Changes in pore structure in a no-till chronosequence of silt loam soils ...

Changes in pore structure in a no-till chronosequence of silt loam soils, southern Ontario A. J. VandenBygaart1, R. Protz1, and A. D. Tomlin2 1Department

of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1; e-mail: [email protected]; 2Southern Crop Protection and Food Research Centre, Agriculture and Agri-food Canada London, Ontario, Canada N5V 4T3. Received 11 May 1998, accepted 22 October 1998. VandenBygaart, A. J., Protz, R. and Tomlin, A. D. 1999. Changes in pore structure in a no-till chronosequence of silt loam soils, southern Ontario. Can. J. Soil Sci. 79: 149–160. Many research studies have dealt with the influences of minimum or notill soil management practices on the major physical, chemical, biological and morphological properties in the soil profile. However little work has been done on the assessment of the rates of changes in pore properties as management practices are converted from conventional to no-till (NT) methods. Short-term changes in soil micromorphology attributed to conversion to no-till from conventionally tilled management are evaluated in this paper. As the number of years in no-till increased there was a decrease in the number pores of 30- to 100-µm diameter in the no-till soils. However, pores from 100- to 500-µm diameter increased in number only after 4 yr of NT. The pores of this size are important for water storage, transmission and root development. The decline in the number of these pores after no-till initiation followed by the increase after 4 yr may explain why crop yields tend to be lower only after the first few years after implementing no-till. The 100- to 500-µm diameter pores may be crucial for the proper development of roots in wheat and corn. The no-till soils had greater numbers of horizontally oriented elongated macropores in the top 5 to 15 cm of the soil profile due to the lack of tillage and annual freeze-thaw processes. These pores may inhibit proper drainage and root penetration. Rounded macropores increased with the number of years the soil was in no-till as these pores were maintained each year due to the lack of tillage and greater faunal activity in the no-till soils. Key words: Micromorphology, image analysis, earthworms, no-till, soil structure VandenBygaart, A. J., Protz, R. et Tomlin, A. D. 1999. Évolution de la porosité dans une chronoséquence de loams limoneux conduite en régime de semis direct dans le sud de l’Ontario. Can. J. Soil Sci. 79: 149–160. De nombreuses recherches ont porté sur les influences des pratiques de travail minimum du sol ou de semis direct sur les grandes propriétés physiques, chimiques, biologiques et morphologiques du profil des sols. On s’est relativement peu intéressé à l’évaluation des taux de changement des propriétés des pores lorsqu’on passe des méthodes de travail classique au semis direct. Dans cet article, nous évaluons les changements à court terme de la micromorphologie du sol résultant de cette reconversion. À mesure qu’augmentent les années en semis direct, on constate une diminution des pores de 30 à 100 µm de diamètre dans les sols. En revanche, le nombre des pores de 100 à 500 µm de diamètre ne commence à s’accroître qu’après 4 années de semis direct. Les pores de ce calibre sont importantes pour le stockage et pour la transmission de l’eau ainsi que pour la croissance racinaire. Le recul du nombre de ces pores dès le début de la pratique du SD, suivi de leur accroissement après 4 années, peut éventuellement expliquer pourquoi les rendements culturaux ne commençaient généralement à baisser qu’après les quelques premières années de la mise en place de la pratique du SD. Les pores de 100 à 500 µm de diamètre joueraient un rôle crucial pour le bon développement des racines du blé et du maïs. Les sols conduits en SD contenaient plus de macropores allongées horizontales dans les 5 à 15 premiers cm du profil à cause de l’absence de travail mécanique et des processus annuels de gel-dégel. Ces pores pourraient nuire au drainage et à la pénétration des racines. Les macropores arrondies croissaient en nombre avec les années de conduite du sol en semis direct, du fait qu’elles se maintiennent d’une année à l’autre grâce à l’absence de travail mécanique du sol et à la plus grande activité faunique dans les sols ainsi conduits. Mots clés: Micromorphologie, analyse d’image, vers de terre, semis direct, structure du sol

