Functional Ecology

2003

17

, 516–525

© 2003 British Ecological Society

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Blackwell Publishing Ltd.

Observational and simulated evidence of ecological shifts within the soil nematode community of agroecosystems under conventional and organic farming

CH. MULDER,† D. DE ZWART, H. J. VAN WIJNEN, A. J. SCHOUTEN and A. M. BREURE

Laboratory for Ecological Risk Assessment, RIVM – PO Box 1, NL-3720 BA Bilthoven, the Netherlands

Summary

1.

Soil sustainability implies a sufficient diversity and abundance of organisms to performsoil functions and to resist environmental stress. Previous studies have shown theimportance of functional biodiversity for soil organisms.

2.

Soil samples have been collected within the framework of a long-term monitoringprogramme in the Netherlands. Nematological and microbiological techniques werecombined to facilitate a more comprehensive understanding of possible below-groundeffects of land management.

3.

A possible bias due to stochastic circumstances was investigated. The Mantel testshowed that the diversity at species level is largely related to air temperature, but at genuslevel the effect of temperature disappears. No direct influence of rainfall on the soilbiodiversity was found in our model.

4.

To extrapolate our data to a national level, habitat–response relationships for soilorganisms have been derived. Generalized linear models (GLMs) and Monte Carlosimulation allowed the estimation of the probability of occurrence at a given abundancefor 95 nematode genera.

5.

Our study describes the influence of abiotic conditions and land use intensity on thecomposition of nematode communities in grasslands on sand. The results obtainedreveal a major influence of pH and livestock density on the diversity of the nematodecommunity at both taxonomic levels as well as at different trophic levels (feeding habits).The presence and abundance of soil nematodes decrease with cattle pressure.

6.

Functional diversity decreases with increasing management intensity. It is shownthat the Shannon diversities of bacterial feeding nematodes and fungal feeding nema-todes are strictly related to cattle pressure, whereas the bacterial biomass occurring underorganic farming scores higher than in other farming systems.

Key-words

: Bacterial biomass, functional diversity, intensive farming system, livestock density, predator–preyinteractions

Functional Ecology

(2003)

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, 516–525

Introduction

Since Roman times, European civilization has mani-pulated and domesticated grazing animals successfully(Vera 2000). However, short-term economic gains forfarmers have increasingly affected the stocking capa-city of large areas and caused, among others, a declinein soil organic matter (Dormaar & Willms 1990) anda reduction in the amount of litter (Naeth

et al

. 1991).In combination with increased nutrient availability(DiTommaso & Aarssen 1989; Bobbink, Hornung &Roelofs 1998), the abuse of temperate grasslands, both

above and below ground, has affected the natural resil-ience of many soil ecosystems. Several efforts have beenmade to address the impact of the selective pressure ofgrazing herbivores on nutrient availability and vegeta-tion (e.g. macroherbivory and plant succession byVan Wijnen, Van der Wal & Bakker 1999; and microher-bivory and plant competition by Verschoor

et al

. 2002).Nevertheless, the indirect effects of herbivory on soil eco-systems remain uncertain. Therefore, field studies to obtaininsights into the relations between soil biodiversity andecosystem functioning are high research priorities(Brussaard

et al

. 1997; Wall, Snelgrove & Covich 2001).Interactions between vegetation and soil microfauna

are extremely complicated, making the progress ofbelow-ground investigations on multiple trophic levels

†Author to whom correspondence should be addressed. E-mail:christian.mulder@rivm.nl

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slow. For instance, rough estimates of the soil nematodecommunity composition were regarded as sufficientfor most nature management and policy processes.Especially in grasslands, nematodes have been foundto play an important role in the transfer of energy andmatter through the soil food web because of theircentral and diverse trophic positions (Hunt

et al

. 1987;Dawson, Grayston & Paterson 2000; Hunt & Wall2002). Different functional groups can be distinguishedwithin the nematode community (Beare

et al

. 1992;Yeates & Bongers 1999): nematodes may belong to theprimary consumer group (plant feeders), the second-ary consumer group (bacterivores and fungivores), orthe tertiary consumer group (predators and omnivores).

