Response of soil food-web structure to defoliation of different plant
species combinations in an experimental grassland community
J. Mikola
a,*, G.W. Yeates
b, D.A. Wardle
a, G.M. Barker
c, K.I. Bonner
aaLandcare Research, P.O. Box 69, Lincoln 8152, New Zealand bLandcare Research, Private Bag 11-052, Palmerston North, New Zealand
cLandcare Research, Private Bag 3127, Hamilton, New Zealand
Received 14 January 2000; received in revised form 23 May 2000; accepted 20 June 2000
Abstract
We established a greenhouse experiment based on replicated mini-ecosystems to evaluate the effects of defoliation of different plant species combinations on soil food-web structure in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata), were subjected to the following defoliation treatments: no defoliation of any species (control) and selective trimming of all possible one-, two- and three-way combinations of the species either to 27 cm height (weak defoliation) or to 15 cm height (strong defoliation) above the soil surface three times over a 10-week period. Successive defoliations removed the largest amounts of shoot mass from systems in whichT. repenswas included among the defoliated species becauseT. repensdominated aboveground plant biomass. At the ®nal harvest shoot mass was lowest in treatments that included defoliation ofT. repens, while total root mass was on average lower in strongly than in weakly defoliated systems and did not differ between the control and defoliation treatments. Total shoot production was not affected by defoliation. Microbial basal respiration and soil NO3-N concentration differed between the
combinations of defoliated species; e.g. microbial respiration was on average 32% lower in systems in which onlyL. perennewas defoliated than in systems in which onlyT. repenswas defoliated. Microbial biomass and soil NH4±N concentration were not signi®cantly affected by
defoliation treatments. Enchytraeid abundance differed signi®cantly between the combinations of defoliated species: in systems in which onlyL. perennewas defoliated enchytraeid abundance was on average 88% lower than in systems in which all species or onlyT. repenswere defoliated. Enchytraeid abundance was also positively associated with total defoliated shoot mass. Abundances of both bacterial-feeding and fungal-feeding nematodes were affected by the combination of defoliated species; e.g. the abundance of bacterial feeders was on average 52% lower in systems in which onlyT. repenswas defoliated than in systems in which bothP. lanceolataandT. repenswere defoliated. Fungal-feeding nematodes were also more numerous in strongly than in weakly defoliated systems and positively associated with total defoliated shoot mass. Herbivorous nematode abundance was not signi®cantly affected by defoliation treatments. The results show that the response of soil food webs to defoliation can be affected by which combination of species in a plant community is defoliated. Further, it seems that the role of the combination of species that are defoliated may for some components of the soil biota (e.g. fungal-feeding nematodes) be explicable simply in terms of the total mass of foliage removed. However, for other components of the soil biota (e.g. bacterial-feeding nematodes and enchytraeids) species-speci®c properties of different plant species in the combination of defoliated species are also clearly important, over and above simple mass removal effects of defoliation.q2001 Elsevier Science Ltd. All rights reserved.
Keywords: Decomposers; Defoliation; Grassland; Mini-ecosystem; Soil food web
1. Introduction
It is becoming increasingly appreciated that aboveground herbivory may substantially in¯uence the structure of belowground food webs and that soil food-web responses
need to be known in order to better understand the role of herbivory in determining community and ecosystem-level properties (Bardgett et al., 1998). One of the main processes linking aboveground grazers and soil systems is defoliation of plants, which is known to alter the proportion of resources that plants allocate to root growth and exudation (Bentley and Whittaker, 1979; Detling et al., 1979; Richards, 1984; Miller and Rose, 1992; Holland et al., 1996), which in turn is likely to affect nutrient uptake by plants (McNaughton and Chapin, 1985; Ruess, 1988). However, few experimental studies have investigated how
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* Corresponding author. Present address: Department of Biological and Environmental Science, University of JyvaÈskylaÈ, P.O.Box 35 (YAC), FIN-40351 JyvaÈskylaÈ, Finland. Tel.:1358-14-2604199;
fax:1358-14-2602321.
defoliation effects are propagated through soil food webs and those that have do not show consistent trends. The response of the soil microbial biomass to defoliation is positive in some studies (Holland, 1995; Mawdsley and Bardgett, 1997) and neutral in others (Wardle and Barker, 1997). Similarly, defoliation has been shown to have both positive and negative effects on the components of soil fauna (Stanton, 1983; Ingham and Detling, 1984; Seastedt et al., 1988).
