Soil nematode populations in a grassland plant diversity experiment run for seven years

Applied Soil Ecology 48 (2011) 174–184

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Soil nematode populations in a grassland plant diversity experiment run for seven years Maria Viketoft a,∗ , Björn Sohlenius b a b

Department of Ecology, Swedish University of Agricultural Sciences (SLU), Box 7044, 750 07 Uppsala, Sweden Swedish Museum of Natural History, Department of Invertebrate Zoology, Box 50007, 104 05 Stockholm, Sweden

a r t i c l e

i n f o

Article history: Received 7 January 2011 Received in revised form 19 March 2011 Accepted 22 March 2011 Keywords: BIODEPTH Functional diversity Plant composition Plant species richness Soil nematodes Sweden

a b s t r a c t Plant species identity and diversity may greatly influence the composition of the nematode fauna. The abundance of various nematode populations was investigated in a field experiment on plant diversity. 58 plots in an arable field planted with plant species growing alone or together up to a richness of 12 species were sampled after seven years for analysis of composition of the nematode fauna. Two additional control plots without vegetation were also sampled. Plant species identity was generally more important than plant diversity for the composition of the nematode fauna. Only the omnivorous Aporcelaimidae was positively related to plant species richness, and the fungal-feeding Aphelenchus and the bacterial-feeding Prismatolaimus were affected by functional diversity. Some nematode populations were strongly influenced by plant species composition, e.g. the plant-feeder Tylenchorhynchus maximus was clearly coupled to the grass Phleum pratense. Nematode species within a feeding group sometimes had a rather specific abundance patterns under various plant species and plant species combinations. This was especially the case with the plant-feeding nematodes some of which obviously were directly influenced by the host suitability of specific plant species. Other nematode species were probably more influenced by indirect effects of plants on edaphic and nutrient conditions including biotic interactions from other components in the soil organism community. Yet other nematode species were little influenced by the kind of vegetation in the different plots. Our results show that to fully understand plant community effects on the nematode fauna there is a need to go further than just a division into nematode feeding groups. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Plants can affect the soil organism community through several mechanisms (Wardle, 2002). Nematodes are an abundant component of the soil community, and may reflect soil processes including root production and microbial activity. Individual plant species may differ in their effect on the soil microclimate and on the quantity and quality of material returned to the soil as root exudates and plant litter. It has been demonstrated that various plant species have great influence on the abundance of nematodes (Wardle et al., 2003; De Deyn et al., 2004; Viketoft et al., 2005). Especially the effects of plant species identity can be great when plant-feeding nematodes are considered. Also the functional group of plants may have an impact on the nematode fauna. For instance, legumes may promote certain rapidly growing bacterial-feeders and forbs may promote fungal-feeders (Viketoft et al., 2005; Sohlenius et al., 2011), which in turn reflect the effects of plant species on the fungal and bacterial activity (Coleman et al., 2004).

∗ Corresponding author. Tel.: +46 18 672346; fax: +46 18 672890. E-mail address: [email protected] (M. Viketoft). 0929-1393/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2011.03.008

Theoretically, aboveground plant diversity may promote belowground diversity by increasing the variety of food resources (litter quality and composition), the range of environmental conditions (temperature, humidity), or the structural complexity of the habitat (Anderson, 1995). If there are a great number of specific connections between particular plant and nematode species an increased diversity of plant species should be coupled to an increase in numbers of nematode species. The interaction between above and belowground diversity has been in focus of several recent investigations. However, so far current studies indicate only a weak association between number of plant species and number of nematode species (De Deyn et al., 2004; Viketoft et al., 2009). Most studies investigating the effect of plant diversity on soil nematode communities have only considered the nematode diversity and/or functional groups of nematodes (Porazinska et al., 2003; Wardle et al., 2003; De Deyn et al., 2004; Viketoft et al., 2009). Division into feeding groups is uncertain, because of poorly documented food resources for some groups of nematodes (Yeates, 2003). Besides, functional groups are aggregated units and functional groups defined with respect to one particular function may not be the same as those defined with respect to another function (Bengtsson, 1998). In addition, when there are marked long-term

