Directory UMM :Data Elmu:jurnal:A:Applied Soil Ecology:Vol16.Issue2.Feb2001:

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Effect of mesofaunal activity on the rehabilitation

of sealed soil surfaces

Marcus Langmaack

, Stefan Schrader

a,∗

_{, Katharina Helming}

a_{Institute of Zoology, Technical University, Spielmannstrasse 8, D-38106 Braunschweig, Germany} b_{Department of Soil Landscape Research, Center for Agricultural Landscape and Land Use Research (ZALF),}

Eberswalder Strasse 84, D-15374 Müncheberg, Germany

Received 14 February 2000; received in revised form 12 July 2000; accepted 14 July 2000

Abstract

The impact of soil mesofauna on the rehabilitation of soil surfaces sealed by rainfall was investigated in a long-term laboratory experiment. Fifteen undisturbed soil monoliths from the Ap horizon of a Gleyic Podzoluvisol/Haplic Luvisol derived from loess were obtained after conventional tillage and seedbed preparation. The soil of this site is known to be susceptible to surface sealing as a result of rainfall activity. All monoliths were defaunated in a microwave oven and then inoculated with mesofauna, some with 300 individuals of Collembola and others with 200 individuals of Enchytraeidae. Additional monoliths were left uninoculated for comparison. Ten monoliths were then treated with simulated rainfall (intensity: 29 mm h−1_{; time: 60 min) to form a surface seal. The roughness of all 15 monoliths was measured using a non-contact laser}

scanner immediately and after 6 and 18 months. Differences in the soil surface roughness were assumed to indicate mesofaunal activities and intrinsic soil processes. Soil surface roughness was significantly different between monoliths with and without rain impact. Monoliths subjected to rainfall showed significant differences in soil surface roughness between those with and without mesofauna as well as between monoliths inoculated with Collembola and Enchytraeidae. The roughness differences detected between unsealed monoliths were not significant. Over the entire experimental time of 18 months the relative changes in sealed uninoculated monoliths were much lower than the alterations as a result of mesofaunal activities. The results show that within a few months the activities of Collembola and Enchytraeidae distinctly contribute to the rehabilitation of sealed soil surfaces and the development of a finely structured soil surface microrelief. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Enchytraeidae; Collembola; Soil surface crusts; Soil surface roughness; Soil rehabilitation

1. Introduction

Soil surface sealing is of paramount importance in surface runoff and soil erosion processes. In the temperate climate of Central Europe, runoff and soil erosion predominantly occur on cultivated land

∗_{Corresponding author. Tel.:}₊_{49-531-391-3237;} fax:+49-531-391-8198.

E-mail address:[email protected] (S. Schrader).

with non-permanent vegetation cover (Kwaad, 1991). Freshly-tilled unvegetated soils typically have a high water infiltration capacity such that runoff occurs infrequently with intensive rainfall events. However, with exposure to rainfall over time a soil crust devel-ops on exposed soil surfaces which prevents infiltra-tion. During rain events soil aggregates break apart when the kinetic energy of raindrops impacting soil surfaces is converted into radial forces that result in a splash off of soil particles (Al-Durrah and Bradford,

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1982; Le Bissonais et al., 1989). The detached soil particles form a smooth dense seal a few mm thick on the soil surface (Roth and Helming, 1992; Fohrer et al., 1999). Despite the thinness of the layer, the hydraulic conductivity of such a seal is usually more than one order of magnitude lower than the unsealed soil surface (McIntyre, 1958; Ela et al., 1992; Roth et al., 1995). The result of the sealing process is an exponential decrease in the infiltration rate and a sub-sequent increase in soil erosion and runoff. After the soil seal dries, a surface crust forms which restricts infiltration as well as water, heat and air exchange between soil and atmosphere until the crust is broken (Valentin and Bresson, 1992; Cresswell et al., 1993). The latter process may be initiated physically through swelling and/or shrinking, biologically through root growth and/or soil faunal activity or mechanically through tillage and traffic operations. This paper fo-cuses on the effect of soil faunal activity on structural changes in sealed soil surfaces.