Many recent research studies have dealt with the influences of minimum or no-till soil management practices on the major physical, chemical, biological and morphological properties in the soil profile. Much of this work has focussed on the changes to the physical properties of the soil which inherently influences infiltration, moisture retention and soil erosion potential (Edwards et al. 1988; Wollenhaupt et al. 1995; Azooz and Arshad 1996). There has also been significant work on the influence of no-till management on the structural and morphological characteristics of the soil. Pagliai et al. (1983) used the image analysis of thin sections to compare the size, shape and orientation of the pore structure in soil under no-till relative to conventionally tilled soils. Although they conclud-

ed that the overall porosity was much greater in the conventionally tilled soils, there was a greater proportion of pores of size range 30 to 500 µm diameter. Shipitalo and Protz (1987) evaluated the influence of tillage and no tillage on the soil morphological properties and porosity using the image analysis of intact soil blocks. They found that total porosity in the no-till was about half of that found in the conventionally tilled soils. However, this was somewhat counterbalanced by two to nine times more bioporosity in the no-till that was attributed to a greater earthworm population in the no-till. The measurement of the orientation of pores is possible using existing image analysis software. Pagliai et al (1983) and Shipitalo and Protz (1987) noted greater horizontally 149

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Table 1. Particle size, pH, % CaCO3, and organic matter in the soils of the studyz Watershed

pH

% CaCO3

% OM

% Sand

% Silt

% Clay

4 years NT 4 year CT Pair

6.3 7.0

0.9 1.2

4.1 4.1

20.8 20.1

59.6 55.9

19.6 24.0

6 years NT 6 year CT Pair

7.3 7.2

2.3 1.9

4.2 3.6

14.6 12.8

63.1 62.4

22.2 24.8

11 years NT 11 year CT Pair

7.0 7.4

2.3 3.2

3.9 4.4

21.8 27.4

57.9 56.0

20.3 16.6

zMean

for 33 samples in top 20 cm for each watershed.

oriented macropores in no-till soils relative to conventionally tilled ones. However, the distribution of pore orientation with depth in the profile was not evaluated in either study. Pagliai et al. (1995), in two alluvial soils in Italy, found greater numbers of transmission pores, which resulted in a greater hydraulic conductivity compared to a conventionally tilled counterpart soil. Storage pores (0.5 to 50 µm) were found to increase in the no-till as determined by mercury intrusion porosimetry. Little, if any, research, however, has evaluated the morphological changes occurring over time after no-till is implemented. Information on the rates of changes occurring over time could allow for better management of the soils. For example, if the number of transmission pores of size 50 to 500 µm is reduced in the first 4 yr of no-till, this will alter the water status of the soil, and therefore the soil may be better suited for one crop type than another. If the number of transmission pores increases after 4 yr, again this may alter the moisture status of the soil and could influence the suitability of the soil for certain crops. The objective of this study was to analyze and interpret the differences in the soil micromorphology with time on an 11-yr chronosequence of no-till soils. This was accomplished by the visual inspection and comparison of soil thin sections from the no-till and conventionally tilled soils, coupled with assessing the main factors affecting the pore size distribution, shape and orientation, using automated computer image analysis of soil thin sections. MATERIALS AND METHODS Sampling and Thin Section Preparation Three no-till (NT) and conventional till (CT) paired watersheds in similar soil types, textures and crop rotation were sampled for their soil properties after harvest and before fall tillage (in the CT soils), in September and October, 1994. The three NT watersheds had been under their existing management regime for 4, 6 and 11 yr. Each of these watersheds had adjacent CT paired watersheds. All soils were in the wheat phase of a corn-wheat-soybean rotation during sampling. The watersheds were approximately 100 m in length and 1.5 ha in area. Each of the CT watersheds were moldboard ploughed. The NT watersheds were also moldboard ploughed prior to the initiation of the conservation management. The effective depth of tillage varied from 20 cm to beyond 30 cm in some of the lower slope positions due to colluvial additions of eroded soil materials from upslope.