Soil organisms are responsible for a suite of soilprocesses, such as decomposition of organic matterand nutrient cycling (Moore & De Ruiter 1991; Wardle& Giller 1996). These soil processes are regarded asmajor components in the global cycling of materials,energy and nutrients in ecosystems (Scheu & Setälä2002; Wardle 2002). For example, in many agroecosys-tems the upper soil biomass (25 cm top-soil layer)processes over 50 000 kg of fresh organic material ha

−

1

year

−

1

(Bloem & Breure 2003). This processing includesthe decomposition of organic matter by microorganismsas well as consumption and production within the foodweb. From the viewpoint of functional biodiversity,the determination of the extent of these processes is asyet unknown. Unfortunately, soil processes fluctuateso strongly in time and space that direct measurementsmay not be effective. Therefore, we aim to derive suchfunctional information from the species composi-tion (occurrence and abundance) in different soils. Herewe present the first results of this research. The objec-tives of the present study at 70 Dutch dairy farms withgrasslands on sandy soils are:

1.

to infer whether a significant correlation existsbetween farming systems and nematode commun-ities, corrected for seasonal fluctuations in the soilecosystem;

2.

to determine the habitat–response relationship ofthe soil nematodes in managed grasslands in orderto assess the impact of livestock on below-groundbiodiversity;

3.

to estimate the environmental pressure due to dif-ferent farming systems on nematode and bacterialcommunities in order to assess the resilience of thesoil ecosystem;

4.

to analyse any cascade effect across trophic levelsin the soil food web in order to provide empiricalevidence for sustainable land use.

Materials and methods

Based on the infrastructure of the Dutch Soil QualityNetwork (DSQN), a monitoring programme was

designed to obtain information regarding (a)biotic soilstatus and trends for decision-making and develop-ment policies. Established in 1993, the DSQN covers asampling programme representing 70% of the surfacearea of the Netherlands. A complete sampling ‘round’takes 5 successive years and comprises 200 locations.Farms constitute the basic sampling units and com-posite soil samples are collected across the entire farm.The sampling design comprised 20 farms acting aspseudo-replicates of each land use and soil type. Thefirst round of measurements and sampling of grass-lands on sand (conventional and intensive farmingsystems) took place in 1993 and 1994, whereas the sec-ond round was performed in 1999 and 2000. Organicfarms were sampled in 2000 only. The bacterial biomasswas measured during the second phase (1999/2000).The majority of the investigated sites is rural and mostlocations are no-tillage farms with pH

(KCl)

around 5.Each farm has been characterized according to farm-ing system and the livestock units (where one livestockunit is given as the amount of cattle, pigs and poultryexcreting an average of 41 kg N ha

−

1

year

−

1

). The inves-tigated farming systems can be described as follows:

1.

Certified organic farming (mixed biodynamic),using compost/farmyard manure and no use ofbiocides, averaging 60 ha and 1·7 livestock units.

2.

Conventional farming, using mineral fertilizers, amuch smaller amount of farmyard manure, averaging45 ha and 2·4 livestock units.

3.

Semi-intensive farming, using both organic andmineral fertilizers, averaging 25 ha and 3·2 livestockunits.

4.

Intensive farming, using biocides and fertilizers,averaging 20 ha and 5·1 livestock units.

On each site 320 soil cores (diameter 2·3 cm

×

10 cm)were collected randomly across the farm. A compositesample was obtained by mixing the different cores in aplastic container, and approximately 500 g of soil wascollected in glass jars. The sample treatment followedindependent analyses for plant available nutrients(C, N and P), pH

(KCl)

values, soil nematodes and micro-organisms. Nematodes were extracted, within 1 week,from 100 g of wet soil using the funnel elutriation methodcomplemented with sieving and cottonwool extraction(Oostenbrink 1960; modified). Two clean suspensionsin 10 ml water were screened with a stereoscope tocount the individuals. Permanent mounts in formalde-hyde on mass slides contained about 150 individualsthat were identified by light microscope at 400–600magnification, following the keys of Bongers (1988).Taxa identified were further assigned to functionalgroups according to their life strategies (Bongers 1990)and feeding habits (Yeates

et al

. 1993). The Shannon–Wiener Index of generic diversity was calculated independently for five nematode feeding

