Changes of pore morphology, infiltration and earthworm community in a loamy soil under different agricultural managements

Catena 54 (2003) 637 – 649 www.elsevier.com/locate/catena

Changes of pore morphology, infiltration and earthworm community in a loamy soil under different agricultural managements Mathieu Lamande´ a, Vincent Hallaire a,*, Pierre Curmi a,b, Gue´nola Pe´re`s c, Daniel Cluzeau c a

INRA-Centre de Rennes, Unite´ Sol et Agronomie Rennes Quimper, 65, route de Saint-Brieuc, 35042 Rennes, France b French Institute of Pondicherry, 11, Saint-Louis Street, PB 33, Pondicherry 605001, India c CNRS, UMR 6553 ‘‘Ecobio’’—Laboratoire d’Ecologie du Sol et de Biologie des Populations, Station Biologique, 35380 Paimpont, France

Abstract Earthworm activity produces changes at different scales of soil porosity, including the mesoporosity (between 1.000 and 30 Am eq. dia.) where both water retention and near-saturated infiltration take place. At this scale, the structural changes are poorly described in temperate agricultural systems, so we do not yet fully understand how these changes occur. The present study was conducted to determine the relationships between the morphology of the mesopores, which is mainly affected by earthworm activity, and the hydrodynamic behaviour (near-saturated infiltration) of topsoil under different agricultural managements inducing a large range of earthworm populations. Investigations were carried out at the soil surface in three fields under different management practices giving rise to three different earthworm populations: a continuous maize field where pig slurry was applied, a rye-grass/maize rotation (3/1 year, respectively) also with pig slurry, and an old pasture sown with white clover and rye-grass. Pore space was quantified using a morphological approach and 2D image analysis. Undisturbed soil samples were impregnated with polyester resin containing fluorescent pigment. The images were taken under UV light, yielding a spatial resolution of 42 Am pixel 1. Pores were classified according to their size (which is a function of their area) and their shape. Hydraulic conductivity K(h) was measured using a disc infiltrometer at four water potentials: 0.05, 0.2, 0.6, and 1.5 kPa. The abundance and ecological categories groups of earthworms were also investigated.

* Corresponding author. Tel.: +33-223-485-429; fax: +33-223-485-430. E-mail address: [email protected] (V. Hallaire). 0341-8162/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0341-8162(03)00114-0

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Continuous soil tillage causes a decrease in both abundance and functional diversity (cf. maize compared with old pasture) when soil tillage every 4 years causes only a decrease in abundance (cf. rotation compared with old pasture). There were no relationships between total porosity and effective porosity at h = 0.05 kPa. Image analysis was useful in distinguishing the functional difference between the three managements. Fewer roots and anecic earthworms resulted in fewer effective tubular voids under maize. There were fewer packing voids in the old pasture due to cattle trampling. Greater hydraulic conductivity in the pasture phase of rotation may arise from a greater functional diversity than in the maize and absence of cattle trampling compared with the pasture. We point to some significant differences between the three types of agricultural management. A better understanding is required of the influence of agricultural management systems on pore morphology. This study provides a new methodology in which we consider the earthworm activity as well as community in order to assess the effects of agricultural management on soil structure and water movement. D 2003 Elsevier B.V. All rights reserved. Keywords: Soil structure; Hydraulic conductivity; Image analysis; Cropping systems; Earthworms

1. Introduction Understanding the role of fauna on soil physical properties is important in developing sustainable agricultural managements, and during the last decade, important efforts were devoted to the description and quantification of the direct and indirect effects of soil invertebrates on the major processes of the soil, in particular, the formation and conservation of the physical structure. In temperate regions, the earthworms in term of biomass constitute the principal component of the total faunal biomass (Lee, 1985). Earthworm populations are affected by agricultural management (Binet et al., 1997; Paoletti et al., 1998; Chan, 2001), and earthworms have a large influence on soil physical properties through their burrowing and casting activities. Also known as ‘‘ecosystem engineers’’ (Jones et al., 1994), earthworms produce structural features at three different scales of soil porosity. Much work deals with the characterisation of burrow networks created by earthworm species (Capowiez et al., 1998; Je´gou et al., 1999). In relation to macropore space (>1 mm), burrow networks act as preferential flow paths (Bouche´ and Al-Addan, 1997; Trojan and Linden, 1998). At a smaller scale, earthworms may change the pore space between mineral and organic particles, i.e. the microporosity, and the stability of soil structure (Shipitalo and Protz, 1989; Blanchart et al., 1993; Chauvel et al., 1999). Packing voids within cast deposits control soil mesoporosity, in which large amounts of water and solutes are transported and retained. Because these structural features have been little studied in temperate agricultural systems, we are here primarily concerned with the mesoscale aspects of pore morphology. The effects of earthworm activity and agricultural management on soil physical properties have been studied, but only in terms of preferential flow paths (Ehlers, 1975; Sveistrup et al., 1997). Hallaire and Curmi (1994) and Kribaa et al. (2001) showed the main role of morphology in linking the effective porosity with movement of water and solutes. The effect of agricultural management systems on soil physical properties has