The effects of soil macrofauna on soil structure, in-filtration and water fluxes have been extensively inves-tigated especially for earthworms (see reviews by, e.g. Lee and Foster, 1991; Makeschin, 1997), termites (Lo-bry de Bruyn and Conacher, 1990) and ants (Lal, 1988; Lobry de Bruyn and Conacher, 1994). Particularly in arable soils the burrowing activity of earthworms sig-nificantly impacts soil structure and properties. Lavelle (1994) applied the concept of ‘ecosystem-engineers’ to describe the bioturbative activities of earthworms and termites. Lavelle (1994) and Lavelle et al. (1997) applied the Jones et al. (1994) idea of ‘physical ecosys-tem engineering by organisms’ in detailing the direct and indirect effects of burrowing and cast production on soil physical and chemical processes. The available access to biotic and abiotic habitat resources by other soil organisms is also a result of these macrofaunal activities.

In addition to these smaller macrofauna, other studies on bioturbation focussed on the impact of larger macrofauna, e.g. mammals such as ground squirrels (Citellus spec.), moles (Talpa europaea) (review: Graff and Makeschin, 1979) and badgers (Meles meles) (Neal and Roper, 1991). While capable of moving soil volumes as great as 120 t ha−1_a−1_, the single permanent burrow system, limited popula-tion densities and frequencies of these larger species in mid-European arable land suggest that the more

prevalent, smaller macrofauna may more significantly impact soil structural and physical properties (Graff and Makeschin, 1979; Neal and Roper, 1991).

Although less intensively researched than macro-fauna, the impact of mesofauna on soil fertility through consumption of soil organic matter and casts of larger animals is well-accepted (e.g. Dawod and FitzPatrick, 1993). The effects of their burrowing activities in compacted soils have also been observed (Schrader et al., 1997). Enchytraeidae, in particular, have been found to be important bioindicators for abiotic charac-teristics in the soil environment (Röhrig et al., 1998; Graefe and Schmelz, 1999). Moreover, air permeabil-ity and hydraulic conductivpermeabil-ity were shown to increase as a result of enchytraeid activities (Didden, 1990; van Vliet et al., 1998). In the case of Collembola, Heisler et al. (1996) measured a significant increase in aggregate stability of their casts in comparison to soil aggregates of the same size. However, the description or quantification of these effects creates methodolog-ical problems as great as these animals are small. Until now few studies have considered mesofauna to be ‘ecosystem engineers’ as described by Schrader (1999) for Collembola and Enchytraeidae.

The analysis of mesofaunal activities requires methods that allow a non-destructive characterisation of mesofauna-related soil structural changes. Babel and Vogel (1989) and Rusek (1986) used micro-morphological methods to investigate the effects of Collembola and Enchytraeidae on soil microstruc-ture. Micromorphological methods are very effective in the visualisation of soil structural properties at a high resolution (Bresson and Boiffin, 1990). How-ever, this method is destructive to soil structure and precludes long-term, repeated analyses of soil struc-tural changes. X-rays at a resolution of about 1 mm are non-destructive and highly effective in studying soil structure and hence, a well-established analytical tool in soil science research (Heijs et al., 1995; Lang-maack et al., 1999). However, for purposes of charac-terising mesofaunal activity, a resolution much higher than 1 mm is required. Although, the technology for computed tomography for analysis of soil pores to 10–50mm has already been developed (Reinken et al.,

1995), the appropriate equipment is difficult to locate. Schrader et al. (1997) used a laser microrelief meter to analyse the burrowing activities of Enchytraeidae and Collembola in compacted soils. This non-contact

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and non-destructive equipment scans the morphology of the soil surface at a resolution of 0.3 mm (Helming, 1992). Without any intervening extraneous influences morphological changes observed on the soil surface are indicative of sub-surface soil structural changes as a result of faunal activities. The effects of collembolan and enchytraeid activities on the topography of previ-ously sealed soil surfaces were investigated through the use of a laser microrelief meter in the present study. This research was inspired by the results of Kladi-vko et al. (1986), Roth and Joschko (1991) and Mando and Miedema (1997) which demonstrated the structural changes of crusted soils and the reduc-tion of surface runoff due to earthworm burrowing and termite activities. The purpose was to investi-gate whether soil mesofauna contribute to, or even accelerate the rehabilitation of sealed soil surfaces through their burrowing activities. Soil core samples were inoculated with Collembola and Enchytraei-dae in defined numbers and subjected to a simulated rainstorm. The soil type chosen is highly suscepti-ble to surface sealing processes. Alterations in the artificially created seal were detected by measuring the topography of the soil surface immediately af-ter the rainstorm application and at 6 and 18 month intervals.