′ = −H p pi i i * )Σ ln (

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habits (

p

i

is the proportion of each of the

i

taxa belong-ing to one given feeding habit). The biomass of thesedifferent functional levels was estimated from measure-ments of specimens grown in litterbags (A. J. Schouten,unpublished data; cf. Bradford

et al

. 2002).All samples for microbiological analyses were stored

for 1 month at a temperature of 12

°

C and 50% waterholding capacity. Measurements of soil respirationrequired long incubations under standardized condi-tions to eliminate the influence of short-term localfluctuations. Respiration can be measured as basalactivity, where the CO

2

evolution under standardizedconditions in the laboratory is measured withoutaddition of substrates, or as substrate-induced, wherea glucose substrate is added to soil and utilized bymicroorganisms. The increased CO

2

evolution in thefirst hours, compared with an unamended control, is ameasure of the (responsive) microbial biomass. Thetotal amount of bacterial cells was determined by fluo-rescent staining, direct microscopy and image analysis(Bloem, Veninga & Shepherd 1995). The carbon con-tent of the microbial biomass (

C

mic

) was taken to be48·5% dry mass (cf. Cohen

et al

. 1980).

Air temperature, rainfall and soil humidity fluctuatestrongly from day to day. Thus, air temperatures and/orrainfall may influence the occurrence and abundanceof (certain) nematode taxa during the sampling period.The Royal Netherlands Meteorological Institute(KNMI, De Bilt) supplied the daily air temperaturesand precipitation (minima, maxima and averages forthe latest 28 days before sampling) near each invest-igated farm. For purposes of calculation, the calendaryear was taken to begin on 1 March instead of 1 January.Julian Days (Julian Weeks) refer to integer numberscorresponding to the absolute count of whole days(weeks) that have elapsed each year since 1 March. Toavoid confusion, this term has been merely chosen as aproxy to consolidate data for phenological analyses,and should not be seen as a conversion of the samplingdays in either the Julian or the Gregorian Calendars.

A partial Mantel analysis has been carried out toassess whether the association between sampling periodand habitat–response is stronger than could result fromchance (algorithm based on Douglas & Endler 1982).This preliminary approach is intended to yield anincreased understanding of the actual relationshipsbetween soil nematodes and their environmentalpatterns, but a more elaborate statistical analysisis needed. We decided to use the relative Euclideandistances in the one-tail evaluation to infer possiblerelationships between phenological characters andenvironmental characters. Under the

null

hypothesis(

H

0

= 0, no difference from random), the observedMantel’s value is expected to fall near the mode of therandomized matrix. We used the same set of entities (95Julian Days) in a matrix describing the nematode

community structure (95 genera, matrix Y), a matrixof ecological distances computed for the land usevariables (livestock density, soil acidity and plantavailable phosphorus in the matrix X), and a matrix ofclimatological distances (rainfall and temperature in thematrix W).

The nematode abundance (AB

i

) of the

i

th taxon wasanalysed with stepwise regression of a generalizedlinear model (McCullagh & Nelder 1989) as follows:

ln(AB

i

) =

a

+

c

1

LSU +

c

2

pH +

c

3

P

2

O

5

+

d

1

(LSU)

2

+

d

2

(pH)

2

+

d

3

(P

2

O

5

)

2

eqn 1

This formula allows us to relate the abundance AB foreach nematode genus to the environmental predictorslivestock units (LSU), soil reaction (pH) and plantavailable phosphorus (P

2

O

5

), while ignoring theircross-products. The reliability of these predictor termshas been further increased by transforming AB to adensity score between 0 and 1. It is assumed in theGLM that the error distribution is Poisson distributed(Mulder & De Zwart 2003).