M. Viketoft, B. Sohlenius / Applied Soil Ecology 48 (2011) 174–184

shifts in specific nematode populations it is critical that the species are identified and the shifts assessed in ecosystem terms (Yeates, 2003), as soils are a heterogeneous environment that promotes functional redundancy at the species level (Setälä et al., 2005). In this article, we examine relatively long-term effects of plant species diversity, functional diversity, and plant species composition on soil nematodes using the experimental grassland plots of the pan-European BIODEPTH project (BIODiversity and Ecological Processes in Terrestrial Herbaceous ecosystems) in Sweden. These plots had been maintained close to the original plant species composition for eight growing seasons at the time of sampling. This is longer than most other experimental studies on the effects of plants on soil biota. In a previous study (Viketoft et al., 2009), we found that plant species identity is most important for nematode feeding groups, but the hypothesis that species or functional diversity of plants will affect nematode diversity or composition also received some support. Here we report on plant composition and richness effects on individual nematode taxa. Specifically, we address the following questions: 1) Are individual nematode taxa influenced by plant species or functional diversity? 2) Is plant species identity the most important determinant for variation in abundance of nematode populations in a specific soil? 3) How do different taxa within a nematode feeding group change their abundance under a range of plant species or plant species combinations? 2. Materials and methods 2.1. Site description and experimental design The field site was located at the experimental fields of the Swedish University of Agricultural Sciences in Umeå, northern Sweden (63◦ 45 N, 20◦ 17 E, 12 m a.s.l.) and was established in 1996 as part of the pan-European BIODEPTH project (Hector et al., 1999). The site was an agricultural field for at least 35 years prior to establishment of the experimental grassland, and during the last ten years the main crop had been barley. The soil is classified as silt loam (4.1% clay, 57.9% silt, 38.0% fine sand) and after 1995, no fertilizers or biocides were applied to the investigated plots. More detailed information about the site is found in Mulder et al. (2002). In 1996, a field experiment with different levels of plant diversity (0–12 plant species) was established (Mulder et al., 2002). The different plant species were all common in leys and semi-natural grasslands in Sweden, and were divided into the three plant functional groups grasses, legumes and non-leguminous forbs based on differences in growth form (grasses vs. forbs) and the ability to fix nitrogen (legumes vs. grasses and forbs). The plots (2.2 m by 5.0 m) were laid out in two blocks because of a small elevation gradient. In the present study, 60 of the plots containing 0, 1, 2, 4, 8 and 12 plant species were investigated (Table 1). The plots were sown with a density of 2000 seeds m−2 . The desired plant species composition and diversity was maintained by manual weeding and by re-sowing when necessary. Between the plots 1.5 m wide walkways were sown with Phleum pratense which were cut regularly. All plots were harvested once a year to measure plant productivity and other variables (see Mulder et al., 2002). In the present study, we used the variables plant biomass >5 cm above-ground 2003 (Table 1) and root biomass 2003, the year in which the nematode samples were taken, as indicators of plant productivity (dry weight after drying at 60 ◦ C for 24 h). For calculating plant functional diversity, we used six traits obtained from measurements in the

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monoculture plots in 1997, 1998 and 2003, or for plant secondary and defence compounds (e.g. tannins, phyto-oestrogen, oxalic acid) from the literature. The traits were: (1) root biomass (1997), (2) above-ground biomass (mean of 1997 and 2003), (3) total nitrogen concentration in vegetation (% of dry matter in biomass, mean of 1997 and 2003), (4) canopy height (measured in 1998 only), (5) cover of target species (2003), and 6) secondary and defence compounds (presence/absence). For more detailed information see Viketoft et al. (2009). 2.2. Nematode sampling Nematodes were sampled in the beginning of August 2003 (block 2), just before plant biomass harvest, and in the beginning of September 2003 (block 1). This means that the block effect in our statistical analyses also includes possible effects of sampling date. In each plot, six cores (diam. 2.3 cm) were randomly taken with a soil corer to a depth of 10 cm. In plots with sparse vegetation cover the cores were taken in the vicinity of the plants. The samples were placed in plastic tubes, sealed with a cap, transported to the laboratory in a cooling box, stored at 4 ◦ C and extracted within one week after sampling. A subsample of 8 g soil (wet weight) was taken from each individual core sample for extraction using a modified Baermann method (Viketoft et al., 2005), resulting in six extractions for each plot. From each extraction, the total nematode numbers were counted under low magnification (50×). After counting, the suspensions from two extractions were pooled and about 200–300 nematodes were identified to genus or species level under higher magnification (200×). The nematodes were placed into seven different semi-taxonomic feeding groups based on Sohlenius (2002). A modification used in the present study is that the bacterialfeeders belonging to Rhabditida are divided into one group of rapidly growing species, Rhabditida r-strategists (c-p 1 according to Bongers, 1990), and another with moderate growth rate, Rhabditida K-strategists (c-p 2-3). For the taxonomic analysis of the fauna a large number of permanent slides were made and a special effort had been given to identify specimens to species level. The slides are deposited at the section of Invertebrate Zoology at the Swedish Museum of Natural History. All analyses were made on mean values for each plot to avoid pseudo-replication, resulting in n = 2 for each treatment. 2.3. Data treatment and statistical analyses The effects of plant composition and richness effects on total nematode abundance, richness of nematode genera, and community composition at the level of nematode functional groups have been reported previously (Viketoft et al., 2009). Here we investigate the effects of plant composition and richness on individual nematode taxa. We used principal component analysis (PCA) to summarise the variation of the nematode community in the 60 plots (two plots with no plants were included in the ordination but not used in the subsequent analyses). Only the nematode taxa present in at least half the samples or clearly common under certain plant species were included, a total of 23 taxa. Prior to the PCA, species abundances were transformed to relative proportions of the total nematode abundance to minimise density effects and then square-root transformed (Lindberg and Bengtsson, 2006). Pearson correlations between PCA scores and nematode abundances were used to determine which nematode taxa were significantly related to the principal components. Sequential Bonferroni correction was applied to examine the statistical significance of the correlations. The sample scores along the first two principal components were used in the statistical procedure described below to

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Table 1 Plant species number, composition and aboveground plant biomass (herbage above 5 cm in 2003) of the 30 studied plant communities. Each monoculture and mixture was studied in two plots (one per block). G = grass, L = legume, F = forb. No. sown species