2. Material and methods 2.1. Experimental design

In autumn 1996, 15 undisturbed soil monoliths (h:5 cm,∅: 11 cm,_A: 95 cm2) were obtained from the

Ap-horizon of a Gleyic Podzoluvisol/Haplic Luvisol derived from loess (FitzPatrick, 1986; FAO, 1988) (approximate German equivalent: Pseudogley-Parabra-unerde aus Loess) after basic tillage and seedbed preparation practices were performed. According to AG Boden (1994) the soil is classified as a sandy loamy silt (Uls) with 26.2% sand, 60.9% silt and 12.9% clay. The arable land site is located in the sloping Solling area (Lower-Saxony, Germany) and very susceptible to surface sealing and soil erosion. The annual mean precipitation is 810 mm.

As described by Schrader et al. (1997) the mono-liths were defaunated in a microwave oven for 6 min at 750 W. Comparing the convenient defaunation

techniques microwaving, deep freezing and biocide application, Huhta et al. (1989) recommended micro-waving as most effective. Only nematodes occasion-ally recovered from microwaved treatments.

Six of the 15 monoliths were inoculated with Collembola (300 individuals ofFolsomia candida ≡

30 000 m−2_{), six with Enchytraeidae (200 individuals} ofEnchytraeus minutusandE. lacteus≡20 000 m−2)

and the remaining three were not inoculated. These densities are within the upper range for typical meso-faunal populations in arable land (Tischler, 1965; Didden, 1993).

At the end of the experiment all monoliths were checked for the presence of mesofauna. Control, i.e. uninoculated monoliths were still without Enchytraei-dae and Collembola, while both taxa were reduced in abundance in inoculated monoliths.

2.2. Rainfall simulation

To create a surface seal, soil monoliths were sub-jected to a simulated rainstorm generated with a rainfall simulator as described by Roth and Helm-ing (1992). In brief, the rainfall simulator consists of a 1 m2 _{drop-producing unit with 2500}

capillar-ies mounted 7 m above the box containing the soil monoliths. The drop size distribution was randomised around a median drop size of 2.89 mm. The rain-fall kinetic energy produced was 25.80 J m−2_mm−1_,

which corresponds to about 99.5% of natural rainfall kinetic energy at 30 mm h−1 _{rainfall intensity (Laws} and Parsons, 1943).

Rainfall was applied to 10 of the 15 monoliths (4 with Collembola, 4 with Enchytraeidae, 2 uninocu-lated) for 60 min to instigate the formation of a sealed surface. The actual rainfall intensity was 29 mm h−1_. The soil monoliths were placed under the rainfall de-vice at a 5% slope to prevent the soil surfaces from ponding and to allow for sampling of the generated surface water runoff. The arrangement of the soil monoliths under the rainfall simulator with hoses for runoff collection is shown in Fig. 1.

Thus, the experiment comprised the following treatments: four sealed and two unsealed monoliths inoculated with Collembola, the same number of monoliths inoculated with Enchytraeidae, and two sealed and one unsealed uninoculated (no fauna) monoliths.

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Fig. 1. View of the platform (slope 5%) with soil monoliths placed 7 m below the drop-producing unit of the rainfall simulator. Soil surface run-off from each monolith was collected by hoses.

2.3. Soil surface scanning

Immediately after rainfall application the surface topography of all monoliths was measured with a non-contact laser microrelief meter, as described by Helming (1992). In brief, the laser microrelief meter consists of an aluminium frame supporting a sledge with a mounted laser probe, consisting of a laser source and an optical sensor. The x- and y-direction movement of the laser probe is software-controlled and driven by two stepping motors. As the laser probe moves, height values are collected and stored in a computer. This scanning movement allows the topography of areas from a few cm2 _{to a maximum}

of 1 m2 _{to be digitised with 0.3 mm horizontal and}

0.2 mm vertical resolutions, respectively. The result is a surface digital elevation model (DEM) with regular grid spacing.

A 7 cm×7 cm area at the centre of each monolith

surface was digitised to measure the surface topogra-phy. A grid spacing of 0.3 mm resulted in a DEM of 52 900 height values per area. For analysis purposes

the dimensionless roughness index ‘specific surface area’ (SSA), representing the ratio of the surface area measured to the projected surface on a plane perpen-dicular to the laser beam, was calculated based on the procedures of Helming et al. (1993). Briefly, the method consists of summing up the surface area of each 3 mm×3 mm grid square. The surface area of

each grid square was determined by calculating the area of the four triangles derived from the four ele-vation points of the grid plus a linearly interpolated midpoint. In previous studies the SSA index was de-termined to be a suitable parameter for the character-isation of soil surface topographies (Helming et al., 1993; Schrader et al., 1997).