Results

Several farms were investigated over 4 years, startingon 1 March 1993. The organic farms were sampled in1999, and the others were sampled twice (during 1993and 1999, or during 1994 and 2000). A careful assess-ment of the background noise (natural variation) thatcan be ascribed to fluctuating microclimate was requiredto extend the validity of our data beyond any chanceeffect of weather before and during the sampling period.Although air temperature increased almost steadilyfrom 1 March to June, reaching a maximum in latesummer (Fig. 1, primary

y

-axis), the detectable bio-diversity at genus level (secondary

y

-axis) fluctuated lessthan at species level (Fig. 2a). The weekly consolidatedspecies diversity of soil nematodes approached, in fact,a Gaussian distribution over the 4 years with a remarkablemaximum in May and June. In contrast, no trends canbe recognized in the random distribution of the nutri-ents other than the expected increase of P-availabilitywith the intensity of land management (Fig. 2b).

We investigated the influence of stochastic circum-stances (climate fluctuations and field sampling condi-tions) using partial correlation analysis and path analysis.These concerned the influence of air temperature andprecipitation during the 1, 2, 3 and 4 weeks beforefield sampling on the distribution of the results. TheMantel test (

t

-distribution with 95 degrees of freedom)points to contrasting trends. In fact, it shows a positiveassociation between the soil biodiversity matrix andthe air temperature matrix (

t

= 0·86,

P

= 0·39), buta negative association between the soil biodiversitymatrix and the rainfall matrix (

t

=

−

0·47,

P

= 0·64).These correlation coefficients state that air temper-ature directly affected the soil biodiversity (as expected,owing to increased activity and catchability of soil

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Ecological shifts within the soil nematode community

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Fig. 1. Temperature ( ) vs detected nematode diversity (�) during the sampling period (1993, 1994, 1999 and 2000). Averagedaily air temperature on the primary y-axis (highly significant trendline, n = 95). Average nematode diversity at genus level onthe secondary y-axis.

Fig. 2. Weekly fluctuations of nematode species diversity (a) and plant available P (mean mg P2O5 l−1) for each farming system

(b) during the growing season across the investigated years. A trendline (highest bars) is provided only for total nematode species(= colonizers, intermediate + persisters). The number of species that can be ascribed to r-like strategists is more darkly shaded in (a).Diamonds ( ) show the plant P-availability in the study area (b). The lines represent the means of the four farming systems.

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organisms), but the rainfall affected the sampling resultsonly indirectly (lower sampling efforts under worseweather conditions). Although significant, the effect oftemperature on soil biodiversity was of opposite signduring March (negative partial correlation) and April(positive partial correlation). There was no relationbetween the average temperature of | 1–7 | or | 1–14 | daysbefore the sampling data and the nematode occur-rence. Trendlines across the average air temperaturesof | 1–21 | and | 1–28 | days before the sampling have thesame degree of significance (

r

2

= 0·83). Figure 1 showsthat the effects of temperature on the nematode diver-sity at genus level were not significant over the 4 years.This seems in contrast with the well-known correlationof other organisms – such as microarthropods – withair temperature. Mites sampled show, however, a cor-relation between the air temperature during | 1–21 | daysbefore sampling and their abundance (

r

2

= 0·84 witha time shift of 10 days in 1999), but not with theirbiodiversity (Ch. Mulder, data not shown).

The GLM shows that the probability of occurrenceof a given genus can be seen as a cumulative functionof independent predictors, such as soil acidity, plantavailable nutrients and the amount of livestock units.

Five types of trend can be distinguished: none (a com-mon outcome in the phosphorus model, as the simulatednutrient availabilities are neither uniform nor continu-ous), linear positive or negative, optimum quadraticcurve with

x

2

< 0, and quadratic curve with

x

2

> 0(cf. Klepper

et al

. 1999). As linear trends and quad-ratic optimum curves are common results, we focusedonly on genera with an explained deviation higher than60%. In Fig. 3, all bacterial-feeders (Bf) belongingto the subclass Secernentea show an obvious optimumwithin the observed pH-range, while only the Alaimi-dae (Enoplida, Adenophorea) show a wide ecologicalamplitude.