Soil pH was determined in a 0.01 M CaCl2 solution using a hand-held pH meter. Organic carbon was determined by the modified Walkley-Black method (Tiessen and Moir 1993) and converted to percent soil organic matter using the 1.724 factor. The percent CaCO3 equivalent was measured by the approximate gravimetric method of Raad (1978). Particle size determination used the sieve-pipette method by Kilmer and Alexander (1949). The soils were silt loam-textured Orthic Gray-Brown Luvisols (43°N, 81°W) located in Huron and Middlesex Counties, in southwestern Ontario. General properties of the soils during sampling are listed in Table 1. More specific data on these soils are given by VandenBygaart (1998, unpublished Ph.D. thesis). A soil profile was excavated at three slope positions (lower, middle, upper) in each of the six watersheds. In each soil profile, at each slope position, six intact soil samples for micromorphology were taken in 5 cm increments to 30 cm depth using 9.0 × 7.5 × 5.0 cm Kubiena tins. Bulk density cores (100 cm3) were taken in 5cm increments from the surface to 30-cm depth. Vertically and horizontally oriented thin sections from each depth increment were polished to 30 µm thickness and imaged for RGB colours (256 levels of grey per wavelength) at a resolution of 4096 × 3072 (12.5-µm pixel) by a digital scanner (Array Technologies). The 12.5-µm size pixel coupled with the large thin section image allowed for sufficient representation of the pore space greater than the 30-µm thickness of the slide in these soils. This is supported by the establishment of the representative elementary area (REA) of 2 × 2.5 cm for thin sections in order to account for the pore sizes of interest (i.e. 30 µm to 3000 µm) (VandenBygaart and Protz 1999). Some CT sites did not allow for adequate sampling to the 30-cm level due to stoniness, and, therefore, statistical analysis was performed only on samples in the top 20 cm. Image Analysis To account for the similarity of the spectral properties of transmitted light between voids and anisotropic minerals (i.e. quartz) during image analysis, both transmitted and reflected images were collected (6 wavelengths: RGB channels in reflected and transmitted light). Cluster analysis (Kmeans) was used to classify the images into 30 different classes of pedofeatures based on the spectral properties in the six wavelengths using EASI/PACE™ remote sensing image analysis software (PCI™ Inc., Richmond Hill,

VANDENBYGAART ET AL. — PORE STRUCTURE IN A NO-TILL CHRONOSEQUENCE

No-till 11 years Lower Slope

0-5 cm

151

Conventional tillage Lower Slope

A

G

B

H

C

5-10 cm

D

I

10-15 cm

15-20 cm

E

20-25 cm

J

F

K

25-30 cm 1 cm

Fig. 1. Digital images of vertically-oriented thin sections from NT (11 yr) and CT silt loam soils in southwestern Ontario (lower slope position).

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Fig. 2. Bulk density with depth in the NT and CT soils of this study. Bulk density pooled for slope position and years in NT. Bars represent ± standard error of means.

Table 2. ANOVA table for interaction effects of management, depth, slope and NT age a. 30–100 µm, b. 100–500 µm, c. 500–1000 µm and d. 1000–3000 µm Summary of all Interaction Effects; design: 1, management; 2, depth; 3, slope; 4, NT age Interaction effect

df effect

MS effect

df error

MS error

F

P-level

a.

12 13 14 134

3 2 2 4

12369.78 141473.11 68081.59 44452.08

70 70 70 70

17926.93 17926.93 17926.93 17926.93

0.69 7.89 3.80 2.48

0.561 0.001 0.027 0.052

b.

12 13 14 134

3 2 2 4

3575.70 16387.41 6197.81 2422.20

70 70 70 70

1223.42 1223.42 1223.42 1223.42

2.92 13.39 5.07 1.98

0.040 0.000 0.009 0.107

c.

12 13 14 134

3 2 2 4

31.05 131.24 47.91 46.00

70 70 70 70

8.04 8.04 8.04 8.04

3.86 16.33 5.96 5.72

0.013 0.000 0.004 0.000

d.