The alterations of the created seal were monitored by measuring the topography of the soil surface with the non-contact laser microrelief meter immediately after the rainstorm application (time t0) as well as

6 (timet6) and 18 months (time t18) later. Between

the measurements, the soil monoliths were stored at 20◦_{C in dark and humid conditions (Schrader et al.,} 1997). For comparison of the different treatments

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and measurement dates the relative differences of the SSA values were calculated. By definition the lowest possible value of SSA is 1.0 (totaly plane surface): we therefore subtracted 1.0 from all SSA values be-fore determining the percentage differences between treatments. The Mann–Whitney-U-test was used for statistical analysis.

3. Results

Water saturation of the soil occurred 11 min and sur-face runoff 16 min after rainstorm simulation began. After 16 min the infiltration rate decreased rapidly and the runoff rate increased. At the end of the exper-iments soil loss as a result of surface runoff averaged 92.28 g m−2_{. After the rainfall simulation a seal had} developed on the soil surfaces of the monoliths.

The differences in the mean specific surface area (SSA) values between unsealed (1.45) and sealed (1.31) soil surfaces immediately after the rainstorm impact (timet₀) were statistically significant (Fig. 2). The smoothening of the soil surface after the rainstorm is reflected in the mean SSA of sealed monoliths, which is 31.1% lower than for unsealed monoliths.

Macroscopically no noticeable soil surface differ-ences between Enchytraeidae and Collembola were visible at eithert6ort18. Effects of 6 months of

enchy-traeid activity on the previously sealed and unsealed

Fig. 2. Box-plot of the specific surface area (SSA) of sealed and unsealed monoliths at the start of the experiment (t0). Each box

represents half of the values and the bars represent ranges recorded, the solid line marks the median inside each box. The different letters indicate that the treatments are significantly different based on the Mann–Whitney-U-test,p≤0.05.

surfaces are shown as an example in Fig. 3. In both these cases, the burrowing activity of Enchytraeidae resulted in a rougher surface with a higher SSA. In particular, the initially sealed surface appears to be more finely structured, i.e. rougher after 6 months of enchytraeid activity (Fig. 3). In this case, SSA increased by 17.2% during the first 6 months of the experiment. After the same time period, the unsealed surface showed an increase in SSA of 24.1% att₆.

When the SSA data for sealed and unsealed treat-ments over the entire period of the experiment (t0–t18)

are pooled, the influence of the inoculated mesofauna on the sealed surfaces in comparison with uninoc-ulated treatments becomes more apparent. In the sealed treatments, the mean SSA was significantly higher for Enchytraeidae and Collembola inoculated monoliths compared to monoliths without inoculated fauna (Fig. 4). Furthermore, the higher mean SSA for Collembola compared to Enchytraeidae was also sta-tistically significant. The mean SSA of uninoculated monoliths was 34.3% lower than for monoliths with Collembola and 25.8% lower than for monoliths with Enchytraeidae. Mesofaunal activity had no effect in the case of unsealed monoliths (Fig. 5).

Differences in the mean SSA-values of the sealed monoliths(≡1SSA)can be due to the surfaces

be-coming rougher (positive1SSA) or smoother (nega-tive1SSA) in all cases from date to date measurement (Table 1). No clear direction in roughness develop-ment was determined either for uninoculated or inoc-ulated sealed monoliths. Nevertheless,1SSA showed lower differences for uninoculated monoliths com-pared to those with collembolan and enchytraeid ac-tivity over the entire experimental period ((t18−t0) in

Table 1

Differences of mean SSA-values(≡1SSA) of sealed

uninocu-lated monoliths and monoliths inocuuninocu-lated with Collembola and Enchytraeidae for the first experimental period (t6−t0), the

sec-ond experimental period (t18−t6) and for the entire experimental

period (t18−t0). Positive values indicate an increase and negative

values a decrease in soil surface roughness

No fauna Collembola Enchytraeidae

t6−t0 +0.025 +0.013 −0.008

t18−t6 −0.020 −0.053 +0.048

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Fig. 3. Examples of surface characteristics of two soil monoliths inoculated with Enchytraeidae. Top: sealed surfaces. Bottom: unsealed surfaces. t0 =start of the experiment, t6 =6 months later. Visualisation based on digital elevation models with 0.3 mm grid spacing

obtained with a non-contact laser scanner. Specific surface area (SSA) is an index of roughness. The grey levels indicate elevation levels from 1 (light=low) to 21 mm(dark=high).