Functional groups

Species composition

Abbreviation

0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 4 4 4 4 4 4 8 8 8 12 12

– G G G G L L L L F F F F GG GF LF FF LL GL 2G2F 2G1L1F 2G2L 2L2F 1G1L2F 1G2L1F 4G2L 2F 4F2G2L 4L2G2F 4G4L4F 4G4L4F

– Dactylis glomerata L. Festuca ovina L. Phalaris arundinacea L. Phleum pratense L. cv. Jonatan Lotus corniculatus L. Trifolium pratense L. cv. Betty Trifolium hybridum L. cv. Stena Trifolium repens L. cv. Undrom Achillea millefolium L. Leucanthemum vulgare Lam. Ranunculus acris L. Rumex acetosa L. DgPa FoAm LvLc RaRua ThTr PpTp DgFoAmRua DgPaLcLv PaPpLcTp ThTrLvRa PpTpRaRua FoThTrAm AllG ThTrLvRa AllF PaPpLcTp AllL DgFoAmRua All1 All2

No Dg Fo Pa Pp Lc Tp Th Tr Am Lv Ra Rua DgPa FoAm LvLc RaRua ThTr PpTp DFAR DPLL PPLT TTLR PTRR FTTA GTTLR FPPLT LDFAR ALL1 ALL2

determine the effects of plant diversity and plant community composition on nematode community composition. We also analysed the abundance of the 23 nematode taxa individually. To reveal variability and effect of treatments, linear correlation coefficient between plot pairs of the same treatment was calculated. If there is a clear effect of treatment, nematode abundance in parallel plots with the same plant species should be significantly correlated. Mean value and coefficient of variation (CV = standard deviation/mean value) were also calculated for all 60 plots, i.e. including the plots with no plants. A large CV value indicates an aggregated distribution which might be an indication of treatment effects. We think that CV values above 1.0 might be considered in this context. A high CV combined with no, negative, or low positive values of correlation coefficient may indicate that the high variability could be caused by other factors than treatment. These could be unknown variation of edaphic factors or a strong aggregative distribution pattern due to colonization processes. To test the effects of plant community composition, plant species and functional diversity we used ANOVA. The statistical procedure in this paper follows Spehn et al. (2005) for a single site. Hence, we used Type I sum of squares throughout and included independent variables in sequential order (Table 2). If “Community” or other independent variables were significant, we investigated this further by singly adding aboveground plant biomass, root biomass, presence of grass, legumes, forbs, and each of the 12 plant species after the block factor (Table 2). To account for the multiple tests of plant identity effects, the significance level was set to 0.01 for the individual plant species. In the above analyses the plots with no vegetation were not included. We checked visually which transformations best approached normality for each variable. Plant species richness was ln-transformed (following the original BIODEPTH studies (Spehn et al., 2005)), biomasses were square-root transformed and nematode abundances were log(1 + N)-transformed.

Plant biomass (g dw m−2 ) Grasses

Forbs

Legumes

0 86.8 103.3 421.9 153.5 3.6 1.9 1.7 3.0 1.6 1.6 2.4 0.4 220.7 65.7 3.7 0.7 1.6 138.8 79.6 241.6 200.5 2.8 192.0 5.9 359.2 293.2 63.1 377.7 175.8

0 3.1 1.9 2.7 36.0 4.0 0 0 0 128.7 48.6 79.8 17.7 1.4 44.4 69.9 95.5 1.7 0.4 87.5 53.2 2.2 90.1 20.1 198.5 6.1 55.3 80.7 12.9 48.9

0 0 0 0 0 300.2 429.6 76.4 112.7 0 0 1.0 0 0.7 0 323.9 0 59.9 312.2 0.6 101.5 491.5 33.8 479.3 109.6 101.6 549.6 888.2 490.9 252.1

For the PCA, CANOCO version 4.5 (Microcomputer Power, Ithaca, NY, USA) was used and all other statistical tests were performed using SAS for Windows 9.1 (SAS Institute Inc., Cary, NC, USA). Means presented in figures were calculated using non-transformed data (±SE). 3. Results 3.1. Nematode community composition Totally about 60 taxa of nematodes were found and of these were 23 examined more closely (Table 3). The relative composition of the semi-taxonomic feeding groups is indicated in Fig. 1. Obligate plant-feeders were the dominant feeding group in most treatments followed by Rhabditida K-strategic bacterial-feeders. The nematode community composition in monocultures of grass species varied less than monocultures of legumes and forbs (Fig. 1). The variability among treatments with 8–12 plant species was less than among the other treatments. The first two PCA axes together explained 63% of the variation in the composition of the nematode communities (Fig. 2). Both PC1 and PC2 were strongly affected by plant community (p < 0.0001). The first axis (PC1) explained 49% and was negatively related to shoot and root biomass as well as the presence of grasses, while the second axis (PC2) explained 14% and was negatively related to forbs and positively to the grass P. pratense. However, there were still effects of community after including these variables. PC1 was negatively correlated with high abundances of the root-feeding Paratylenchus and positively correlated with high abundances of both fungal- and bacterial-feeders (Fig. 2). On the other hand, PC2 was positively correlated with high abundances of the plantfeeders Tylenchorhynchus maximus and Pratylenchus and negatively to the fungal-feeder Aphelenchus (Fig. 2). We interpret the result as showing that the plant-feeder Paratylenchus responded positively

