopores, which would also explain a lower density, as reported by Amato and Ruggiero (1994).
The literature aiming to characterize the relevance of soil mechanical impedance to root growth is discussed in relation to bulk density and resistance to penetrometer insertion (Barley and Greacen, 1967; Russell, 1977; Bennie, 1996). Both features present a high variability, depending on the specific methods used (Amato and Ruggiero, 1994; Greacen, 1986). Their relationship with root growth is also variable (Cockroft et al., 1969). The relations between soil bulk density and root growth have been discussed by Jones et al. (1991), who used the soil sand percentage by weight to predict the moist bulk density at which there is no inhibition of rooting (BDO) and that at which rooting is severely impaired (BDX). The bulk density values reported in our work for the S treatment, and for the sandy clay soil in the other treatments, were in all cases lower or around BDO. In these soils, therefore, no large effect of bulk density on root growth is predicted. Bulk densities for the peds and the C treatment reached values corresponding to those limiting root growth (higher than BDO). In CC, they are even higher than BDX, thus explaining the severe impairment of root growth found in the compacted clay.
In any case, characterizing soil mechanical impedance with a single parameter does not allow accounting for soil features which may provide ways for root penetration even where the bulk soil strength is high. Among them, there is the macroporosity due to shrinkage cracks, earthworm pores or previous root channels. In this study, the reported decrease in density values with increasing soil volume, especially in the clay-loam soil, suggests that cracks were present in which localized root growth could take place.
It should also be pointed out that local soil conditions (like strength in a soil layer) may affect the suitability of a particular soil region for root colonization. But the actual presence of roots in that zone will also depend on whole plant factors, like general water status or nutrition (Tardieu, 1989) and the time-course of stress development. The conditions in adjacent soil regions will also play a role. Tardieu (1988a)(1989) reported that a soil region where soil strength is not limiting per se, may have low root density because of local compaction directly above (shadow effect). In our experiment, root density was greater in the peds of the clay-loam treatment compared with peds in the other treatments. The observation may be discussed in terms of compensatory growth (since in C roots did not have more hospitable soil to grow into), rather than in relation to differences in local ped conditions.
Root length density values were quite variable, but results show how some of the variation in root sampling can be eliminated if a structured (nonrandom) component is identified and separated from the total variability, as we did by separating roots found in peds from those in the bulk soil. This treatment, of course, requires that appropriate sampling schemes be used. Similar treatment was shown by Tardieu (1988a) to reduce the coefficient of variation for length density of maize roots grown in soil with compacted inter-rows.
Effects of structural status on root clustering and water content were documented. Roots grew quite uniformly in the S treatment and in the bulk soil or shrinking cracks of the other treatments, but they did not penetrate peds beyond 2 cm from the surface unless biopores or secondary cracks were present.
In all structured treatments (C, S + LA, and S + SA) water gradients across peds, away from roots, were detected. Tardieu (1987)(1988b) documented a higher soil water content in soil regions where the root density of maize was low. Pardo et al. (2000) found unextracted water in the center of peds not completely colonized by chickpea (Cicer arietinum L.) roots, but they were not able to measure variations of water content with distance from root clusters. In the present experiment, we documented such variations using time-domain reflectometry with small probes. The lack of techniques for measuring water content in situ on a small scale for a soil with a high content of clay prompted us to choose this technique, although problems of excessively high dielectric constant values were found in calibration of small probes such as those used in this experiment (Amato and Ritchie, 1995). This problem could render data from our experiment questionable, at least quantitatively. Nevertheless, the consistency of measurements, even at high v, suggests that the determined values were not an artifact of the TDR technique. It may be concluded, therefore, that root clustering was associated with less extraction of water. The gradients in water content found across peds were responsible for the low water extraction reported for the S + LA and C treatment, and to a smaller extent in Treatment S + SA. The water left in peds was on average 69% of the initial water for the large peds (S + LA), 58% for the small ones (S + SA), and 60% for the C treatment.
Received for publication April 28, 2000.
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REFERENCES
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INTRODUCTION