Table 1). Neglecting the sign (+/−) of the1SSA data and calculating the relative differences between the measurements (1SSA (%)) provides the most striking indication of the changes in the surface roughness for the sealed monoliths (Fig. 6). Except for Collembola at t0–t6,1SSA (%) was always higher for mesofaunal

inoculated monoliths. The greatest changes in the soil surface due to enchytraeid activity appear to have oc-cured within the first period. Nearly the same degree

of relative change was measured in the collembolan and enchytraeid treatments during the second period (t6−t18). The unincoluated monoliths also

experi-enced nearly the same degree of relative change in period one (t0−t6) as in period two (t6−t18). Over the

total experimental period (t0−t18) the relative changes

in uninoculated monoliths were much lower(≈0.4%)

than the alterations as a result of mesofaunal activities

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repre-sents half of the values and the bars represent ranges recorded, the solid line marks the median inside each box. Treatments in-dicated by different letters are significantly different based on the Mann–Whitney-U-test,p≤0.05.

half of the values recorded, the bars represent ranges and the solid line marks the median inside each box. Treatments are not signif-icantly different based on the Mann–Whitney-U-test,p≤0.05.

4. Discussion

The smoothening of the surface, as reflected by the decreasing SSA values is a precondition for the sealing process which occurs after a rainstorm event (Zobeck

Fig. 6. Relative changes of soil surface roughness (1SSA (%)) of sealed monoliths with no inoculated fauna and inoculated with Collembola and Enchytraeidae for the first experimental period (t0−t6), the second experimental period (t6−t18) and for the entire

experimental period (t0−t18).

and Onstad, 1987; Rudolph et al., 1997). The topog-raphy of the soil surface is therefore a good indicator for the existence of a surface seal.

Collembola and Enchytraeidae are clearly able to affect the morphology of soil surfaces through their burrowing activities leading to superficially deposited casts. Generally, this cast deposition results in a de-crease or inde-crease in the SSA, indicating a smoother or rougher surface, respectively. A higher SSA in inoculated monoliths has also been demonstrated in previous experiments conducted with compacted soil monoliths (Schrader et al., 1997). In the case of unsealed surfaces in the present study no sig-nificant difference in SSA between inoculated and uninoculated monoliths was visible (Fig. 5). On the other hand, it can be concluded for sealed soil sur-faces that mesofauna are able to penetrate the seal (Fig. 4). The minero-organic casts of Collembola and Enchytraeidae create a finely structured topography, i.e. a rougher surface which results in an increased SSA. Moreover, irrespective of the surface roughness these fine cast aggregates of both mesofaunal groups enhance the stability of the soil structure at the mi-croscale (Trappmann, 1953; Heisler et al., 1996).

The relative changes in SSA from Collembola and Enchytraeidae burrowing activities in sealed mono-liths were distinctly higher than in uninoculated monoliths. The change in the surface seal is the most significant development, regardless of the direction of change. Usually, one would expect a rougher sur-face as a consequence of mesofaunal activity and seal disruption. However, a change towards a smoother surface is also possible when mesofaunal activity

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leads to a reorganisation of surface inhomogeneities into smaller scale inhomogeneities which are below the measurement scale of resolution (Schrader et al., 1997). The change itself demonstrates that mesofauna are able to alter the otherwise static sealed surface and thereby initiate and enhance the rehabilitation of sealed soil surfaces. An increase in the infiltration capacity of the soil will be the consequence of the sur-face changes. On a larger scale, similar rehabilitation processes were investigated for earthworms (Kladi-vko et al., 1986) and termites (Mando and Miedema, 1997). The activities of both macrofaunal groups led to improved infiltration rates (Mando et al., 1996).

Positive1SSA values (Table 1) indicate an increase in surface roughness by cast deposition, whereas nega-tive values occur if for example small cracks are filled with fine mesofaunal casts resulting in a decreasing surface roughness. Despite the fact that 1SSA was negative for Collembola and positive for Enchytraei-dae for the entire experimental period (Table 1), their activity may be regarded as similar in terms of the alteration of the surface ignoring the direction of this alteration. This becomes more clear after calculating

1SSA in (%) terms (Fig. 6). Thus, for sealed

sur-faces the1SSA (%) for the total experimental period

(t0−t18) is nearly the same for Enchytraeidae and

Collembola in the present experiment. The results of this research suggest that the contribution of meso-fauna to soil structure is only apparent in cases in which the soil is exposed to exogenous physical and/or mechanical impacts and corresponding changes in soil properties and structure occur. When surface seal-ing (this study) or soil compaction (Schrader et al., 1997) take place, mesofaunal activities may counter-act such changes in the soil structure and accelerate the rehabilitation processes of the soil surface.