Other trophic groups show either a steep lineardescendent trend (

Epidorylaimus

) or a log-logistic ascen-dant trend (

Aporcelaimus

), or suggest a minimum(

Xiphinema

). Twenty taxa including plant pathogenslike

Pratylenchus

and bacterial-feeding cosmopolitangenera such as

Acrobeles

,

Acrobeloides

,

Cephalobus

,

Eucephalobus

,

Panagrolaimus

and

Plectus

show nosignificant pH-response at all (modelled fluctuationrange < 0·01). A much lower number of nematode taxa(11 genera) were not sensitive to livestock density (cattlepressure can be seen as a proxy for grazing, tramplingand P-enrichment), including

Achromadora

(feedingon unicellular eucaryotes, including diatoms, fungalspores and yeasts), again the bacterivores

Acrobeles

,

Acrobeloides

,

Cephalobus

and

Eucephalobus

, and theplant-feeding genus

Meloidogyne

. In contrast withthe previous pH-related trends, Fig. 4 shows mainlysigmoidal-like decreasing trends. Only two genera showincreasing trends (the bacterivore

Chiloplacus

acrossthe whole gradient and the carnivore/omnivore

Thonus

under semi-intensive and intensive management). Thesteepest trendline in the probability of occurrence at agiven abundance (‘density score’) is shown by theTeratocephalidae (i.e.

Metateratocephalus

and

Terato-cephalus

), a taxon that appears extremely sensitive tolivestock density at the boundary between organic/conventional and intensive farming (Fig. 4, approxim-ately 2·8 livestock units).

Bongers (1990) proposed the Maturity Index (MI),based on the life histories of local nematode commu-nities, as a reliable tool to assess the stability of the soilecosystem. However, the calculated MI (1·07 < MI < 2·49)shows no significant relationships with either livestockdensity or pH in our grasslands. In contrast, the Shannon–Wiener diversity index calculated for bacterial +hyphal-feeding habits provides a subtle assessment atfunctional level (Fig. 5). A further direct comparisonbetween the biomass of the bacterial community and theabundance of bacterial-feeding soil nematodes (Fig. 6)helps to explain the shifts between the

H

′ values ofbacterial-feeding and hyphal-feeding nematodes. Thebacterial biomass was highest in organic farms, and lowestat the intensively managed farms (Fig. 7). If we assignthe average of the bacterial biomass from organic farmsas 100%, conventional, semi-intensive and intensivelymanaged farms differ 5, 17 and 19%, respectively, from

Fig. 3. Generalized linear model of the pH predictor for a selection of soil nematodes.Independent density scores for the expected abundance of each taxon on the y-axis.Genera allocated to feeding habits according to Yeates et al. (1993): bacterial-feeders(Bf), hyphal-feeders (Hf), plant-feeders (Pf), carnivores (Ca) and omnivores (Om).The taxa are ranked from Cuticularia to Xiphinema according to their life strategy, fromcolonizers (r-strategy) to persisters (K-strategy) according to Bongers (1990).

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the organic farming system. The Cmic standard deviationwas the highest in organic farms, as can be seen by thescattered plots at the left side of Fig. 6 and in the SD-bars of Fig. 7.