12 13 14 134

3 2 2 4

16.72 22.50 2.44 17.13

70 70 70 70

3.35 3.35 3.35 3.35

5.00 6.72 0.73 5.12

0.003 0.002 0.485 0.001

Ontario, Canada; http://www.pci.on.ca). Those classes representing the voids in the sections were selected by visual comparison with the soil slide. The final image was exported to the morphometric image analysis package Sigma Scan Pro™ (SPSS®, Chicago, IL, USA; http://www.spss.com).

Sigma Scan Pro™ was used to measure the area, perimeter and orientation properties of the voids in each of the images. Void areas were converted to an equivalent pore diameter (e.p.d.) where e.p.d. = 2(area/π)0.5. Pore shape in two dimensions is related to pore area and perimeter and is

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SF 0.04 = rounded The orientation of elongated pores was measured in Sigma Scan Pro™ by calculating the angle of the long axis relative to the horizontal for each elongated pore. The long axis was determined by joining the two furthest pixels in the elongated pores with a line, and calculating the length of that line. Statistical Analysis Analysis of variance was utilized to determine if there were significant differences (P ≤ 0.05) between the means for pore sizes of 30–100 µm, 100–500 µm, 500–1000 µm, and 1000–3000 µm in the NT and the CT soils. Tukey’s HSD test was then performed on each of the pore sizes where significant differences were determined between slope position, management and number of years in NT conservation and the interaction effects. Management (NT and CT) was the main plot factor in the analysis, while pore shape (elongated, irregular, rounded) was pooled into pore size, and thin section orientation (horizontal and vertical) was treated as a replicate and pooled at each depth, for the analysis of variance statistical analysis. Statistical analysis was carried out using STATISTICA™ (Statsoft®, Inc., Tulsa, OK, USA; http://www.statsoft.com). Since the CT sites were ploughed each year, the interaction effects involving the number of years in NT had to include the management effect (i.e. years in NT × management, and years in NT × management × slope position). This was due to only the NT data being related to the number of years in NT (i.e. the CT sites could not be treated individually unless the management interaction was included in the analysis). For the pore morphometric analysis of the study, thin sections samples with different orientations, different depths and slope positions were pooled together, with the exception of the orientation analysis where only vertically-oriented thin sections were used. Only pore sizes 100 µm diameter and greater were considered in the morphometric analysis. This is because the shape analysis of pore sizes smaller than 100 µm is problematic. Below 100 µm the number of pixels in an object approaches the size of the pixel, and as a result the object is forced into the rounded size class. Thus the shape factor is of no value due to the object approaching the shape of the square pixel. Fig. 3. Number of pores per cm2 for (a) 100–500 µm, (b) 500–1000 µm, and (c) 1000–3000 µm size classes for depth × management interaction in the top 20 cm of the NT and CT soil profiles of the study. Data were pooled for slope position and years in NT. Bars with different letters in each size class significantly different at P ≤ 0.05 (Tukey’s HSD).

measured by a shape factor (SF): SF = A/P2 where A is pore area and P is pore perimeter (Pagliai 1995). Pore perimeter is calculated by counting the number of pixels along the outside of the pore boundaries. Three shape types can be inferred from the SF (Pagliai 1995):

RESULTS AND DISCUSSION Micromorphic Observations CHANGES IN SOIL MICROMORPHOLOGY AFTER 11 YEARS OF NT. Figure 1 shows the digital images of vertically-oriented thin sections from the 11 year NT and CT paired site. The characteristic differences between the two tillage treatments after 11 yr can be analyzed by visual interpretations of the soil micromorphology. The 11-yr watershed pair was chosen as representative for comparison of the micromorphology between the tillage treatments. Further interpretations of the rates of the micromorphological changes occurring in the NT will be evaluated utilizing the automated image analysis results.

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Fig. 4. Number of pores per cm2 for (a) 30–100 µm, (b) 100–500 µm, (c) 500–1000 µm, and (d) 1000–3000 µm size classes for slope × management interaction in the top 20 cm of the NT and CT soil profiles of the study. Data were pooled for depth and years in NT. Bars with different letters in each size class significantly different at P ≤ 0.05 (Tukey’s HSD).