Plant species differ in their resource allocation to shoot and root growth when defoliated (Bentley and Whittaker, 1979; Richards, 1984; Wilsey et al., 1997), and in their ability to sustain microbes and microbial-feeding nematodes in their rhizospheres (Grif®ths, 1990; Grif®ths et al., 1992; Wardle and Nicholson, 1996). Further, Mawdsley and Bardgett (1997) have shown that the magnitude of the response of soil microbes to defoliation may depend on the plant species defoliated. It is therefore reasonable to expect that the effect of defoliation on soil food webs in a multi-species plant community depends on which species, or combination of species, in the community is preferentially consumed and thus suffers the greatest defoliation.
To better understand the effects of defoliation on soil food webs in grasslands, and especially differential effects of defoliation of different plant species in the grassland community, we established a greenhouse experiment based on replicated grassland systems composed of mixtures of three plant species Ð white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.), and plantain (Plantago lanceolata L.). We defoliated different combinations of these species at two intensities with the aim of testing the extent to which the effect of defoliation on soil food-web structure depends on the combination of
defoliated species, and whether the combination of the species which are defoliated is able to in¯uence the effect of defoliation intensity on belowground systems.
2. Materials and methods
We performed a greenhouse experiment based on 90 experimental units; each unit consisted of a plant commu-nity established in a white plastic container (height 32 cm, bottom 21£21 cm; drainage holes in the bottom). Soil
(with a C content of 4.7%, a N content of 0.4%, and a pH of 6.0) was collected from a cropping ®eld at Lincoln, New Zealand, in late June 1998, passed through a 6-mm sieve, thoroughly mixed, and 2.20 kg (dry weight) added to each container (forming a 5 cm deep layer). No organisms were removed from or added to the soil prior to the experiment. In the greenhouse lighting was not controlled but followed natural photoperiod length (see Fig. 1). During winter months temperature was maintained above 108C, and in summer ventilation and fans were used to maintain tempera-ture close to outdoor values (see Fig. 1). Containers were placed on metal trays, and irrigated periodically by ®lling the trays with water.
For establishing plant communities, we chose three common grassland species and of these cultivars that have a relatively upright growth habit and a positive response to defoliation; i.e. white clover (Trifolium repens L. cv Grasslands Kopu), perennial ryegrass (Lolium perenne L. cv Grasslands Nui) and plantain (Plantago lanceolata L. cv Ceres Tonic) (see Barker et al. 1993; Hay and Newton 1996; Stewart, 1996). Plants were raised from seeds sown into vermiculite in plastic propagation trays, and at the age of nine weeks (beginning of week 1 in the experiment, mid July 1998) two seedlings of each species were planted into each container. These plants were allowed to grow for further 22 weeks before imposing the treatments. Contain-ers were weeded twice to remove unwanted plants, and before imposing the actual treatments plants were trimmed twice; at week 18 L. perenne and P. lanceolata were trimmed to15 cm above the soil surface to reduce competi-tive suppression of T. repens, and at week 28 all species were trimmed to 27 cm above the soil surface. Plants were regularly checked for fungal attack throughout the experi-ment, and spray applications of the fungicides triforine (as Saprol) and chlorothalonil1thiophanate (Taratek 5F) were made to plants in each container at weeks 12, 18, 19 and 37. Similarly, the insecticide pirimicarb (Pirimor 50) was applied as a spray to plants in each container at weeks 18, 37 and 39 in response to observed aphid infestations in the greenhouse.