Discussion

Most efforts made to study the impact of the loss of plantdiversity on productivity of plant communities arefocused on above-ground effects in the field and understrictly controlled conditions (Tilman, Wedin & Knops1996; Lawton et al. 1998; Wardle 2002). Yet, a possiblestrong legacy of land use history has to be kept in mind.Although the cattle pressures on grasslands which wereinvestigated on two occasions show an average reduc-tion of almost one livestock unit between the first andthe second round (∆2000−1993 = −0·85 ha−1 year−1, aver-age for all farmlands within 7 years), below-groundeffects on the nematode community that can still beascribed to the land management remain evident inour GLM curves. We show some consistent habitat–responses, especially within the bacterial-, fungal- andplant-feeding genera. It is likely that these GLM curvesare related not only to changes in nutrient cycling anddiversity of cryptobiota, but also to a combination ofbiotic and abiotic factors such as changing land use,biodiversity enhancement and succession. Like otherorganisms, soil nematodes can be ranked along a gra-dient referring to their reproductive strategies, fromlarger (persistent) K-strategists adapted to stable environ-ments because of their long life cycles towards colon-izing r-strategists that respond quickly to favourableconditions (Bongers 1990, 1999; Ferris, Bongers & DeGoede 2001). The fast-reproducing bacterial-feedersare mostly assumed to respond secondarily to any vari-ation in the input of detritus with immediate changesin their abundance (Moore & Hunt 1988; Moore & DeRuiter 1993), whereas the primary consumers (plant-feeding nematodes) will mainly respond to shifts inthe (above-ground) primary production (cf. Korthalset al. 2001). Proportions of plant-feeding nematodesshowed contrasting responses to changes in the land

Fig. 5. Frequency distribution of the Shannon–Wiener diversity index (x-axis) for the hyphal-feeding group (primary y-axis)and the bacterial-feeding group (secondary y-axis) at genus level (clusters of 025 units).

Fig. 4. Generalized linear model of the livestock predictor for a selection of soilnematodes. Nematode density scores on the y-axis. Feeding habits according to Yeateset al. (1993), ranked as in the previous figure (eight bacterial, two hyphal, threeplant-feeding and five carnivore + omnivore taxa plotted). Further explanation inthe text.

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management and the host plants. For instance, the plant-feeding genus Xiphinema, known for its sensitivity toallelopathic plants (Anaya 1999), suggests in Fig. 3an induced distribution related to the occurrenceof the Yorkshire-fog grass (Holcus lanatus). In soilscovered by this grass, the fungal-based energy channel oflitter decomposition may, in fact, play an importantrole under low pH conditions due to a combination ofhigh concentrations of water-soluble phenolics andmycorrhizal fungi (cf. Kidd & Proctor 2000; Mulder2001). Therefore, at the population level, primary con-sumers such as Xiphinema reflect a combination ofindirect effects of pH on their host plants (cf. Inghamet al. 1985; Yeates 1987) and direct effects on their hostplants (mutual exclusion/competition of plant spe-cies). A further selective pressure by herbivores seemsecologically unlikely for a Xiphinema–Holcus mutualinteraction (<6% fluctuation along the entire livestockgradient).

On the other hand, the GLM model for livestockdensity (Fig. 4) confirms the shifts among bacterial-feeding genera noted by Ferris, Venette & Lau (1996) insoils under organic and conventional management.However, the modelled insensitivity of cephalobid nema-todes such as Acrobeles, Acrobeloides and Eucephalobusto pH and farming systems (not plotted) seems in con-trast with the negative response of these species tocompost mulch (Porazinska et al. 1999) and their pos-itive correlation with bacterial phospholipid fatty acid(PLFA) analysis (Yeates et al. 1997). Possible explana-tions for these contrasting results are: a sensitiveresponse to vegetation manipulation (Korthals et al.2001), a high sensitivity to bulk density (Yeates, Dando& Shepherd 2002), a quick but ephemeral reaction tomulch (Bulluck, Barker & Ristaino 2002), or randomdistribution of organic amendments in the larger grass-lands, regardless of the livestock density (cf. standarddeviation of organic farms in Fig. 7). Considering thedifferent extent of the dairy farms investigated (see Studyarea), it seems ecologically plausible that an independ-ent combination of these processes may occur in thefour management systems. This is not unlikely, sinceomnivorous (and carnivorous also) genera (althoughwith delayed responses to environmental changes)show a high sensitivity to soil acidification and live-stock density. Yet not all Cephalobidae behave in auniform manner. Although most of these nematodesare known to be predominant under improved pas-ture/organic management (Yeates 2003), Fig. 4 shows arather unexpected correlation of Chiloplacus withheavily grazed sites. Moreover, the parallel trends ofChiloplacus and Thonus (the only other genus posi-tively related to livestock density) mirror the alreadysuggested link between cephalobid preys and predatorsdiscussed by Yeates & Wardle (1996). As observationalevidence for effective control of bacterial grazingis still lacking (Scheu & Setälä 2002), the increasingoccurrence of these two soil nematodes in ‘stressed’environments is a promising sign of a propagationof predatory effects under different grass-dominatedagroecosystems.