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Table 2 ANOVA of the effects of plant species richness, functional richness, and plant community composition on nematode communities in a grassland plant diversity experiment eight growing seasons after establishment. Independent variables

df

Error term

Basic model

Block ln SR SRcat FD Community

1 1 3 1 23

Residual Community Community Community Residual

Extended model

Block Aboveground or root biomassa Grass or legumes or forbs (presence/absence)a One of the 12 plant species (presence/absence)a ln SR SRcat FD Community

1 1 1 1 1 3 1 23/22

Residual Residual Community Community Community Community Community Residual

Independent variables were added sequentially (Type I sum of squares): ln SR – ln(plant species richness) as a continuous variable (tests for linear effects of plant species richness); SRcat – plant species richness as category (1, 2, 4, 8, 12; tests for additional non-linear effects of plant species richness), FD – plant functional diversity, total length of the dendrogram branches required to join the species in a multi-variable trait space (Viketoft et al., 2009); community – 29 different plant communities with each n = 2 (see Table 1). Residual df is 28 in the basic model. a Only one of these 17 variables were added in each extended model.

to plant productivity in general, while some of the other plantfeeding nematodes are more dependent on specific suitable host plants. 3.2. Individual nematode taxa Among the obligate plant-feeders, Paratylenchus (predominantly Paratylenchus projectus) strongly dominated the community and contributed to half of the total nematode numbers (Table 3). The coefficient of variation of mean values (CV) was large among the obligate plant-feeders (Table 3), the correlation coefficients between plots of the same treatment was in general high and all genera were affected by plant community (Table 4). Paratylenchus reached its highest abundance under the monoculture of Trifolium repens, but it was also abundant and dominant in most treatments except Trifolium hybridum, Leucanthemum vulgare and no plants, where it was almost absent (Fig. 3a). In general, the abundance of Paratylenchus tended to be high in the presence of grasses (p = 0.02),

in particular Dactylis glomerata (p = 0.04) (Fig. 3a), and was positively affected by shoot (p = 0.02) and root (p = 0.01) biomass, as was also seen in the PCA (Fig. 2). Within the genus Pratylenchus at least 4 species were recognized (crenatus, fallax, pratensis and penetrans). However, it was not possible to separate these species in the suspension because this requires analysis in high magnification of specimens mounted on microscopic slides. For Pratylenchus, the correlation coefficient among plot pairs did not indicate any significant effect of treatment but the ANOVA showed an effect of plant community (Table 4). The abundance of Pratylenchus tended to be greatest in the presence of a grass, P. pratense (p = 0.04), but lowest in the presence of forbs (p = 0.03) (Fig. 3b). For example, no Pratylenchus was found under L. vulgare and only a few under Achillea millefolium. Both Tylenchorhynchus species were clearly affected by plant community composition (Table 4). A large number of Tylenchorhynchus dubius was found under legumes (p = 0.01), while root biomass (p < 0.0001) and presence of forbs (p = 0.02) covaried with

Table 3 Nematode community parameters: abundances of individual nematode taxa (mean of all treatments), coefficient of variation (CV = standard deviation/tot. mean), contribution to total number (percent); frequency of occurrence (percent of treatments). Taxon

Feeding categorya

Tot. mean (No./g dw)

CV

Contribution (%)

Occurrence (%)

Tylenchorhynchus dubius Tylenchorhynchus maximus Pratylenchus Paratylenchus Tylenchus Filenchus Boleodorus Ditylenchus Aphelenchus avenae Aphelenchoides Dorylaimellus Rhabditis s.l. Panagrolaimus Acrobeloides Acrobeles ciliatus Cephalobus persegnis Chiloplacus Cervidellus Eucephalobus mucronatus Eucephalobus oxyuroides Prismatolaimus Mesodorylaimus Aporcelaimidae

OPF OPF OPF OPF RH RH RH FF FF FF FF Rh-r Rh-r Rh-K Rh-K Rh-K Rh-K Rh-K Rh-K Rh-K AdBf Dor Dor

2.3 0.8 10.0 94.9 0.2 10.6 2.6 0.3 5.1 5.0 0.5 0.8 0.7 32.1 1.4 3.1 1.4 0.9 0.3 0.7 4.7 0.6 2.0

2.2 3.4 1.0 1.9 1.3 0.7 1.5 1.3 1.1 0.9 1.3 1.3 1.6 0.6 1.6 0.9 1.6 1.9 1.2 1.3 0.8 2.4 0.5

1.2 0.4 5.4 51.5 0.1 5.8 1.4 0.2 2.8 2.7 0.3 0.4 0.4 17.4 0.8 1.7 0.8 0.5 0.2 0.4 2.5 0.3 1.1

93.5 35.5 96.8 96.8 58.3 100 87.1 58.3 96.8 100 90.0 56.7 93.3 100 90.3 100 96.7 80.0 46.7 86.7 100 86.7 100

a OPF = obligate plant-feeders, RH = root-hair feeders, FF = fungal-feeders, Rh-r = Rhabditida r-strategic bacterial-feeders, Rh-K = Rhabditida K-strategic bacterial-feeders, AdBF = Adenophorea bacterial-feeders, and Dor = Dorylaimida omnivores/predators.