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often been quantified with near-saturated infiltration measurements (Ankeny et al., 1990; Meek et al., 1992; Azevedo et al., 1998; Heddadj and Gascuel-Odoux, 1999; AnguloJaramillo et al., 2000). Our objective here is to evaluate the effect of various agricultural managements and natural earthworm populations on the physical properties of topsoil. We looked for relationships between the morphology of the mesopores and the rate of near-saturated infiltration. We measured in situ hydraulic conductivity at multiple water potentials near saturation with tension infiltrometers and characterised the morphology of a large part of mesopores by image analysis of a soil affected by both agricultural treatments and earthworm activity.

2. Materials and methods 2.1. The experimental design The experiment was carried out at the experimental station of the Lyce´e Agricole de Kerbernez, in western Brittany, France (latitude 47j57VN, longitude 4j8VW). Agriculture in this region consists mainly of intensive milk production characterised by rotations of maize and pasture. The climate is of temperate oceanic type, with an average annual precipitation of 1200 mm and a mean annual temperature of 11.4 jC. The soil is a Humic Cambisol (FAO) of loamy texture with a high concentration of organic matter in the first 30 cm (Table 1) developed on granitic saprolite. The trial comprised three plots, each 9 m wide and 16 m long, that were managed as follows: (i) continuous maize treated with pig slurry for 22 years, (ii) a pasture phase (1st year) of a rye-grass/maize rotation (3/1 year) also with pig slurry for 22 years, (iii) old pasture sown with white clover and rye-grass maintained over a period of 9 years. Physical measurements and soil sampling were performed from soil surface on the most representative zone in the topsoil, determined by mapping the structural features of the given ploughed soil horizon (Manichon and Roger-Estrade, 1990; Curmi et al., 1996) just before maize seeding (March) after 6 months without tillage. 2.2. Earthworm community Natural earthworm community was extracted in each field using the formaldehyde method on 1 m2 (Bouche´, 1972; Cluzeau et al., 1999); after three sprayings of

Table 1 Soil characteristics of the top soil in the three studied fields

Maize Rotation Old pasture

< 2 Am (%)

2 – 20 Am (%)

20 – 50 Am (%)

50 – 200 Am (%)

200 – 2000 Am (%)

Organic matter content (%)