In the 11-yr NT soil, the top 1–2 cm is dominated by a spongy structure in which the soil matrix continuity is broken by many interconnected packing voids (Bullock et al. 1985) (Fig. 1A). Between 3 and 5 cm depth the structure is dominated by horizontal-trended packing pores (RingroseVoase and Bullock 1984) with well-developed interconnectivity (Fig. 1B). The functional properties of a soil structure dominated by packing pores include rapid infiltration of water and easy root penetration and migration (RingroseVoase and Bullock 1984). Figure 1C marks an earthworm channel that has been intersected by the slicing of the thin section. Earthworm channels are also functional as conduits for roots and water (Lee and Foster 1991; Edwards et al. 1995). From 5 to 15 cm the structure is platy with planar voids separating horizontally oriented and elongated aggregates (Fig. 1D). This structure likely develops due to the settling of tillage pores (Ringrose-Voase and Bullock 1984). However, freeze-thaw processes may also contribute to platy structure due to the creation of ice lenses under freezing conditions. When the ice lenses thaw the voids are left intact with a horizontal orientation (Williams 1968).

Beyond 15 cm depth there are abundant channels that are created by earthworms and roots. As the soil is no longer being ploughed these macropores are maintained and stay intact. Earthworm numbers increased sharply after the initialization of NT at the sites in this study (VandenBygaart 1998). This is supported by many other researchers who have noted marked increases in earthworm numbers after implementing NT (Edwards and Lofty 1982; House and Parmelee 1985; Edwards et al. 1995). The greater earthworm numbers also likely aid in adequate drainage, as bulk density increased significantly in the top 25 cm of the NT soils (Fig. 2). The earthworms are not disrupted by annual tillage in the top 30 cm in the NT, and are therefore allowed to continue to create and maintain burrows that extend below 30 cm. This likely further aids hydraulic conductivity in NT soils (Edwards and Shipitalo 1998). Recent studies have clearly concluded that the permanent macropores created by earthworms are major contributors to adequate drainage in NT soils during intensive rainfall events (Edwards et al. 1988; Edwards and Shipitalo 1998). However, the burrows have been implicated as promoting advanced movement of surface-applied agricultural chemi-

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Fig. 5. Number of pores per cm2 for (a) 500–1000 µm and (b) 1000–3000 µm size classes for slope × management × year in NT interaction in the top 20 cm of the NT and CT soil profiles of the study. Data were pooled for depth. Bars with different letters in each size class significantly different at P ≤ 0.05 (Tukey’s HSD).

cals to groundwater before the chemicals can be decomposed (Edwards et al. 1988). Surface soil crusts were evident in four of the nine surface micrographs of the CT soils (Fig. 1G.). Surface crusts can develop into surface seals, which reduce infiltration and increase surface runoff (Kwaad and Mucher 1994; Reichert and Norton 1995), and are indicative of aggregate instability (Bajracharya et al. 1996). Beneath the surface crust, vughs can develop due to the trapping of air after drying (Pagliai et al. 1989) (Fig. 1H). Annual tillage results in large packing pores due to the loose packing of aggregates in the plough layer (Fig. 1I). Fissures develop beneath the plough layer where compaction allows shrink-swell processes to create an influence

on the structure (Ringrose-Voase 1991). This may be the only means of developing structural units (Ringrose-Voase and Bullock 1984) that would otherwise leave the soil with a massive structure beneath the plough layer. Intact earthworm macropores may be present beneath the plough layer in the CT soils (Fig. 1K). However, the burrow is dissected by the plough and is evacuated, leaving the earthworm without a permanent shelter. The earthworm must create another burrow or is killed by the plough or scavenged by birds at the soil surface after tillage (Tomlin and Miller 1988). In the NT soils, the lack of disruption, the maintenance of an insulative cover and an available abundant food source due to crop residue, allows for an environment more conducive to earthworm survival (Edwards et al. 1995; Edwards and Shipitalo 1998).