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Fig. 1. Relative composition (%) of nematode communities per treatment: obligate plant-feeders (OPF); root-hair feeders (RH); fungal-feeders (FF); Rhabditida r-strategic bacterial-feeders (Rh-r); Rhabditida K-strategic bacterial-feeders (Rh-K); Adenophorea bacterial-feeders (AdBF); Dorylaimida omnivores/predators (Dor). For treatment codes see Table 1.

low abundance of this nematode species (Fig. 3c). A large number of T. maximus was found only in the presence of the grass P. pratense (p < 0.0001) (Fig. 3d) and including this variable in the model made the community effect insignificant. The relative abundance of T. maximus, i.e. number/g biomass of P. pratense, was much greater in monocultures than when this grass species were grown together with other plant species (0.08–0.16 T. maximus/g dw harvested aboveground biomass of P. pratense in monoculture and 0.008–0.02 specimens/g dw P. pratense in plots with 4–12 plant species). Among the root-hair feeders, the genera Filenchus and Boleodorus were both affected by plant species composition (Table 4). Filenchus was the third most abundant taxon (Table 3), with the greatest abundance found in the monoculture with A. millefolium (Fig. 4a). The abundance varied more between mono-

Fig. 2. A principal component analysis (PCA) of nematode communities in 30 different plant communities in a long-term grassland plant diversity experiment; 0 plant species (open circles), 1 plant species (filled circles), 2 plant species (open triangles), 4 plant species (closed triangles), 8 plant species (open squares), and 12 plant species (filled squares). For treatment codes see Table 1. Only nematode taxa that significantly correlated with the principal components are shown (Pearson correlations, Sequential Bonferroni correction applied).

cultures than between treatments with a mixed plant species composition. In general, the abundance of Filenchus increased with root biomass (p < 0.0001) but decreased in the presence of legumes (p = 0.005), in particular T. hybridum (p = 0.02). Although the abundance of Boleodorus varied rather much among plots, there was a tendency for increased abundance in the presence of forbs (p = 0.098), and decreased abundance in the presence of grasses (p = 0.078) (Fig. 4b). None of the fungal-feeders was significantly affected by the treatments when correlation coefficient was considered but Aphelenchus appeared to be both affected by plant functional diversity and plant community in the ANOVA analyses (Table 4). Both of these effects seem to be caused by presence of forbs and grasses, forbs (p = 0.0005) increasing the abundance of Aphelenchus while grasses (p = 0.007) decreasing it. In particular, inclusion of the grasses Phalaris arundinacea and P. pratense and the forb A. millefolium made the effect of functional diversity insignificant. The greatest abundances of Aphelenchus were found under T. repens, A. millefolium, L. vulgare and the 2-species plot with L. vulgare and Lotus corniculatus (Fig. 4c). The abundance of Aphelenchoides only tended to be affected by plant community in the ANOVA analyses (Table 4). The greatest abundance was found under the grass D. glomerata, while the lowest abundances were found under legumes (Fig. 4d). Rather high abundances were also found under forbs and in plots without vegetation. The coefficient of variation (CV) of mean values was relatively large among the r-strategic rhabditids (c-p 1) (Table 3). Both Panagrolaimus and Rhabditis s.l. were affected by plant community composition (Table 4), and increased their abundances in the presence of legumes (p < 0.0001 and p = 0.0017, respectively). However, Rhabditis had its highest abundance under L. corniculatus while Panagrolaimus was most common in the monoculture of T. repens (Fig. 5a and b). For both genera, after including presence of legumes in the model the community effect was no longer significant. The K-strategic rhabditids (c-p 2-3) consisted mainly of cephalobids and were dominated by the genus Acrobeloides, which was the second most abundant nematode taxon and contributed 17% to the total number of nematodes (Table 3). Some of the K-strategic rhabditids were affected by plant composition (Table 4). The highest abundances of Acrobeloides were found under T. repens and Festuca ovina (Fig. 5c). In general, the abundance of Acrobeloides tended to be low in the presence of the forb Rumex acetosa (p = 0.048). The variation in abundance of Acrobeloides among monocultures

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Table 4 Correlation coefficients between plot pairs and results from analyses of variance of the effects of plant species richness (ln SR), functional richness (Fdiv), and plant community composition (Type I sum of squares: df, F-value, p-value). Significant p-values are indicated in bold. Taxon

p

ln SR

Fdiv

Community

0.8063 0.8262 0.4851 0.6469

<0.001 <0.001 n.s. <0.01

1, 0.06, 0.81 1, 0.00, 0.95 1, 0.19, 0.66 1, 2.01, 0.17

1, 0.28, 0.60 1, 1.53, 0.23 1, 1.58, 0.22 1, 1.74, 0.20

23, 5.13, <0.0001 23, 5.47, <0.0001 23, 7.95, <0.0001 23, 5.00, <0.0001

Root-hair feeders Tylenchus Filenchus Boleodorus

0.3625 0.6378 0.2195

n.s. <0.05 n.s.