Defoliation treatments were started at week 32 (mid December 1998): 15 treatments were set up at this time, i.e. a non-defoliated control and a factorial design of seven species defoliation treatments £ two trimming heights. The species defoliation treatments consisted of J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
trimming of each of the three species singly in the sward as well as all the possible two- and three-way species combi-nations, while the trimming heights were either 27 cm (later referred to as weak defoliation) or 15 cm (strong defolia-tion) above the soil surface. The combinations of defoliated species are subsequently referred to using the following letters: P for Plantago, L for Lolium and T for Trifolium (see Figs. 2±6). These defoliation treatments were imposed on the grassland community three times Ð at weeks 32, 36 and 40. Each treatment was replicated six times and the experiment arranged in a replicate block design; each of the six replicate blocks was positioned on a separate irrigation tray throughout the experiment. All live plant material cut from the defoliated plants was sorted into species, dried and weighed. Plant parts, such as leaves and ¯ower stems, were not separately weighed, and trimmed material was collectively referred to as shoot J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
Fig. 2. Total defoliated shoot mass (mean11 SE;n6in relation to defoliation treatments in a three-species grassland system. White and hatched bars represent systems that were defoliated 27 and 15 cm above soil surface, respectively, PPlantago lanceolatadefoliated, LLolium perennedefoliated, TTrifolium repensdefoliated, PLbothP. lanceo-lataandL. perennedefoliated, etc.
mass. At each of the three defoliation events, dead plant tissue was removed from the containers to reduce poten-tial fungal growth on plants. Consequently, all shoot mass and production data of plants are based on live
shoot mass collected at each of the three defoliation events and at the ®nal harvest.
Each container was destructively harvested at week 42 (late February 1999), two weeks after the ®nal defoliation J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
Fig. 4. Shoot production (mean11 SE;n6;consisting of both defoliated and harvested shoot material, in a three-species grassland system. Treatment labels and statistical symbols are as for Fig. 3.
was performed. All aboveground biomass was ®rst removed, sorted into species, dried and weighed. To esti-mate root mass, four subsamples of soil (totalling 430 g dry weight (d.w.) equivalent) were taken from each container; the roots were then extracted by washing, dried and weighed. Of soil fauna we determined the abundance of micro- and mesofauna; they were extracted from six subsamples of soil (totalling 110 g d.w.) using a variant of the tray method described by Yeates (1978). Total nema-todes, enchytraeids and rotifers were ®rst counted live at 40£ magni®cation before ®xing the suspension by the addition of an equal volume of boiling 8% formaldehyde. Subsequently an average of 120 nematodes per sample were identi®ed to nominal genus, typically using 400£ magni®-cation, and allocated to trophic groups according to Yeates et al. (1993). Finally, for microbial and inorganic N measurements, six subsamples of soil (totalling 270 g d.w.) were taken from each container and passed through a 4-mm mesh sieve prior to analysis. KCl-extractable NH4-N
and NO3-N concentrations of the soil were determined using
Technicon Autoanalysis. Microbial basal respiration and substrate-induced respiration (SIR, a relative measure of active microbial biomass) were determined as described by Wardle (1993), based on the approach by Anderson and Domsch (1978). Brie¯y, a 7.4 g (dry weight basis) subsample of sieved soil, with a moisture content adjusted to 35% (d. w. basis) was placed in a 169-ml sealed container and incubated at 228C. Basal respiration was determined as the total CO2-C released between one and four hours of
incubation, measured using infra-red gas analysis. SIR was measured in the same way, except that samples were amended with 10 mg glucose per g soil (wet weight) immediately prior to incubation.
Results were statistically analysed using the SPSS statis-tical package (SPSS, 1999). To determine which of the 14 different defoliation treatments produced effects that differed signi®cantly from the non-defoliated control, each of the treatments was compared against the control using Dunnett's test. Effects of the combination of defoliated species and defoliation intensity were further tested using a two-way Analysis of Variance (ANOVA; control was excluded from these tests). After ANOVA, differences between the means of the seven combinations of species defoliations were tested using the Student±Newman± Keuls (SNK) test at the signi®cance level of P0:05 If
the effect of the combination of defoliated species was statistically signi®cant, we used a regression analysis to determine whether total variation across experimental units for the dependent variable was signi®cantly explained by total defoliated shoot mass. Homogeneity of variances was checked using Levene's test (Dunnett's test and ANOVA) and residual plots (regression analysis), and to satisfy the assumption of homogeneity of variance, a loga-rithmic or a square-root transformation was applied to dependent variables whenever necessary.