pHwater

16.1 17.7 17.0

21.4 19.6 17.5

20.4 23.6 17.5

13.2 13.1 18.0

28.9 26.6 30.0

4.18 4.43 4.20

6.20 5.65 5.90

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formaldehyde solution (10 l per spraying with different concentrations: 0.25%, 0.25%, 0,4%), earthworms were collected at the soil surface. Three replicates were performed in each field surrounding the sites chosen for physical measurement and soil sampling. Earthworm communities were characterised by their abundance (number collected per m2) and their ecological group. This last parameter is based on earthworm morphology and behaviour (localisation in soil, feeding behaviour), and corresponds to three ecological groups (Lee, 1959; Bouche´, 1972, 1977) whose burrow systems were described (Kretzschmar and Aries, 1990; Lee and Foster, 1991; Lavelle, 1997): epigeic (range, 1 – 2.5 mm in diameter, live and feed above the soil surface, create no or few burrows), anecic (range, 4 – 8 mm in diameter, feed at the ground surface, live in semipermanent burrows, more or less vertical and opened to the soil surface), and endogeic (range, 2 –4.5 mm in diameter, ingeste soil, dig extensive systems of temporary burrows that they immediately refill with their casts, the burrows are mostly subhorizontal oriented and very ramified through the soil but rarely open to the surface). In order to link earthworm activity to soil properties, the earthworm communities were also characterised by their functional diversity that combines the ecological group and growth stage (juvenile, adult) (Pe´re`s et al., 1998); six functional classes are defined. 2.3. Physical measurements Hydraulic conductivity K(h) was measured using a disc tension infiltrometer with an 80-mm-diameter base, which determined tension at the soil surface as described in Ankeny et al. (1990, 1991). Steady-state infiltration rates were measured at four soil water potentials h: 0.05, 0.2, 0.6, and 1.5 kPa. Flow was measured from 1.5 to 0.05 kPa. The disc of the infiltrometer was positioned on the undisturbed surface covered with a thin layer of sand to obtain a flat surface in the soil with maize. We gently removed the upper root zone in the rotation and old pasture fields in order to place the disc of the infiltrometer (respectively, 2 and 3 cm from the surface). The flow was measured at a given potential for about 1 h to reach steady state. We used methylene blue in water (0.4 g l 1) to dye the effective porosity at water potential h = 0.05 kPa. We estimated the unsaturated hydraulic conductivity curve at several tensions by computing multiple supply potentials with the same disc, as proposed by Reynolds and Elrick (1991) and Ankeny et al. (1991), assuming Wooding’s solution for three-dimensional axisymmetric infiltration (Wooding, 1968). We estimated total porosity from measurements at the same sites by weighing cylindrical samples having a volume of 250 cm3 (four replicates) assuming a solid density of 2.65 g cm 3. 2.4. Image analysis of macropore space Pore space descriptions were made using undisturbed soil blocks (10  10  8 cm) taken vertically beneath the locations of the infiltration measurements. Soil samples were dried and impregnated with a polyester resin containing fluorescent dye (Ringrose-Voase, 1996). The blocks were then cut in four horizontal polished sections (7  7 cm) at four depths (1, 3, 5, and 7 cm). For each section, four areas (2.2  3.1 cm) were analysed using OPTIMAS software with a spatial resolution of 42 Am pixel 1. We chose a spatial

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resolution corresponding to the pore space involved in infiltration measurements (pore size between 0.018 and 28.3 mm2), so we cannot measure the smallest part of the mesoporosity. One grey-level image was taken with a CCD camera under UV light on which the

Fig. 1. Method of image treatment (magnification  2.5): (a) grey-level image acquired under UV light (the porosity is bright and the solid material is dark); (b) grey-level image acquired under white light (quartz, feldspar, and plagioclase are bright, the porosity and solids are dark); (c) segmented image of quartz, feldspar, and plagioclase; (d) intersection of images (c) and (a); (e) segmented image of (d); (f) segmented image of (d) only stained pores.

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Table 2 Pores classification according to size and shape Shape classes

Tubular void Is < 38 Crack Isa[38; 89] Packing void Is>89

Size classes (10

6

m2)

1

2

3

4

[0.018; 0.031]

[0.032; 0.196]

[0.197; 1.77]

>1.77

T1 C1 P1

T2 C2 P2

T3 C3 P3

T4 C4 P4

Is: elongation index.

solid phase appears dark and the porosity bright. Because some mineral fragments (quartz, feldspar) appear bright under UV light, another grey-level image was taken under white light on which the porosity is dark and coarse fragments are bright. We segmented and inverted the image with coarse fragments and masked the coarse fragments on the greylevel UV image. We then segmented the grey-level UV image into a binary image and removed the porosity unaffected by methylene blue using a hand-made mask (Fig. 1). The stained pores were classified from the final binary image according to their size and shape. Pore size was measured from surface area on the binary image. Four size classes were determined, corresponding to the effective pore size at the water potentials used for infiltration measurement: from 0.018 to 0.031, from 0.032 to 0.196, from 0.197 to 1.77, and up to 1.77 mm2. Pore shape was measured using the elongation index Is (perimeter2/ area), and three shape classes were determined to distinguish between tubular voids, cracks, and packing voids (Hallaire and Curmi, 1994). The thresholds used for the size and shape classes are given in Table 2.

