Earthworm assemblages in an ecotone between forest and arable ﬁ eld and their relations with soil properties

– The objective of this work was to assess the effects of a forest-ﬁ eld ecotone on earthworm assemblages. Five sites (blocks) differing in the type of crop rotation used in the ﬁ eld were studied in Central Bohemia, Czech Republic. In each block, sampling was carried out in seven parallel rows perpendicular to a transect from a forest (oak or oak-pine) to the centre of a ﬁ eld, both in spring and autumn 2001–2003. Individual rows were located in the forest (5 m from the edge), in the forest edge, and in the ﬁ eld (at 5, 10, 25, 50 and 100 m distances from the forest edge). The density and biomass of earthworms were lowest in the forest, increased markedly in the forest edge, decreased again at 5 or 10 m distance from the forest edge and then continuously increased along the distance to the ﬁ eld boundary. The highest number of species was found in the forest edge and in the ﬁ eld boundary. Individual species differed in their distribution along the transect. Both density and biomass of earthworms were correlated with distance from forest edge, soil organic matter content, soil porosity, and water inﬁ ltration rate.


Introduction
In Central Europe, most lowland forests are fragmented due to the long-lasting human activity.Patches of forests are mostly surrounded by agricultural land.Ecotones between forests and arable fi elds or grasslands form a remarkable part of the landscape mosaic.Therefore, the research on ecotones is important for a better understanding of the effects of landscape pattern on ecosystem functions, and could also add to a sound scientifi c justifi cation of regulatory protection of biological diversity and of landscape management planning.
Most studies on biodiversity of ecotones have dealt with plants, birds, small mammals and several groups of insects.Soil fauna has been scarcely studied in this context.Some data are available on the diversity of nematodes (Háněl, 2000;Imaz et al., 2002), enchytraeids (Nowak, 2004) and collembolans (Slawski & Slawska, 2000) across ecotones of different scales (micro or mesoecotones).There are only a few studies comparing earthworm assemblages in arable land and adjacent forests (Whalen, 2004;Smith et al., 2008) or dealing with earthworm dispersal in the agricultural Pesq. agropec. bras., Brasília, v.44, n.8, p.922-926, ago. 2009 landscape (Mather & Christensen, 1995).Three studies on earthworms in ecotone systems have been published so far.Arujo & Lopez-Fernandez (1999) examined earthworm populations in forest-savanna ecotones in Venezuela.In Sweden, Lagerlöf et al. (2002) compared earthworm assemblages of a non-cultivated fi eld boundary and of the adjacent arable fi eld at different distances from the boundary.Makulec (2004) studied earthworms in a midfi eld shelterbelt, an adjacent arable fi eld and an ecotone between both ecosystems in western Poland.However, some additional data are available for comparison from studies dealing with changes of earthworm assemblages during the secondary succession in abandoned arable fi elds (Scheu, 1992;Pižl, 1999;Slávecz & Csuzdi, 2007).
The aim of this study was to examine the effect of forest edges on species richness and quantitative parameters of earthworm assemblages in the agricultural landscape of Central Europe.As our study was conducted within a multidisciplinary project investigating the effects of fi eld margins on water dynamics in arable soils, particular attention was paid to the agriculturally used part of the studied ecotones.