Changes in sealed soil surfaces as a result of in-trinsic soil processes are reflected in the1SSA (%)

of uninoculated monoliths (0.4%). Like macrofaunal activities these processes also contribute to a reha-bilitation of sealed soil surfaces. In general, swelling and shrinking are two main intrinsic soil processes depending on the water balance and clay content of the soil (Dexter, 1988). However, such processes ap-peared to be of little significance over the 18 months experimental period in the present case where ditions were kept constant. Consequently, the con-tribution of mesofaunal activity to the rehabilitation

of soil surface seals should not be underestimated. The impacts of mesofaunal activity on soil structure should be considered in addition to those of macro-fauna when physical processes, such as flux processes are evaluated. The experiments of Didden (1990) and van Vliet et al. (1998) reveal a general increase in air permeability and saturated hydraulic conductivity as a result of enchytraeid burrowing activity.

The burrowing activities of Collembola and Enchy-traeidae both directly and indirectly affect soil phys-ical and chemphys-ical processes (e.g. Trappmann, 1953; Didden, 1990; Heisler et al., 1996; Schrader et al., 1997; van Vliet et al., 1998). As a result the access to biotic and abiotic habitat resources for other soil organisms is controlled, or at least strongly influ-enced by these prominent members of the mesofaunal community. Accordingly, the burrowing activities of Collembola and Enchytraeidae in general, which can for example lead to the rehabilitation of sealed soil surfaces are consistent with the concept of ‘physical ecosystem engineering by organisms’ (Jones et al., 1994) and should be included in the category of Lavelle’s (1994) ‘ecosystem engineers’.

Acknowledgements

We thank Prof. Dr. H. Diestel (TU Berlin) who allowed us the use of the laser relief meter. We also thank Prof. Dr. M. Renger (TU Berlin) for the use of the rainfall simulator. We thank Mrs. M. Konder-mann for breeding the Collembola and Enchytraeidae. Ms. Sarah Ellgen is kindly acknowledged for her improvement of the language. Finally, we are grate-ful for receiving research funds from the German Research Foundation (Deutsche Forschungsgemein-schaft, DFG).

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and measurement dates the relative differences of the

SSA values were calculated. By definition the lowest

possible value of SSA is 1.0 (totaly plane surface):

we therefore subtracted 1.0 from all SSA values

be-fore determining the percentage differences between

treatments. The Mann–Whitney-U-test was used for

statistical analysis.

3. Results

Water saturation of the soil occurred 11 min and

sur-face runoff 16 min after rainstorm simulation began.

After 16 min the infiltration rate decreased rapidly

and the runoff rate increased. At the end of the

exper-iments soil loss as a result of surface runoff averaged

92.28 g m

−2

_{. After the rainfall simulation a seal had}

developed on the soil surfaces of the monoliths.

The differences in the mean specific surface area

(SSA) values between unsealed (1.45) and sealed

(1.31) soil surfaces immediately after the rainstorm

impact (time

t

₀

) were statistically significant (Fig. 2).

The smoothening of the soil surface after the rainstorm

is reflected in the mean SSA of sealed monoliths,

which is 31.1% lower than for unsealed monoliths.

Macroscopically no noticeable soil surface

differ-ences between Enchytraeidae and Collembola were

visible at either

t

or

t

. Effects of 6 months of

enchy-traeid activity on the previously sealed and unsealed

Fig. 2. Box-plot of the specific surface area (SSA) of sealed and unsealed monoliths at the start of the experiment (t0). Each box represents half of the values and the bars represent ranges recorded, the solid line marks the median inside each box. The different letters indicate that the treatments are significantly different based on the Mann–Whitney-U-test,p≤0.05.

surfaces are shown as an example in Fig. 3. In both

these cases, the burrowing activity of Enchytraeidae

resulted in a rougher surface with a higher SSA. In

particular, the initially sealed surface appears to be

more finely structured, i.e. rougher after 6 months

of enchytraeid activity (Fig. 3). In this case, SSA

increased by 17.2% during the first 6 months of the

experiment. After the same time period, the unsealed

surface showed an increase in SSA of 24.1% at

t

₆

.