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Soil Depth × Management Interaction. Figure 3 shows the results of image analysis for the interaction effect of soil management and soil depth of pore size classes from 100 to 3000 µm. There are significantly greater numbers of pores in the top 20 cm in the CT soils relative to NT (Fig. 3). Therefore there is a loss of pore volume occurring from the surface through to 20 cm depth. These results are consistent with the observations of horizontally trended platy structure in the NT soil (Fig. 1) and a higher overall bulk density in the top 25 cm of the NT soils (Fig. 2). Slope Position × Management × Years in NT Interaction. When the thin sections were pooled for depth and number of years in NT, there were no significant differences between slope position in the NT soils (Fig. 4). However, in the CT there was a general increase in pore numbers for each of the size classes from lower to upper slope positions. When the number of years of NT is included as an interaction effect with slope position and management (Fig. 5), it becomes evident that the 4 and 11 year CT pairs had greater numbers of pores from 500 to 3000 µm diameter in upper slope relative to lower slope positions. Matsui et al. (1994) found that the proportion of pores greater than 1000 µm diameter in the top 5 cm of the soil profile was greater in the upper slope relative to middle and lower slope positions on a CT hillslope in Belgium. They suggested that this was due to strong pedality in the upper slope position and near apedality in the middle and lower slope position. Pedality refers to a structure in which the aggregate units of soil are completely surrounded by void (Bullock et al. 1985). Differences in pedality between slope positions in the CT were not apparent from the thin sections of this study. This may warrant further investigation as to the effect of slope position on the pore size distribution in CT soils.

Fig. 6. Number of pores per cm2 for (a) 30–100 µm, (b) 100–500 µm, and (c) 500–1000 µm size classes for management × year in NT interaction in the top 20 cm of the NT and CT soil profiles of the study. Data were pooled for depth and slope position. Bars with different letters in each size class significantly different at P ≤ 0.05 (Tukey’s HSD).

Image Analysis of Soil Micromorphology FACTORS AFFECTING PORES OF DIFFERENT SIZES. The results of the analysis of variance indicates that soil depth, slope position and the number of years in NT all had significant influences on the pore size distribution of the soils (Table 2). Only those interaction effects that had significant differences (P < 0.05) are included in Table 2 and discussed in the results.

Years in NT × Soil Management Interaction. For pore sizes from 30 to 100 µm the number of pores declined relative to their CT counterparts as years in NT increased (Fig. 6a). However, for pores 100 to 500 µm after 4 yr of NT the pore numbers are significantly lower than the CT pair. After 6 yr the numbers are not different from their CT pairs (Fig. 6b) suggesting that they are increasing in number after 4 yr in NT. Pagliai et al. (1983) found significantly greater numbers of pores < 500 µm in continuous NT versus CT, while Shipitalo and Protz (1987) found a relative increase in proportions of pores in size from 200 to 500 µm in soils in NT for 7 yr relative to those in CT. The relative increase in 100- to 500-µm pore numbers from 4 to 11 yr of NT (Fig. 6b) may result in an amelioration of the porosity as this size range may aid in root growth. Russell (1978) suggested that most feeding roots require pores in size ranges from 100 to 200 µm to grow. In the 500to 1000-µm size range, however, the numbers are significantly lower in the NT than the CT pairs suggesting that these pores are degrading as a result of NT conservation management (Fig. 6c). However, if pores larger than 500 µm dominate the soil structure in soils not having a granular structure then this is indicative of poor aggregation (Pagliai et al. 1983).