Materials and Methods
The research was carried out in Central Bohemia (Czech Republic) near the town Kostelec nad Černými lesy, ca.35 km east of Prague.Five sites (blocks) representing forest-fi eld ecotones with different types of crop rotation were studied.The distances between sites ranged from 0.5 to 3 km.The forest parts of the sites received no forestry intervention during the study, and the fi eld parts were ploughed each year in autumn.Additional data about the sites are given in Table 1.At each site, soil sampling was carried out in seven parallel rows perpendicular to a transect from an oak or oak-pine forest towards the centre of the fi eld (ecotone lines).Individual rows were located in the forest interior (A, at 5 m from the edge), in the forest edge (E), and in the fi eld at distances of 5 (P1), 10 (P2), 25 (P3), 50 (P4) and 100 m (P5) from the edge of the forest.Six samples (each with 625 cm 2 in area and 30 cm in depth) were taken from each row in spring and autumn 2001-2003.The distances between sampling points were dependent upon the sizes of individual sites and were of 25 m in block 3, 30 m in blocks 1 and 2, 35 m in block 5, and 50 m in block 4. Earthworms were collected by hand sorting from the soil samples.In order to expel worms from deeper soil layers, 0.5 L of 0.25% formaldehyde was applied three times within a period of 20 minutes into each hole.A total of 5,402 earthworms were obtained from 1,260 samples.
The lumbricids obtained were preserved in 7% formaldehyde and identifi ed (including the ones in juvenile stages) to the species level.Preserved specimens were weighed to determine earthworm biomass, and no corrections were made for gut content or fresh weight.The earthworm assemblages of individual rows were characterized by density, biomass, composition of species and their relative densities.
Table 1.Geographical position of individual blocks, tree composition in forests, crops planted in the fi elds in the studied years, and basic soil parameters (means±SD, n = 6), measured in spring 2002.
In addition, chemical parameters of soil were measured from soil samples taken from each ecotone row in spring 2002.Soil pH and the contents of organic carbon (C ox ), K, Na, Ca and total, available and water soluble P were measured using standardized methods (Zbíral et al., 1997).Infi ltration rate and soil porosity at depths of 10, 20 and 30 cm were measured in each row in spring and autumn 2001-2003, according to Sklenička et al. (2002) ) methods.
Differences between earthworm densities and biomasses along the transect were tested by the distribution-free Kruskal-Wallis ANOVA and subsequently by the Mann-Whitney-U-Test for the corresponding pairs of rows.Spearman's coeffi cient was used to assess correlations between earthworm density and biomass and selected physical parameters of soil (SPSS 15.0 for Windows).An ordination based on principal component analysis (PCA), calculated using the CANOCO program, was used for visualizing relations among environmental and earthworm assemblage parameters.  A. rosea, D. octaedra, L. rubellus, L. terrestris, and O. lacteum) were found in all blocks.On the other hand, P. antipai, an earthworm scarcely reported in cultivated areas, was only found in one block and the remaining species in two or three blocks.Aporrectodea caliginosa predominated in all blocks (Table 2).The highest number of species was found in the forest edge and in the fi eld boundary (Figure 1).