3. Results
3.1. Plant properties
Trifolium repensdominated aboveground plant biomass and consequently defoliation removed the largest amounts of shoot mass from the systems in which T. repens was included among the defoliated species (Fig. 2). Relative to the control, shoot mass at the ®nal harvest was lower in those defoliation treatments, which included T. repens (Fig. 3a), and was on average higher in the weakly than in the strongly defoliated systems (Fig. 3a; Table 1). Shoot mass of T. repens was lower when defoliated than in the control and was not affected by defoliation ofL. perenneor P. lanceolata(Fig. 3b).Plantago lanceolatashoot mass was signi®cantly lower in the strongly defoliated P and PL treatments than in the control, and was also affected by the defoliation of other species: harvested P. lanceolata shoot mass was higher in the strongly defoliated T and LT treatments and marginally higher P,0:10in the weakly
defoliated L treatment than in the control (Fig. 3c).Lolium perenneshoot mass was lower in the strongly defoliated L, PL and LT treatments than in the control (Fig. 3d). Total root mass did not differ between the control and defoliation treatments (Fig. 3e), but was on average lower in strongly than in weakly defoliated systems (Fig. 3e; Table 1). Root mass appeared to be lower in systems whereT. repenswas defoliated than in those systems where it was not, although J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
the SNK-test could not locate signi®cant differences between treatment means (Fig. 3e, Table 1). Root mass was negatively associated with total defoliated shoot mass (regression analysis:R20:158;P,0:001:
Total shoot production did not differ between the control and defoliation treatments (Fig. 4a), and was not affected by the combination of defoliated species or by defoliation intensity (Fig. 4a; Table 1). Similarly, no differences were found between the control and defoliation treatments with regard to the shoot production ofT. repensandL. perenne, whereas the shoot production ofP. lanceolata was signi®-cantly higher in the strongly defoliated LT and PLT treat-ments than in the control and marginally higher P,0:10
in the strongly defoliated T treatment (Fig. 4b±d).
3.2. Soil microbial properties and inorganic N
No statistically signi®cant differences were found between the control and the defoliation treatments with regard to soil microbial properties or concentrations of inor-ganic N in soil, except that basal respiration was marginally higher P,0:10 in the intensely defoliated T treatment
than in the control (Fig. 5). However, there were statistically signi®cant differences between the combinations of defo-liated species with regard to basal respiration and NO3-N
concentration: basal respiration was on average lower in the
L treatment than in the T treatment, and NO3-N
concentra-tion was lower in the T treatment than in the P, PL and PLT treatments (Fig. 5a and d, Table 1). Neither basal respiration nor soil NO3-N concentration were signi®cantly affected by
total defoliated shoot mass (regression analyses:R20:02;
P0:272 andR
2
0:027;P0:135;respectively).
3.3. Soil fauna
Enchytraeid abundance was signi®cantly higher in the intensely defoliated T and PLT treatments than in the control (Fig. 6a), and on average higher in the T and PLT treatments than in the P and L treatments (Fig. 6a, Table 1). Enchytraeid abundance was also positively associated with total defoliated shoot mass (regression analysis: R20:157; P,0:001: Rotifer abundance did not differ
between control and defoliation treatments (Fig. 6b) and was not signi®cantly affected by the combination of defoliated species or defoliation intensity (Fig. 6b, Table 1). No statistically signi®cant differences between the control and the defoliation treatments were detected with regard to the abundance of nematode trophic groups, except that higher fungivore abundance was detected in the strongly defoliated PLT treatment than in the control (Fig. 7). Further, the ratio of abundance of fungal-feeding nematodes to that of bacterial-feeding nematodes did not J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
Table 1
ANOVA of the effects of the combination of defoliated species and defoliation intensity on plant and soil food-web properties (non-defoliated control excluded from analyses)
Source of variation Signi®cant differences
between combinations (according to SNK-test) Dependent variable Combination of
defoliated species
Defoliation intensity Combination £Intensity
F6,70a P F1, 70 P F6, 70 P
Plant properties
Total shoot mass 27.88 ,0.001 54.11 ,0.001 2.46 0.032 T, PT, LT, PLT,P, L, PLb
Total shoot production 1.41 0.221 2.35 0.130 0.57 0.751
Total root mass 2.42 0.035 7.83 0.007 0.82 0.560
Soil microbes and inorganic N
Basal microbial respiration 2.67 0.022 0.02 0.880 0.93 0.480 L,T
Substrate induced respiration 0.45 0.844 2.04 0.158 0.87 0.520
NO3-N concentration in soil 4.03 0.002 3.04 0.086 0.55 0.771 T,P, PL, PLT
NH4-N concentration in soil 1.34 0.252 0.93 0.338 0.41 0.869
Abundance of enchytraeids and rotifers
Enchytraeids 3.53 0.004 1.74 0.191 0.71 0.639 P, L,T, PLT
Rotifers 1.25 0.292 1.11 0.296 0.81 0.563
Abundance of nematode trophic groups
Bacterivores 2.39 0.037 0.61 0.437 1.58 0.165 T,PT
Fungivores 3.57 0.004 4.76 0.032 0.53 0.787 PLT.others
Fungivores to bacterivores ratio 1.44 0.212 1.95 0.167 0.61 0.720
Herbivores/unit soil mass 0.36 0.903 2.41 0.125 0.46 0.835
Herbivores/unit root mass 0.62 0.718 ,0.01 0.999 0.67 0.675
a Degrees of freedom of treatment and error.