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CHANGES IN PORE MORPHOLOGY ON THE NT CHRONOSEQUENCE Pores 100 – 500 µm.There were significant differences in pore shapes related to the numbers of years in NT and soil management (Fig. 7). There is an increase in the number of rounded pores 100 to 500 µm in diameter in the top 15 cm as the number of years in NT increases (Fig. 7a). However the same trend is not apparent in the irregular pores, whereby there are significantly lower numbers of pores 100 to 500 µm in size relative to the CT soils (Fig. 7b). The increase in the rounded pores from 100 to 500 µm in size is likely due to the lack of disturbance and therefore the maintenance of the pores created by roots and possibly some soil fauna (i.e. enchytraeidae; collembola) that have bodies in this size class. Wheat develops roots in the size range of 100–400 µm diameter (Bruand et al. 1996). Maize roots also tend to lie in this pore size range with the modal class between 100 and 200 µm diameter (Kooistra et al. 1992). Therefore the maintenance of rounded pores after 4 yr of NT may be vital for fine root growth in subsequent years if the soil remains in NT. Initially the use of NT management in Ontario was not recommended on soils of most textures due to the reductions in crop yield (Ketcheson et al. 1983). However, although crop yields were not determined in this study, the maintenance of these rounded pores may be one of the factors involved in NT soils requiring a number of years before their crop production becomes comparable or equivalent to that in CT soils (Nolan et al. 1990). Biopores >500 µm. Rounded soil pores >500 µm are assumed to be those pores created by roots and fauna burrowing through the soil. There is an increase in the number of biopores with time in NT in the top 15 cm relative to their CT counterparts (Fig. 8). However, the number of biopores is significantly greater than CT only after 6 yr of NT (Fig. 8). The greater numbers of biopores > 500 µm diameter is due to the lack of annual disruption of the top 15 cm by the plough. The increase in biopores with time in NT and their greater number than the CT after 6 yr in NT may be another reason why NT soils in Ontario require several years before their crop production becomes comparable to CT soils (Nolan et al. 1990). At least after 11 yr, biopores created by roots and soil fauna are maintained and increase in abundance with time in these soils (Fig. 8). Edwards et al. (1988) measured the number of biopores >400 µm in a soil in NT continuous corn for 20 yr in similar soil textures in Coshocton, Ohio. They concluded that after 20 yr of NT, the rounded biopores > 400 µm were still increasing in abundance. If the maintenance of biopores continues in the NT soils of this study, this should aid in ameliorating the loss of macropores due to the lack of annual tillage in the NT. The pores of these sizes are likely what account for sufficient infiltration after NT is implemented, particularly during intense rainstorms (Edwards et al. 1988). Channels created by L. terrestris earthworms are likely of particular importance since they can create very stable continuous burrows often to depths of 1 m or more (Lee 1985) if the surface foraging and habitat areas are not destroyed by ploughing.

Fig. 7. Number of pores per cm2 for (a) rounded pores and (b) irregular pores 100–500 µm diameter for soil management and years in NT interaction in the top 15 cm in the soils of the study. Data were pooled for depth and slope position. P values by Student’s t-test.

Orientation of elongated pores. The lack of annual ploughing allows for natural physical changes to occur in the profile, particularly those resulting from annual freeze-thaw processes as seasons change in southern Ontario. The settling of the tillage pores created by the plough before NT was initiated also occurs. Ice lens formation in soils often is associated with freeze-thaw processes (Williams 1968). After tillage in spring or fall, the remnant features attributed to ice lens formation are destroyed (Fig. 1). However, if the soils are managed with NT these features remain in the pro-

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Fig. 8. Number of rounded pores per cm2 >500 µm (biopores) related to soil management and the number of years in no-till in the top 15 cm in the soils of the study. Data were pooled for depth and slope position.

Fig. 9. Percentage of elongated pores oriented ± 30° from the horizontal in the upper 30 cm in the soils of the study. Data were pooled for slope position and years in NT. Significance by Student’s t-test.