3. Results and discussion 3.1. Earthworm community Earthworm abundances are significantly different in the three treatments (Student’s ttest, p = 0.05) (Fig. 2): the highest abundance of earthworms is found in the old pasture and lowest in the maize; the increase of anthropic constraints is associated with a decrease in earthworm abundance and species diversity. In the old pasture, the community is dominated by endogeic earthworms (48%) and especially by Aporrectodea caliginosa (Savigny, 1826) (juveniles and adults) and Allolobophora c. chlorotica (Savigny, 1826) (adults). Anecic earthworms are also found (42%), especially Lumbricus friendi (Cognetti, 1904) (juveniles and adults). The low abundance of epigeic species in the old pasture (10%) compared to the pasture phase of rotation (52%) may be explained by cattle trampling (Cluzeau et al., 1992). Although land management conditions associated with maize culture (tillage, pesticide use, and low organic matter return) affect all the earthworm communities, the changes mainly concern the anecic (6%) and epigeic species (4%); tillage may affect the largest individuals (anecic adults and juveniles, and endogeic adults), while the soil cover may affect the epigeic species. In fact, the community under

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Fig. 2. Earthworm communities in the studied three fields: (a) continuous maize, (b) pasture phase of rotation, (c) old pasture.

maize is dominated by endogeic species (90%), especially by A. caliginosa observed mainly at the juvenile stage, while both juvenile and adult anecic are almost totally absent. Pasture after maize especially favours the epigeic species (52%), which are dominated by Lumbricus castaneus (Savigny, 1826) and Lumbricus rubellus castenoides (Bouche´, 1972) (observed at the adult stage), as well as anecic species (33%) dominated by L. friendi. This specific structure could be explained by (1) the high rate of reproduction of the epigeic species, (2) the maintenance of some earthworm species via the cocoons during the maize phase, (3) the restauration of the anecic species due to better environmental conditions (grass cover, no more tillage), and (4) the recolonisation of the site by exogenic earthworms. Thus, functional diversity is higher in the two pasture soils than under maize. Anecic (juvenile and adult) and endogeic (juvenile and adult) species are present in the two pastures, but there are more individuals of the three ecological groups in the old pasture than under the rotation system. 3.2. Bulk density The Student’s t-test on the mean at the 95% confidence interval shows that bulk density is not significantly different between maize (1.21 g cm 3) and the pasture phase of rotation (1.26 g cm 3) (Fig. 3). We expected a higher bulk density with rotation than maize due to ‘‘natural’’ compaction and the lack of tillage. The biological activity is more intense with the rotation and could account for the better aggregation and porosity. Bulk density is significantly higher in the old pasture (1.43 g cm 3) (Fig. 3). We observed a compacted layer on the top of the soil profile in the old pasture that was attributed to cattle trampling. 3.3. Near-saturated infiltration The results of near-saturated infiltration measurements are presented in Fig. 4. We estimate a high hydraulic conductivity in the rotation fields at the two water potentials

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Fig. 3. Bulk density (g cm 3) for the three fields: (a) continuous maize, (b) pasture phase of rotation, (c) old pasture. Circles represent means that are significantly different, which either do not intersect at all or intersect only slightly so that the external angle of intersection is less than 90j.

nearest to saturation (K( 0.05) = 1.85  10 4 m s 1 and K( 0.2) = 1.16  10 5 m s 1). There were many new effective pores with equivalent diameters up to 0.5 mm. Only a few new pores become effective at conditions near saturation in the maize field and the old pasture. At the two water potentials nearest to saturation, water flow is controlled more by gravity than by capillarity. The water potential corresponding to the change of gravity/ capillarity ratio is between 0.35 and 0.3 kPa in the three fields. Jarvis and Messing (1995) estimated this break point between 0.6 and 0.25 kPa, depending on the soil texture, and at 0.25 kPa for a loamy soil. They suggested that this water potential could be used as an operational boundary between mesoporosity and macroporosity (Luxmoore, 1981). Although reducing the bulk density, ploughing did not increase the hydraulic

Fig. 4. Hydraulic conductivity at four water potentials near saturation in the three fields.