Results and Discussion
Three species, A. caliginosa, A. rosea and L. terrestris, were recorded in all rows of the transect.Among them, A. caliginosa showed a signifi cant trend of increasing density from the boundary of the fi eld towards the centre, while A. rosea and L. terrestris preferred the forest edge.Other species were found irregularly in some rows.The endogeic Proctodrilus antipai preferred arable soil, whereas the epigeic Dendrobaena octaedra and Lumbricus rubellus and the endogeic Octolasion lacteum showed strong affi nity to the forest edge (Figure 1).
The position within the transect signifi cantly affected the density and biomass of earthworms.In general, both parameters were lowest in the forest interior, increased markedly in the forest edge, decreased again at a 5 or 10-m distance from the forest edge and then increased successively along the distance from the forest edge (Figure 2).The same pattern was observed in all blocks and years of the study.Earthworm density and biomass were positively correlated with the distance from the forest edge towards the centre of the fi eld and with soil Table 2. List of earthworm species and sub-species and their relative density (%) recorded from individual blocks.
Figure 1.Distribution of earthworms along the transect from oak forest to arable fi eld.Bar width indicates the relative proportion (%) of the total number of individuals.A, E, P1, P2, P3, P4 and P5: ecotone rows located in the forest interior; in the forest edge; and in the fi eld distances of 5, 10, 25, 50 and 100 m from the forest edge, respectively organic matter content, moisture, porosity and water infi ltration rate (Figure 3).Signifi cant correlations between both quantitative parameters of earthworm assemblages and soil porosity up to the depth of 30 cm and water infi ltration rate were found in all years of the study (Table 3).
In accordance with Leopold's theory (Leopold, 1933), which states that diversity and organism density will be higher in ecotones than in adjacent ecosystems, the highest numbers of earthworm species were recorded in the forest edge and in the fi eld boundary.No species showed a preference for the fi eld boundary, whereas fi ve preferred the forest edge.Of those, D. octaedra and L. rubellus belonged to epigeic, A. rosea and O. lacteum to endogeic, and L. terrestris to anecic earthworms.The observed preference of epigeic species for the forest edge agrees with the fi ndings by Scheu (1992), who reported a continuous increase of their biomass during secondary succession in abandoned fi elds until the establishment of woody vegetation.Both epigeic species found in this study live in the litter and humus layers or can sometimes penetrate a little deeper into loose mineral soil (particularly L. rubellus).They are susceptible to soil cultivation, since it destroys the superfi cial soil layer.In general, they are much more abundant in Figure 2. General averages of earthworm density and biomass along the transect from oak forest to arable fi eld.A, E, P1, P2, P3, P4 and P5: ecotone rows located in the forest interior; in the forest edge; and in the fi eld distances of 5, 10, 25, 50 and 100 m from the forest edge, respectively.Bars with equal letters above it indicate that means do not differ by Kruskal-Wallis H Test at 5% of probability (n = 180).
Figure 3. Principal component analysis biplot of earthworm density (Abund) and biomass (Biom), distance from forest edge (Dist) and soil parameters (Por10, Por20, Por30: porosity of the soil at the depth of 10, 20 and 30 cm; Inf, infi ltration rate; Moist, soil moisture; C ox , organic carbon; P tot , P v , P avail : total, water-soluble and available phosphorus. Table 3. Spearman's correlation coeffi cient (n = 60) between earthworm density and biomass and selected physical soil parameters.and ** signifi cant at 5 an 1% of probability, respectively.non-arable habitats than in ploughed ones (Ivask et al., 2007;Smith et al., 2008).This may be the reason for their absence in the fi eld centre.Similarly, the deepburrowing earthworm L. terrestris, which collects food from the soil surface, is adversely affected by heavy soil cultivation that destroys its burrow system and reduces food sources.The preference of A. rosea and O. lacteum, which can withstand ploughing rather well (Whalen, 2004), for the forest edge and periphery of the fi eld could be explained by better moisture conditions, as refl ected by the presence of the amphibious E. tetraedra in the same transect rows.
The forest edge had a positive effect on the density and biomass of earthworms.In general, both parameters were higher in the ecotone zone than in the forest or in adjacent parts of the fi eld.These fi ndings contradict what was found in Sweden by Lagerlöf et al. (2002), who reported lower earthworm density and biomass in the boundary and periphery than in more distant parts of an arable fi eld.Similar to our study, however, they found the highest earthworm density in the fi eld centre.Correspondingly, A. caliginosa was identifi ed as the dominant species in both studies.This endogeic earthworm lives in horizontally oriented semi-permanent burrows in the mineral layers of the soil.It occurs commonly in agro-ecosystems and is known to be resistant to cultivation (Smith et al., 2008).Even after shifting tillage practice from ploughing to reduced-tillage stubble cultivation or direct drilling, followed by the rapid increase of L. terrestris and D. rubidus populations, the populations of A. caliginosa often remained unchanged (Nuutinen, 1992).Studies on earthworm succession in abandoned fi elds showed that A. caliginosa seems to be a species of the open landscape, which declines as soon as afforestation occurs (Scheu, 1992;Pižl, 1999).Slávecz & Csuzdi (2007) reported from Maryland, USA, that the introduced A. caliginosa dominated even in successional forests, whereas another introduced species, L. rubellus, dominated in the mature forests.The highest density of A. caliginosa in the lines most distant from the fi eld boundary found in our study may probably result from lower level of competition, higher soil pH and redistribution of organic food sources through the profi le of the loosened soil.Makulec (2004) found no signifi cant difference between the midfi eld shelterbelt and the ecotone in either composition (three species: A. caliginosa, D. octaedra and L. terrestris) or mean density of lumbricids (58 and 73 individuals m -2 , respectively).Again, A. caliginosa was the dominant earthworm.However, he found a single species, A. caliginosa, at an extremely low density (0.9 individuals m -2 ) in an adjacent maize fi eld.Nowak (2004), who carried out her study in the same area, concluded that the assemblages of enchytraeids in the ecotones were similar to those in the fi elds with respect to species composition, and to those in shelterbelts with respect to the density and body size.

Conclusion
Forest edges may play a positive role in the maintenance of earthworm species richness in agroecosystems.