1, 1.51, 0.23 1, 0.02, 0.88 1, 0.54, 0.47

1, 0.72, 0.40 1, 1.81, 0.19 1, 0.17, 0.68

23, 1.38, 0.21 23, 4.84, <0.0001 23, 2.10, 0.03

Fungal-feeders Ditylenchus Aphelenchus avenae Aphelenchoides Dorylaimellus

0.3524 0.4559 0.0326 0.1891

n.s. n.s. n.s. n.s.

1, 1.45, 0.24 1, 0.01, 0.94 1, 0.36, 0.56 1, 2.12, 0.16

1, 0.01, 0.93 1, 6.07, 0.02 1, 0.51, 0.48 1, 0.50, 0.48

23, 1.79, 0.07 23, 3.99, 0.0003 23, 1.78, 0.07 23, 1.59, 0.12

0.6254 0.6790 0.5005 0.1032 0.3573 0.7405 −0.0927 −0.1322 0.5163 −0.1189

<0.05 <0.01 <0.05 n.s. n.s. <0.01 n.s. n.s. <0.05 n.s.

1, 0.00, 1.00 1, 0.09, 0.77 1, 0.69, 0.41 1, 3.59, 0.07 1, 0.11, 0.74 1, 2.99, 0.10 1, 1.00, 0.33 1, 0.43, 0.52 1, 1.00, 0.33 1, 1.06, 0.31

1, 0.06, 0.81 1, 0.00, 0.96 1, 0.05, 0.82 1, 0.00, 0.97 1, 2.07, 0.16 1, 0.17, 0.69 1, 0.44, 0.51 1, 1.61, 0.22 1, 1.80, 0.19 1, 5.64, 0.03

23, 5.14, <0.0001 23, 3.02, 0.003 23, 2.44, 0.01 23, 1.34, 0.23 23, 1.17, 0.34 23, 3.11, 0.002 23, 1.10, 0.40 23, 0.74, 0.77 23, 3.48, 0.001 23, 0.58, 0.90

0.6593 0.1226

<0.01 n.s.

1, 3.54, 0.07 1, 6.04, 0.02

1, 0.08, 0.77 1, 0.23, 0.64

23, 2.66, 0.007 23, 1.25, 0.28

Obligate plant-feeders Tylenchorhynchus dubius Tylenchorhynchus maximus Pratylenchus Paratylenchus

Bacterial-feeders Rhabditis s.l. Panagrolaimus Acrobeloides Acrobeles ciliatus Cephalobus persegnis Chiloplacus Cervidellus Eucephalobus mucronatus Eucephalobus oxyuroides Prismatolaimus Omnivores/predators Mesodorylaimus Aporcelaimidae

Correl. coeff.

was larger than between treatments with mixed plant combinations. The highest abundances of Chiloplacus and Eucephalobus oxyuroides were both found under T. repens and the 2-species mixture of T. repens and T. hybridum (Figs. 5d and 6a). Their abundances were negatively related to shoot (both p = 0.01) and root biomass (both p = 0.005), and both genera were also negatively affected by presence of grasses (both p = 0.02), and Chiloplacus especially by presence of D. glomerata (p = 0.03) (Fig. 5d). The rest of the K-selected rhabditids did not vary much between treatments (Cephalobus persegnis and Eucephalobus mucronatus) or varied rather irregularly (Acrobeles ciliatus and Cervidellus) and therefore did not show a significant effect of plant composition (Table 4). The genus Prismatolaimus (Adenophorea bacterial-feeder) was affected by functional diversity (Table 4). This effect seems to depend upon grasses (p = 0.02) in general and P. pratense (p = 0.04) in particular (Fig. 6b). The omnivore Mesodorylaimus was affected by plant community (Table 4). The highest abundance of Mesodorylaimus was found under T. repens, but high abundances were also found under A. millefolium, L. corniculatus and the plots with no plants, i.e. plots with very low coverage (Fig. 6c). There was some negative effect of root biomass (p = 0.005), but there was still an effect of community after including this variable in the model. The family Aporcelaimidae was positively affected by plant species richness (Table 4), and this effect seems to be caused by legumes (p = 0.02) in general and T. repens (p = 0.004) in particular (Fig. 6d). There was also a positive effect of shoot biomass (p = 0.02), possibly due to the grasses D. glomerata (p = 0.06) and P. pratense (p = 0.08) that individually tended to have a positive effect, making the effect of plant species richness insignificant. 4. Discussion We generally found plant species composition to be more important for specific nematode taxa than any of the plant diver-

sity measures (Table 4). However, there were three instances where plant diversity also affected individual nematode taxa: both the fungal-feeding Aphelenchus and the bacterial-feeding Prismatolaimus were related to functional diversity of plants and the omnivorous Aporcelaimidae was positively related to plant species richness. The positive relationship between plant species richness and the abundance of the omnivorous family Aporcelaimidae could largely be explained by the presence of the legume T. repens. Since legumes and T. repens, as any other plant species, were more likely to be present in plots with more plant species, the most likely explanation for the increase in the abundance of Aporcelaimidae with plant species richness is the selection probability or sampling effect (Loreau and Hector, 2001; Wardle, 2002) whereby one or a particular set of species have a large effect on the variable of interest. However, other plant species also affected the relation between Aporcelaimidae and plant species richness, and thus legumes is not likely to be the only explanation. Primary production was certainly higher in the species rich plots as is indicated by the increasing plant biomass. As Aporcelaimidae probably is feeding high up in the food web as predators their higher abundances also indicate a higher secondary production in the species rich plots. The relationships of the fungal-feeding Aphelenchus and the bacterial-feeding Prismatolaimus with functional diversity seem to be dependent on the traits of forbs and grasses, and indicate an impact of the identity of single plant species and plant functional groups on nematode communities. All plant species are not equal, and the loss or addition of plant species with certain functional traits has impacts on different parts of the nematode community. Our results suggest that forbs stimulate the fungal community as there is a clear positive effect of this group of plants on an important fungal-feeding nematode (Fig. 2). This was also found in previous studies (Viketoft et al., 2005, 2009). To try to answer the question if it is enough to only study functional groups of nematodes, the present study can be com-