differ between the control and the defoliation treatments (Fig. 7c). Abundances of both bacterivores and fungivores were, however, affected by the combination of defoliated species and defoliation intensity: bacterivores were on aver-age more numerous in the PT treatment than in the T treatment (Fig. 7a, Table 1), and fungivores were more numerous in strongly than in weakly defoliated systems and more abundant in the PLT treatment than in the other combinations of defoliated species (Fig. 7b, Table 1). Variance in bacterial-feeding nematode abundance was not explained by total defoliated shoot mass (regression analysis:R20:025; P0:153; whereas the abundance
of fungal-feeding nematodes and total defoliated shoot mass were positively associated (regression analysis: R20:105;P0:003:Herbivorous nematode abundance,
either per unit root mass or per unit soil mass, and the ratio of fungivore abundance to bacterivore abundance were not statistically signi®cantly affected by the
combination of defoliated species or defoliation intensity (Fig. 7c±e; Table 1).
4. Discussion
Our results show that the response of soil food webs to defoliation can be affected by which combination of species in a plant community is defoliated. In our case, the combi-nation of defoliated species appeared to have more in¯uence on the soil food web than did defoliation intensity, but it did not appear to modify the effect of defoliation intensity. Total defoliated shoot mass was not equal in the different combinations of defoliated species, and in many cases when a dependent variable was signi®cantly affected by the combination of defoliated species, variance in the dependent variable was also signi®cantly explained by total defoliated shoot mass. This means that the signi®cant J. Mikola et al. / Soil Biology & Biochemistry 33 (2001) 205±214
effect of the combination of defoliated species on some soil variables may be explicable in terms of total defoliated shoot mass.
Although shoot mass at harvest differed across defoliation treatments, defoliation did not affect total shoot production, a pattern which appears to be common for grassland species (Wilsey et al., 1997). Since root mass was on average lower in strongly than in weakly defoliated systems, plants increased allocation of resources to shoot growth when defoliation intensi®ed. Higher allocation to shoot than root growth after defoliation has been observed in several studies (Detling et al., 1979; Richards, 1984; Ruess, 1988; Polley and Detling, 1989; but see Milchunas and Lauenroth, 1993), and it appears to be typical for species that are adapted to intense but infrequent defoliation (Wilsey et al., 1997). Our results also show that the three plant species differed in their response to defoliation and that the response was partly determined by the level of the dominance of the species in the community. The least dominant species,P. lanceolata, bene®ted signi®cantly from the defoliation of the other species, which is apparent through the high shoot production of P. lanceolataeven in systems where it, in combination with the other species, was subjected to defoliation. In contrast, T. repens was only affected by direct defoliation and did not bene®t through the defoliation of other species. The overall activity and biomass of soil microbes were little affected by defoliation in our experiment; the only response to treatments was the lower basal respiration in the L treatment than in the T treatment. It has been suggested that increased root exudation and root mortality of defoliated plants could lead to the increased microbial activity and biomass observed in the rhizosphere of defoliated plants (Holland, 1995; Holland et al., 1996; Mawdsley and Bardgett, 1997). This would suggest that microbes were more active in the T treatment than in the L treatment because the greater amount of foliage that was removed from the T treatment induced higher root exudation and mortality in these systems. However, basal respiration was generally not signi®cantly affected by total defoliated shoot mass, which suggests that some species-speci®c property other than simply the amount of defoliated shoot mass determined microbial activity in our experiment. While on average more NO3-N tended to be available in
strongly than in weakly defoliated systems, less NO3-N
was found in the T treatment than in the P and PL treatments and total defoliated shoot mass did not signi®cantly affect NO3-N concentration in soil. This suggests that the
concen-tration of inorganic N in soil is not necessarily purely determined by total defoliated shoot mass but also depends on other species-speci®c properties of the defoliated species.