When the SSA data for sealed and unsealed

treat-ments over the entire period of the experiment (t

–t

)

are pooled, the influence of the inoculated mesofauna

on the sealed surfaces in comparison with

uninoc-ulated treatments becomes more apparent. In the

sealed treatments, the mean SSA was significantly

higher for Enchytraeidae and Collembola inoculated

monoliths compared to monoliths without inoculated

fauna (Fig. 4). Furthermore, the higher mean SSA for

Collembola compared to Enchytraeidae was also

sta-tistically significant. The mean SSA of uninoculated

monoliths was 34.3% lower than for monoliths with

Collembola and 25.8% lower than for monoliths with

Enchytraeidae. Mesofaunal activity had no effect in

the case of unsealed monoliths (Fig. 5).

Differences in the mean SSA-values of the sealed

monoliths

(

≡

1SSA)

can be due to the surfaces

be-coming rougher (positive

1SSA) or smoother

(nega-tive

1SSA) in all cases from date to date measurement

(Table 1). No clear direction in roughness

develop-ment was determined either for uninoculated or

inoc-ulated sealed monoliths. Nevertheless,

1SSA showed

lower differences for uninoculated monoliths

com-pared to those with collembolan and enchytraeid

ac-tivity over the entire experimental period ((t

−

t

) in

Table 1

Differences of mean SSA-values(≡1SSA) of sealed

uninocu-lated monoliths and monoliths inocuuninocu-lated with Collembola and Enchytraeidae for the first experimental period (t6−t0), the sec-ond experimental period (t18−t6) and for the entire experimental period (t18−t0). Positive values indicate an increase and negative values a decrease in soil surface roughness

No fauna Collembola Enchytraeidae

t6−t0 +0.025 +0.013 −0.008

t18−t6 −0.020 −0.053 +0.048

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obtained with a non-contact laser scanner. Specific surface area (SSA) is an index of roughness. The grey levels indicate elevation levels from 1 (light=low) to 21 mm(dark=high).

Table 1). Neglecting the sign (

+

/

−

) of the

1SSA data

and calculating the relative differences between the

measurements (1SSA (%)) provides the most striking

indication of the changes in the surface roughness for

the sealed monoliths (Fig. 6). Except for Collembola

at t

–t

,

1SSA (%) was always higher for mesofaunal

inoculated monoliths. The greatest changes in the soil

surface due to enchytraeid activity appear to have

oc-cured within the first period. Nearly the same degree

of relative change was measured in the collembolan

and enchytraeid treatments during the second period

(t

−

t

). The unincoluated monoliths also

experi-enced nearly the same degree of relative change in

period one (t

−

t

) as in period two (t

−

t

). Over the

total experimental period (t

−

t

) the relative changes

in uninoculated monoliths were much lower

(

≈

0.4%)

than the alterations as a result of mesofaunal activities

(

≈

3%).

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Fig. 4. Box-plot of the specific surface area (SSA) of sealed mono-liths inoculated with Collembola and Enchytraeidae and with no inoculated fauna for the entire 18 months experimental period (t0−t18) based on three dates of measurement. Each box repre-sents half of the values and the bars represent ranges recorded, the solid line marks the median inside each box. Treatments in-dicated by different letters are significantly different based on the Mann–Whitney-U-test,p≤0.05.

Fig. 5. Box-plot of the specific surface area (SSA) of unsealed monoliths inoculated with Collembola and Enchytraeidae and with no inoculated fauna for the entire 18 months experimental period (t0−t18) based on three dates of measurement. Each box represents half of the values recorded, the bars represent ranges and the solid line marks the median inside each box. Treatments are not signif-icantly different based on the Mann–Whitney-U-test,p≤0.05.

4. Discussion

The smoothening of the surface, as reflected by the

decreasing SSA values is a precondition for the sealing

process which occurs after a rainstorm event (Zobeck

and Onstad, 1987; Rudolph et al., 1997). The

topog-raphy of the soil surface is therefore a good indicator

for the existence of a surface seal.

Collembola and Enchytraeidae are clearly able to

affect the morphology of soil surfaces through their

burrowing activities leading to superficially deposited

casts. Generally, this cast deposition results in a

de-crease or inde-crease in the SSA, indicating a smoother

or rougher surface, respectively. A higher SSA in

inoculated monoliths has also been demonstrated

in previous experiments conducted with compacted

soil monoliths (Schrader et al., 1997). In the case

of unsealed surfaces in the present study no

sig-nificant difference in SSA between inoculated and

uninoculated monoliths was visible (Fig. 5). On the

other hand, it can be concluded for sealed soil

sur-faces that mesofauna are able to penetrate the seal

(Fig. 4). The minero-organic casts of Collembola and

Enchytraeidae create a finely structured topography,

i.e. a rougher surface which results in an increased

SSA. Moreover, irrespective of the surface roughness

these fine cast aggregates of both mesofaunal groups

enhance the stability of the soil structure at the

mi-croscale (Trappmann, 1953; Heisler et al., 1996).