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Fig. 3. Abundances of plant-feeding nematodes: (a) Paratylenchus, (b) Pratylenchus, (c) Tylenchorhynchus dubius and (d) Tylenchorhynchus maximus in monoculture and multi-species plots in a grassland diversity experiment in northern Sweden. For treatment codes see Table 1.

pared with the study of Viketoft et al. (2009). In the latter study, obligate plant-feeders were found to respond positively to aboveground plant biomass and the presence of legumes. However, the present study show that instead most obligate plant-feeding taxa

responded positively to grasses and only T. dubius increased in abundance of legumes. In addition, Paratylenchus was the only taxon positively correlated to shoot biomass. The effect found of legumes on r-strategic Rhabditida (Viketoft et al., 2009) seems to

M. Viketoft, B. Sohlenius / Applied Soil Ecology 48 (2011) 174–184

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Fig. 4. Abundances of root-hair feeding nematodes, (a) Filenchus and (b) Boleodorus, and fungal-feeding nematodes, (c) Aphelenchus avenae and (d) Aphelenchoides, in monoculture and multi-species plots in a grassland diversity experiment in northern Sweden. For treatment codes see Table 1.

be consistent within the group as both Rhabditis and Panagrolaimus responded positively to the presence of legumes in the present study. Root-hair feeders as a group were found to respond positively to forbs and root biomass (Viketoft et al., 2009), but the present study show that the individual taxa responded differently as Boleodorus was affected by forbs while Filenchus was affected by root biomass. Finally, Dorylaimida omnivores/predators were

found to be affected by plant community but the reason for this effect could not be resolved (Viketoft et al., 2009). The very varying responses of Mesodorylaimus and Aporcelaimidae in the present study may give a hint why. Therefore, to fully understand the effect of the plant community on the nematode community there is a need to go further than just a division into nematode feeding groups.

0

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Block 1

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Fig. 5. Abundances of bacterial-feeding nematodes: (a) Rhabditis s.l., (b) Panagrolaimus, (c) Acrobeloides and (d) Chiloplacus in monoculture and multi-species plots in a grassland diversity experiment in northern Sweden. For treatment codes see Table 1.

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M. Viketoft, B. Sohlenius / Applied Soil Ecology 48 (2011) 174–184

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Fig. 6. Abundances of bacterial-feeding nematodes, (a) Eucephalobus oxyuroides and (b) Prismatolaimus, and omnivorous/predacious nematodes, (c) Mesodorylaimus and (d) Aporcelaimidae, in monoculture and multi-species plots in a grassland diversity experiment in northern Sweden. For treatment codes see Table 1.

It can be suggested that some nematode species are closely linked to specific plant species or groups of plant species, and that there occur monospecific relationships between plant species and nematodes. If there is strong specific coupling of nematodes to plant

species then there should be positive relationships between plant diversity and nematode diversity. The results from the present investigation rather indicate relatively loose coupling between plants and most nematode species. Therefore it is not surprising

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that only a weak association between number of plant species and number of nematode species was previously found (Viketoft et al., 2009). The strongest coupling in the present study was between T. maximus and the grass P. pratense. Although the relative loose coupling, we did find effects of plant species composition on the nematode fauna. The effect of individual plant species and plant species combinations on microbial production was apparently reflected in the composition of the nematode fauna, especially in the proportions of bacterial and fungal-feeding nematodes and among bacterial-feeders in the proportion of r-strategic species belonging to Rhabditidae and Panagrolaimidae. Differences in response towards plant species among various groups of nematodes were sometimes great. Thus plant-feeders were generally low under L. corniculatus, L. vulgare and A. millefolium, which were plant species where root-hair feeders and fungal-feeders occurred in rather high abundances. For plant-feeding nematodes we are actually dealing with measures of host-suitability as the question of resource size and quality may be factors that are relevant for the degree of coupling between plants and nematode species. For example, T. dubius were much more abundant than T. maximus and occurred predominately on a legume with higher nitrogen content (Fig. 3) despite the lower plant biomass of the legume (Table 1). Within certain nematode groups there were great differences between species indicating relatively specific environmental demands. In conclusion, our study revealed minor effects of plant and functional diversity on individual nematode taxa in this experimental grassland. The effect of plant composition was clearly more evident and occurred in all feeding groups. Taxa within a nematode feeding group could differ in their response indicating a need to go further than just a division into nematode feeding groups to fully understand plant community effects on the nematode fauna. Acknowledgements The study was funded through a PhD grant from Oscar and Lili Lamm’s foundation to Maria Viketoft (grant to Jan Bengtsson). We thank Jan Bengtsson for advice at various times during the study. References Anderson, J.M., 1995. Soil organisms as engineers: microscale modulation of macroscale processes. In: Jones, C.G., Lawton, J.H. (Eds.), Linking Species and Ecosystems. Chapman, Hall, New York, pp. 94–106. Bengtsson, J., 1998. Which species? What kind of diversity? Which ecosystem function? Some problems in studies of relations between biodiversity and ecosystem function. Appl. Soil Ecol. 10, 191–199.