When the influence of weather conditions was mini-mized by using a lower taxonomic resolution, the actualimportance of life strategy and food sources for nema-todes became more evident. With respect to other trophicgroups (colonizing) bacterial-feeding nematodes areexpected to exhibit a highly significant response totemperature (Schouten, Van Esbroek & Alkemade 1998).Our results support the close relationship between theabundance of bacterial-feeding nematodes and the sizeof microbial populations suggested by Bååth, Lundgren& Söderström (1981) and Bulluck et al. (2002). TheShannon–Wiener diversity index for the genera belong-ing to these two feeding habits shows remarkable dif-ferences among the four farming systems (Fig. 5). Usually,

Fig. 6. Scatterplot of the average bacterial biomass (food base, primary y-axis, �) andthe bacterial-feeding nematode biomass (secondary consumers, secondary y-axis, �)along an increasing grazing intensity gradient (livestock units as in Fig. 4). Theexponential trendline is fitted on clusters of 05 livestock unit width (n = 9, α = 0·05;vertical error bars for 5% Cmic).

Fig. 7. Average bacterial biomass and standard deviation (SD) for the investigatedfarming systems. ORG = organic, CONV = conventional, SEMI-INT = semi-intensive,INT = intensive farming.

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values of this index for a community with high resil-ience lie above 1·5, such as our H ′ values for bacterial-feeding nematodes in organic (100%), conventional(90%) and semi-intensive systems (75%). Half of theintensively managed grasslands lie below 1·5 (Fig. 5).

A general comparison between the calculatedShannon–Wiener indices of the decomposer guildspoints to different responses of bacterivorous and fungi-vorous nematodes to factors associated with above-ground grazing intensity. Hyphal-feeding nematodespresent, as expected, much lower H′ values (regardlessof the farming system), suggesting a high vulnerabilityof this feeding habit. Pesticides are commonly used inintensively managed farming systems: next to the useof acaricides, the application of the fungicide pro-piconazole is widespread in the Netherlands (D. DeZwart, unpublished data). As the application of pest-icides detrimentally affects prey species in the decomposercommunity (Scheu 2001; Cowgill et al. 2002) andamong the mycorrhizal symbionts (Elmholt 1991; Baymanet al. 2002), biocides can contribute to explaining ourlow H′ values in (semi)-intensive grasslands (cf.Schweiger & Jakobsen 1998). In fact, 75% of the hyphal-feeders occurring below (semi)-intensive farms haveH ′ = 0. At least for hyphal-feeders, this H ′ value impliesalmost no resilience within this feeding habit.

Two different key processes run in parallel withincreasing grazing intensity. On one hand, the bacte-rial biomass of all our grasslands on sand is decreasedwith heavily grazed sites (Fig. 7) and the bacterialgrowth rate is quite low in comparison with othergrasslands (<20 pmol thymidine h−1 measured byBloem & Breure 2003 in marine clay). The decreaseof bacterial biomass suggests a bottom-up control,whose effects result in a 50% decline in the abundance ofbacterial-feeding nematodes along the livestock gradient(Fig. 6). On the other hand, hyphal-feeding nematodes(Fig. 5) show a much lower resilience to intensive far-ming than bacterial-feeders owing to the much lowernumber of taxa and abundance of individuals. Fungiand bacteria often show contrasting trends in theliterature, as bacterial communities seem to be out-competed by mycorrhizal fungi (Scheu & Setälä 2002).Mäder et al. (2002) show a significant inverse relation-ship between the metabolic quotient of the microbialcommunity (expressed as qCO2 = soil basal respira-tion/soil microbial biomass) and farming pressure,invoking a greater energy efficiency of the more diversemicrobial community. The Nematode Channel Ratio[NCR = Bf/(Bf + Hf)] is helpful to express the energyefficiency in soil decomposition processes (Moore &Hunt 1988; Yeates 2003). The NCR-index allows theallocation to independent energy pathways of the con-tribution of bacterial-feeding nematodes (Bf) and/orfungal-feeding nematodes (Hf) to the total soil nema-tode abundance (NCR narrows 1 in totally bacterial-mediated processes). In our study there is a strong andsignificant decrease of the fungal-mediated process inthe following order: organic (NCR = 0·721) < conven-