The relative changes in SSA from Collembola and

Enchytraeidae burrowing activities in sealed

mono-liths were distinctly higher than in uninoculated

monoliths. The change in the surface seal is the most

significant development, regardless of the direction

of change. Usually, one would expect a rougher

sur-face as a consequence of mesofaunal activity and seal

disruption. However, a change towards a smoother

surface is also possible when mesofaunal activity

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into smaller scale inhomogeneities which are below

the measurement scale of resolution (Schrader et al.,

1997). The change itself demonstrates that mesofauna

are able to alter the otherwise static sealed surface

and thereby initiate and enhance the rehabilitation of

sealed soil surfaces. An increase in the infiltration

capacity of the soil will be the consequence of the

sur-face changes. On a larger scale, similar rehabilitation

processes were investigated for earthworms

(Kladi-vko et al., 1986) and termites (Mando and Miedema,

1997). The activities of both macrofaunal groups led

to improved infiltration rates (Mando et al., 1996).

Positive

1SSA values (Table 1) indicate an increase

in surface roughness by cast deposition, whereas

nega-tive values occur if for example small cracks are filled

with fine mesofaunal casts resulting in a decreasing

surface roughness. Despite the fact that

1SSA was

negative for Collembola and positive for

Enchytraei-dae for the entire experimental period (Table 1), their

activity may be regarded as similar in terms of the

alteration of the surface ignoring the direction of this

alteration. This becomes more clear after calculating

1SSA in (%) terms (Fig. 6). Thus, for sealed

sur-faces the

1SSA (%) for the total experimental period

(t

−

t

) is nearly the same for Enchytraeidae and

Collembola in the present experiment. The results of

this research suggest that the contribution of

meso-fauna to soil structure is only apparent in cases in

which the soil is exposed to exogenous physical and/or

mechanical impacts and corresponding changes in

soil properties and structure occur. When surface

seal-ing (this study) or soil compaction (Schrader et al.,

1997) take place, mesofaunal activities may

counter-act such changes in the soil structure and accelerate

the rehabilitation processes of the soil surface.

Changes in sealed soil surfaces as a result of

in-trinsic soil processes are reflected in the

1SSA (%)

of uninoculated monoliths (0.4%). Like macrofaunal

activities these processes also contribute to a

reha-bilitation of sealed soil surfaces. In general, swelling

and shrinking are two main intrinsic soil processes

depending on the water balance and clay content of

the soil (Dexter, 1988). However, such processes

ap-peared to be of little significance over the 18 months

experimental period in the present case where

ditions were kept constant. Consequently, the

con-tribution of mesofaunal activity to the rehabilitation

The impacts of mesofaunal activity on soil structure

should be considered in addition to those of

macro-fauna when physical processes, such as flux processes

are evaluated. The experiments of Didden (1990) and

van Vliet et al. (1998) reveal a general increase in air

permeability and saturated hydraulic conductivity as

a result of enchytraeid burrowing activity.

The burrowing activities of Collembola and

Enchy-traeidae both directly and indirectly affect soil

phys-ical and chemphys-ical processes (e.g. Trappmann, 1953;

Didden, 1990; Heisler et al., 1996; Schrader et al.,

1997; van Vliet et al., 1998). As a result the access

to biotic and abiotic habitat resources for other soil

organisms is controlled, or at least strongly

influ-enced by these prominent members of the mesofaunal

community. Accordingly, the burrowing activities of

Collembola and Enchytraeidae in general, which can

for example lead to the rehabilitation of sealed soil

surfaces are consistent with the concept of ‘physical

ecosystem engineering by organisms’ (Jones et al.,

1994) and should be included in the category of

Lavelle’s (1994) ‘ecosystem engineers’.

Acknowledgements

We thank Prof. Dr. H. Diestel (TU Berlin) who

allowed us the use of the laser relief meter. We also

thank Prof. Dr. M. Renger (TU Berlin) for the use

of the rainfall simulator. We thank Mrs. M.

Konder-mann for breeding the Collembola and Enchytraeidae.

Ms. Sarah Ellgen is kindly acknowledged for her

improvement of the language. Finally, we are

grate-ful for receiving research funds from the German

Research Foundation (Deutsche

Forschungsgemein-schaft, DFG).

References