Highest abundances of microbi-detritivorous enchy-traeids were found in the T and PLT treatments, in which defoliation removed large amounts of shoot mass. As enchytraeid abundance was also positively associated with total defoliated shoot mass, defoliation in general appeared
to increase enchytraeid abundance in soil, possibly through higher root mortality in strongly defoliated systems. However, since enchytraeid abundance in the PT and LT treatments did not signi®cantly differ from that in the P, L and PL treatments, the response of enchytraeids to defolia-tion cannot solely be explained in terms of the net amount of plant mass removed, but other effects of the composition of defoliated species also appear to be important.
Bacterial-feeding faunae were in general only slightly affected by the treatments; no effects were detected for roti-fers and the only signi®cant effect for the bacterial-feeding nematodes was their lower abundance in the T treatment than in the PT treatment. Only one previous study exists in which the effects of arti®cial defoliation on soil bacter-ial-feeding faunae have been investigated; in that a negative response of bacterial-feeding nematodes to defoliation was found (Stanton, 1983). Our results suggest that the effect of defoliation on bacterial-feeding nematodes may depend on the plant species defoliated. In contrast to bacterial feeders, fungal-feeding nematodes clearly responded to defoliation in our study; they were more numerous in intensely than weakly defoliated systems, more abundant in the PLT treat-ment than in the other defoliation combinations, and also positively associated with total defoliated shoot mass. This shows that defoliation in general enhanced fungivore abun-dance in soil, possibly through higher root mortality and the subsequent enhanced growth of fungi. Some ®eld studies have shown that the ratio of fungi to bacteria (Bardgett et al., 1996; 1997) and the ratio of fungal-feeding nematodes to all soil nematodes (Freckman et al., 1979; Wall-Freckman and Huang, 1998) tend to be higher in grazed than in ungrazed grasslands, which implies that grazing may cause a shift between the bacterial-based and the fungal-based energy channels (see Moore and Hunt, 1988). In our experiment the ratio of fungivorous to bacterivorous nema-todes was not affected by defoliation treatments, which implies that defoliation per se may not cause a shift between the two energy channels.
In contrast to previous studies, in which defoliation and animal grazing have often been shown to increase abun-dances of soil herbivores either per unit soil or root mass (Smolik and Dodd, 1983; Stanton, 1983; Ingham and Detling, 1984; Yeates, 1976; Seastedt et al., 1988; Merrill et al., 1994; but see Leetham and Milchunas, 1985; Wall-Freckman and Huang, 1998), total abundance of herbivor-ous nematodes was not statistically signi®cantly affected by defoliation in our study. It may therefore be that intense defoliation of all species of the plant community is needed to produce signi®cant changes in the abundance of root-feeders.
belowground system independently of the mass of material that was defoliated; for example, microbial activity, inor-ganic N and abundance of bacterial-feeding nematodes were all affected by the combination of defoliated species but not by total defoliated shoot mass. In contrast, the response of fungal-feeding nematode abundance to the defoliation of different species combinations appeared to be mostly explained by its response to the total amount of plant biomass removed by defoliation. Enchytraeid abundance appeared to be affected by both total defoliated mass and by species combination effects over and above this. Effects of species combinations on community and ecosystem response variables have usually been found to be idiosyn-cratic and dif®cult to predict based on the effects of single species; studies which have involved mixing litter from different plant species (Blair et al., 1990; Wardle et al., 1997; Nilsson et al., 1999), combining soil faunal species (Faber and Verhoef, 1991; Mikola and SetaÈlaÈ, 1998), and growing plant species in mixtures (Wardle and Nicholson, 1996) have all proved to have idiosyncratic effects on soil variables. Whether defoliating different combinations of plant species will also bring about idiosyncratic responses in the belowground system over and above the effects of mass of foliage removed, as our study suggests, needs to be further tested in communities in which shoot mass is more evenly distributed across species.
Acknowledgements
This work was supported by a grant from the New Zealand Marsden fund to D.A.W. and G.M.B.
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