file. Lenticular structure associated with ice lens formation tend to form elongated pores oriented horizontally relative to the soil surface (Harris 1985). Water migrates upward during freezing causing heaving pressures and therefore the formation of a platy or lenticular structure (Williams 1968). The creation of these pores was investigated by measuring the proportion of elongated pores (SF < 0.015) oriented ± 30° about the horizontal on the vertically-oriented soil thin sections. The image analysis data were pooled for years in NT and slope position. There were significantly more elongated voids oriented horizontally in the 5- to 10-cm, 10- to 15-cm and 20- to 25cm layers of each of the NT sites relative to the CT pairs (P < 0.05) (Fig.9). The greater faunal activity in the 0- to 5-cm layers likely destroys a portion of the lenticular or platy

structure created by the ice lens formation. The development of the horizontal elongated pores may result in a barrier to adequate drainage through the soil profile (Shipitalo and Protz 1987). This is likely countered by an increase and maintenance in the number of continuous biopores >500 µm e.p.d. associated with earthworms and roots in the NT soils, which may allow for more continuous vertical drainage. SUMMARY AND CONCLUSIONS Image analysis of soil micromorphology has been very useful in the quantification of pedofeatures since its initial development in the 1970s (Jongerius et al. 1972). It can be very useful as a tool in studying the soil micromorphological changes that occur as soil management is altered. In general it was concluded that conversion to NT causes a

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reduction in total pore volume and in total number of pores in the soil profile. However with time since the NT was initiated, there were significant increases in the number of rounded pores >100 µm in diameter. This was likely primarily due to the lack of disruption by tillage allowing for the round pores created by roots and soil fauna to be maintained. Eventually after 6 yr of NT, biopores >500 µm in diameter become significantly greater in number relative to the CT soils, and are likely crucial for the adequate drainage of NT soils. This is because the very large pores created by tillage collapse as a result of settling, compaction and freeze-thaw processes as years in NT increase. There was a greater number of horizontally oriented elongated pores between 5 and 15 cm in the NT soils, also created by annual freezing and the creation of horizontally oriented ice lenses in the winter. These voids may contribute to inadequate drainage under very moist conditions. However, the greater number of biopores >500 µm diameter after 6 yr of NT, which are probably relatively continuous (i.e. burrows created by earthworms and roots, for example), likely counter this effect. These pores will likely continue to develop with time in the NT soils after 11 yr. ACKNOWLEDGEMENTS To Agriculture and Agri-food Canada, NSERC and OMAFRA for funds in support of this research. Thanks to Gary Hamilton and Don Irvine for preparing the thin sections and imaging. The efforts put forth by the reviewers helped to significantly improve the manuscript. The authors thank them for their constructive suggestions. Azooz, R. H. and Arshad, M. A. 1996. Soil infiltration and hydraulic conductivity under long-term no-tillage and conventional tillage systems. Can. J. Soil Sci. 76: 143–152. Bajracharya, R. M., Cogle, A. L. and Yule, D. F. 1996. Surface crusting as a constraint to sustainable management of a tropical alfisol: II. Strength characteristics during crust development. J. Sust. Agric. 8: 4–45. Bruand, A., Cousin, I., Nicoulland, B., Duval, O. and Begon, J. 1996. Backscattered electron scanning images of soil porosity for analyzing soil compaction around roots. Soil Sci. Soc. Am. J. 60: 895–901. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T. and Babel, U. 1985. Handbook for soil thin section description. Waine Research. Wolverhampton, UK. 152 pp. Edwards, C. A. and Lofty, J. R. 1982. The effect of direct drilling and minimal cultivation on earthworm populations. J. Appl. Ecol. 19: 723–734. Edwards, C. A., Bohlen, P. J., Linden, D. R. and Subler, S. 1995. Earthworms in agroecosystems. Pages 185–213 in P. F. Hendrix, ed. Earthworm ecology and biogeography in North America. Lewis Publishers, Boca Raton, FL. Edwards, W. M. and Shipitalo, M. J. 1998. Consequences of earthworms in agricultural soils: Aggregation and porosity. Pages 147–162 in C. A. Edwards, ed. Earthworm ecology. St. Lucie Press, Boca Raton, FL. Edwards, W. M., Norton, L. D. and Redmond, C. E. 1988. Characterizing macropores that affect infiltration into non tilled soils. Soil Sci. Soc. Am. J. 43: 851–856. Harris, C. 1985. Geomorphological applications of soil micromorphology with particular reference to periglacial sediments and

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