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conductivity. Kooistra et al. (1984) suggested that ploughing produces a disconnected macroporosity, while some earthworms produce a more continuous type of porosity. Hydraulic conductivity shows that mesopores formed by settling and biological activity with rotation are more continuous than mesopores formed by settling under maize after 11 months without tillage. Chan (2001) suggested that preferential flow paths (related to deepburrowing earthworm species) are the most important factors controlling water movement in soil (preventing flooding and erosion). Since the connections between macropores are strongly dependent on the presence of mesopores, endogeic species should be as important as deep-burrowing species. The beneficial effect of biological activity on near-saturated infiltration observed in the rotation compared with the maize (where K( 0.05) is divided by a factor 17) could have been reduced by cattle trampling in the old pasture (where K( 0.05) is divided by a factor 19). Near-saturated hydraulic conductivity is useful for evaluating the effect of agricultural management systems on soil hydraulic properties. It provides information on soil infiltration capacity as well as the amount of effective pores of different sizes. 3.4. Morphology of the effective mesoporosity Total effective mesoporosity at water potential h = 0.05 kPa as measured by image analysis (surface of the total dyed porosity) was compared with total porosity estimated from bulk density measurements, assuming 2.65 g cm 3 as solid density (Fig 5). There were no relationships between effective mesoporosity and total porosity for the three fields. The difference in hydraulic conductivity at h = 0.05 kPa between the three fields was not related to a difference in total porosity or in effective porosity. There is no correlation between total porosity and effective porosity. In this case, image analysis is useful in distinguishing the functional difference between those two managements.

Fig. 5. Total porosity estimated from bulk density measurements and effective porosity at water potential h = 0.05 kPa.

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Fig. 6. Pore classification according to size and shape for the three fields: (a) continuous maize, (b) pasture phase of rotation, (c) old pasture. Means of the 16 images (4 images per level  4 levels).

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Effective pore size distribution increases sharply from size 1 to size 4 in the three treatments (0.018 mm2 to more than 1.77 mm2, corresponding to efficient pores size at the water potentials used for infiltration measurement) (Fig. 6). This increase was mainly due to packing voids. Packing voids decrease in the old pasture compared with the rotation and partly transformed into cracks due to cattle trampling (Fig. 6). There are more tubular voids in the old pasture and in the rotation than in maize. Tubular voids are almost negligible in the maize, where biological activity (roots and fauna) is lower. Although very few anecic earthworms are present in the maize, they are responsible for most of the tubular voids larger than 2 mm in horizontal 2D sections. Such anecic species live in permanent vertical burrows. The distribution of effective pores coincides well with the hydraulic conductivity curve. Packing voids seem to have a major role in water flow at conditions near saturation (size classes 3 and 4). At lower potentials, water flow is controlled by tubular voids and cracks. The effective porosity at water potential h = 0.05 kPa was two times less in maize than in the old pasture, while the hydraulic conductivity was similar. In maize, most of the water flow occurs through the pores of equivalent diameter smaller than 200 Am. These pores are not all quantified by image analysis (42 Am pixel 1 resolution) but are taken into account in infiltration measurements at water potential h = 1.5 kPa. These pores correspond to the porosity observed within earthworm casts, but they may correspond also to porosity between enchytraeid casts (Dawod and FitzPatrick, 1993). In cultivated soil at low level of earthworm densities, the enchytraeids can play an important role in creating a stable soil structure and porosity (Topoliantz et al., 2000). But the high pH value in maize (6,2) is not favourable to enchytraeid development (Go´rny, 1984).

4. Conclusion The types of agricultural management and earthworm community both induce changes in the structural features and physical properties of soil. To propose sustainable agricultural management systems, we need to improve our understanding of the processes controlling these changes. In this study, we compare pore morphology, infiltration rate, and earthworm community under three different agricultural managements. We show it is necessary to consider the functional diversity of earthworms as well as their abundance. The packing voids are important in controlling water flow and retention, especially when preferential flow paths such as earthworm burrows or cracks are disconnected by tillage. The present study provides a new methodology that may be used to assess the effects of agricultural managements on soil structure and water movement. In particular, we consider the type of earthworm community as a factor influencing the changes in soil physical properties. References Angulo-Jaramillo, R., Vandervaere, J.-P., Roulier, S., Thony, J.-L., Gaudet, J.-P., Vauclin, M., 2000. Fields measurement of soil surface hydraulic properties by disc and ring infiltrometers: a review and recent developments. Soil and Tillage Research 55, 1 – 29. Ankeny, M.D., Kaspar, T.C., Horton, R., 1990. Characterization of tillage and traffic effects on unconfined infiltration measurements. Soil Science Society of America Journal 54, 837 – 840.

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