Bongers, T., 1990. The maturity index: an ecological measure of environmental disturbance based on nematode species composition. Oecologica 83, 14–19. Coleman, D.C., Crossley Jr., D.A., Hendrix, P., 2004. Fundamentals of Soil Ecology, 2nd ed. Elsevier Academic Press, Boston, 386 p. De Deyn, G.B., Raaijmakers, C.E., van Ruijven, J., Berendse, F., van der Putten, W.H., 2004. Plant species identity and diversity effects on different trophic levels of nematodes in the soil food web. Oikos 106, 576–586. Hector, A., Schmid, B., Beierkuhnlein, C., Caldeira, M.C., Diemer, M., Dimitrakopoulos, P.G., Finn, J.A., Freitas, H., Giller, P.S., Good, J., Harris, R., Högberg, P., Huss-Danell, K., Joshi, J., Jumpponen, A., Körner, C., Leadley, P.W., Loreau, M., Minns, A., Mulder, C.P.H., O’Donovan, G., Otway, S.J., Pereira, J.S., Prinz, A., Read, D.J., Scherer-Lorenzen, M., Schulze, E.D., Siamantziouras, A.S.D., Spehn, E.M., Terry, A.C., Troumbis, A.Y., Woodward, F.I., Yachi, S., Lawton, J.H., 1999. Plant diversity and productivity experiments in European grasslands. Science 286, 1123–1127. Lindberg, N., Bengtsson, J., 2006. Recovery of forest soil fauna diversity and composition after repeated summer droughts. Oikos 114, 494–506. Loreau, M., Hector, A., 2001. Partitioning selection and complementarity in biodiversity experiments. Nature 412, 72–76. Mulder, C.P.H., Jumpponen, A., Högberg, P., Huss-Danell, K., 2002. How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities. Oecologia 133, 412–421. Porazinska, D.L., Bardgett, R.D., Blaauw, M.B., Hunt, H.W., Parsons, A.N., Seastedt, T.R., Wall, D.H., 2003. Relationships at the abovegroundbelowground interface: plants, soil biota, and soil processes. Ecol. Monogr. 73, 377–395. Setälä, H., Berg, M.P., Jones, T.H., 2005. Trophic structure and functional redundancy in soil communities. In: Bardgett, R.D., Usher, M.B., Hopkins, D.W. (Eds.), Biological Diversity and Function in Soils. Cambridge University Press, Cambridge, UK, pp. 236–249. Sohlenius, B., 2002. Influence of clear-cutting and forest age on the nematode fauna in a Swedish pine forest soil. Appl. Soil Ecol. 10, 261–277. Sohlenius, B., Boström, S., Viketoft, M., 2011. Effects of plant species and plant diversity on soil nematodes – a field experiment on grassland run for seven years. Nematology 13, 115–131. Spehn, E.M., Hector, A., Joshi, J., Scherer-Lorenzen, M., Schmid, B., Bazeley-White, E., Beierkuhnlein, C., Caldeira, M.C., Diemer, M., Dimitrakopoulos, P.G., Finn, J.A., Freitas, H., Giller, P.S., Good, J., Harris, R., Högberg, P., Huss-Danell, K., Jumpponen, A., Koricheva, J., Leadley, P.W., Loreau, M., Minns, A., Mulder, C.P.H., O’Donovan, G., Otway, S.J., Palmborg, C., Pereira, J.S., Pfisterer, A.B., Prinz, A., Read, D.J., Schulze, E.D., Siamantziouras, A.S.D., Terry, A.C., Troumbis, A.Y., Woodward, F.I., Yachi, S., Lawton, J.H., 2005. Ecosystem effects of biodiversity manipulations in European grasslands. Ecol. Monogr. 75, 37–63. Viketoft, M., Palmborg, C., Sohlenius, B., Huss-Danell, K., Bengtsson, J., 2005. Plant species effects on soil nematode communities in experimental grasslands. Appl. Soil Ecol. 30, 90–103. Viketoft, M., Bengtsson, J., Sohlenius, B., Berg, M.P., Petchey, O.L., Palmborg, C., Huss-Danell, K., 2009. Long-term effects of plant diversity and composition on soil nematode communities in model grasslands. Ecology 90, 90–99. Wardle, D.A., 2002. Communities and Ecosystems: Linking the Aboveground and Belowground Components. Princeton University Press, New Jersey. Wardle, D.A., Yeates, G.W., Williamson, W., Bonner, K.I., 2003. The response of a three trophic level soil food web to the identity and diversity of plant species and functional groups. Oikos 102, 45–56. Yeates, G.W., 2003. Nematodes as soil indicators: functional and biodiversity aspects. Biol. Fertil. Soils 37, 199–210.