tional (NCR = 0·845) < semi-intensive (NCR = 0·887)≤ intensive system (NCR = 0·898). Regardless of thefarming system, the NCR index shows a strong posi-tive correlation with the soil acidity of the investigatedgrasslands (r = 0·26, α < 0·01, n = 116).

It is our opinion that the reduction of mycorrhizalfungi in nutrient-rich open environments (Mulder,Breure & Joosten 2003) and the lower ability ofmycorrhizal-infected plants to tolerate macroherbivory(Hartnett & Wilson 2002) imply a succession to non-mycorrhizal grasses in intensively managed grasslands(cf. Bardgett et al. 2001). The consistent results of thestudies cited above and our own, support the hypo-thesis that both the bacterial and the fungal communitiesare negatively related to fertilized, not nutrient-limitedagroecosystems. Under these conditions, effects oftrampling by cattle become important for bacterial-feeding nematodes, since trampling affects soil aerationand moisture content (and consequently mineraliza-tion rate according to Breland & Hansen 1996) andstrongly reduces the mobility of waterfilm-relatedbacterial-feeders (Yeates, Dando & Shepherd 2002).Bacterial-feeders responded consistently with a muchhigher proportion in organic grasslands with lowlivestock density (cf. Wardle et al. 1999).

Conclusions

Agroecosystems in the Netherlands are usually heavilyfertilized. Management practices such as high stock-ing density cause shifts in functional groups of soilmicroorganisms and may affect the nutrient dynamicsof the soil. Ecological modelling suggests that a strong,selective, human-induced pressure is acting on mosttaxa, indicating decreased ecosystem resilience (lowerbiodiversity within functional groups) as a result ofincreased management intensity. It is evident that manytaxa are endangered as even cosmopolitan, unspecial-ized nematodes such as the Teratocephalidae (Secer-nentea) appear suppressed under intensive management.In organic farming systems, Cmic and bacterial-feedingnematodes such as Metateratocephalus and Terato-cephalus show, in fact, a positive influence of manuringon microflora and bacterivorous nematodes. Theseeffects are comparable with those previously describedby McSorley & Frederick (1999) and Neher (1999).Considering the habitat–response relationship of otherbacterial-feeding Secernentea, such as Heterocephalo-bus (characterized by a very narrow optimum in thepH–response curve of Fig. 3), our approach may helpto enable the recognition of cascade effects across thefood web. Such taxa can become reliable bioindicatorsof soil quality and sustainability owing to their greatsensitivity to environmental stress. The use of theShannon diversity for both bacterial- and hyphal-feedingnematodes is strongly recommended as an indicator ofchanges associated with sustainable agriculture underconventional and organic management regimes. Thedifferent rates of population decrease within these

524C. Mulder et al.

© 2003 British Ecological Society, Functional Ecology, 17, 516–525

trophic groups were sufficiently robust to encompassfour farming systems and weather fluctuations.

Acknowledgements

The authors would like to thank Jaap Bloem, An Vosand Meint Veninga (Alterra, Wageningen) for themicrobiological measurements, Nick Van Eekeren (LouisBolk Institute, Zeist) for information on organic farmsfrom the ‘BIOVEEM’ project, Diana Wall (NaturalResource Ecology laboratory, Colorado) for encourag-ing comments, and Tom Aldenberg (RIVM, Bilthoven)for statistical advice. Tom Bongers (Wageningen Uni-versity and Research Centre) and Gregor W. Yeates(Landcare Research, Palmerston North) are gratefullyacknowledged.

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Received 14 January 2003; revised 18 March 2003; accepted27 March 2003