Nature © Macmillan Publishers Ltd 1998

8Key species groups that affect major eco-

logical processes are vital componentsof community diversity. Many such keygroups are found in the soil, including themycorrhizal fungi that may connect plantsinto a functional “wood-wide web”1. Arbus-cular mycorrhizal associations are formedby fungi of the order Glomales with 90% ofland plant families, and many arbuscularmycorrhizal fungi are thought to have abroad host range2. Here we show that,despite this broad host range, the divers-ity of arbuscular mycorrhizal fungi is strik-ingly low in arable sites compared with awoodland.

The arbuscular mycorrhizal fungi thatcolonize roots cannot be reliably identifiedbelow the genus level except by molecularmethods. We examined roots from fiveabundant woodland plant species at foursites within a broadleaved wood dominatedby oak (Quercus petraea, colonized by ecto-

mycorrhizal fungi), and sycamore (Acerpseudoplatanus, colonized by arbuscularmycorrhizal fungi), at Castle Howard,North Yorkshire, UK. Partial fungal small-subunit ribosomal RNA sequences wereamplified, cloned and screened for differ-ences in restriction pattern by restriction-fragment length polymorphism (RFLP). Wesequenced selected clones to determinetheir phylogenetic position (Fig. 1). Forcomparison, we sampled pea, maize andwheat crops on three farms within a 55-kmradius of the woodland site.

There are three families in the Glomales,represented in our samples by the generaGlomus, Acaulospora and Scutellospora, andthese are readily distinguished as distinctsequence clusters (Fig. 1). Morphologicalstudies confirmed that these three generawere present in these roots3. In arable sites,92% of sequences represented Glomusmosseae or closely related taxa, whereas

those from woodland were much morediverse (Fig. 1). This was true even thoughthe arable sites were separated by up to66 km and three host species were sampled.The combined woodland samples had amuch higher diversity of RFLP types (Shan-non–Weiner H40.144) than the combinedarable samples (H40.398). In both woodand field, we often obtained identicalsequences from different plant species, sug-gesting that the broad host range exhibitedby many cultured arbuscular mycorrhizalfungi2 may also be realized in nature.

Given the broad host range of somearbuscular mycorrhizal fungal taxa, thechange in sequence composition and lowdiversity of the fungi in arable fields isprobably not a result of plant monocultureper se, but reflects other aspects of the agro-nomic regime such as ploughing, fertiliza-tion or fungicide application. In all thearable fields, regardless of host plant orlocation, the dominant arbuscular mycor-rhizal fungal type was a putative G. mosseaenot found in the woodland. This speciessporulates abundantly and colonizes readilyfrom spores, which may be more importantin a field that is ploughed annually than inwoodland4.

Arbuscular mycorrhizal fungi differwidely in their biological properties, andpresumably have several different roles inecosystems5. The low taxonomic diversity ofarbuscular mycorrhizal fungi in arablefields indicates that their functional contri-bution may be less there than in woodland.It has been suggested that low ecosystemdiversity may be associated with impairedfunction6,7 and reliability8,9. Our resultsshow that microbes need to be consideredin any assessment of the effects of agricul-ture on biological diversity and that inten-sive arable agriculture may be operating atminimum levels of diversity for at least onekey functional group.T. Helgason, T. J. Daniell, R. Husband,A. H. Fitter, J. P. W. YoungBiology Department, University of York,PO Box 373, York YO10 5YW, UKe-mail: th7@york.ac.uk

1. Simard, S. W. et al. Nature 388, 579–582 (1997).

2. Smith, S. E. & Read, D. J. Mycorrhizal Symbiosis (Academic, San

Diego, 1996).

3. Merryweather, J. & Fitter, A. H. New Phytol. 138, 117–129

(1998).

4. Clapp, J. P., Young, J. P. W., Merryweather, J. W. & Fitter, A.H.

New Phytol. 130, 259–265 (1995).

5. Newsham, K. K., Fitter, A. H. & Watkinson, A. R. J. Ecol. 83,

991–1000 (1995).

6. Chapin, F. S. et al. Science 277, 500–504 (1997).

7. Tilman, D. et al. Science 277, 1300–1302 (1997).

8. McGrady-Steed, J., Harris, P. M. & Morin, P. J. Nature 390,

162–165 (1997).

9. Naeem, S. & Li, S. B. Nature 390, 507–509 (1997).

10.Simon, L., Lalonde, M. & Bruns, T. D. Appl. Environ. Biol. 58,

291–295 (1992).

NATURE | VOL 394 | 30 JULY 1998 431

Ploughing up the wood-wide web?scientific correspondence

FFiigguurree 11 Neighbour-joiningphylogeny of arbuscularmycorrhizal fungal DNAsequences amplified fromplant roots sampled atwoodland (green letters)and arable (red letters)sites. Fungal sequencesare identified by the hostplant from which theywere isolated. A, Ajuga; H,Hyacinthoides; E, Epilobi-um; G, Glechoma; R,Rubus (from the woodlandsite, sampled in July 1996).P, Pisum and Te, Triticumfrom Escrick; Th, Triticumfrom High Mowthorpe; Z,Zea from Bedale (arablesites, sampled at threetime points during 1997).Orange branches corre-spond to the arbuscularmycorrhizal fungus familyGlomaceae, purplebranches to Gigaspo-raceae, yellow branches toAcaulosporaceae, anddark blue branches to thetwo taxa that do not clus-ter with any of these fami-lies. Partial small-subunitribosomal DNA fragments of about 550 base pairs were amplified using Pfu DNA polymerase and primersNS31 (ref. 10) and AM1 (58-GTT TCC CGT AAG GCG CCG AA-38, designed to amplify fungal and excludeplant DNA sequences). Cloned products were digested with Hinf I and AluI and selected samplessequenced. Named sequences are from GenBank or from library cultures sequenced in our laboratory. Boot-strap values of ¤90% are shown at the nodes. RFLP types defined for analysis are shown on the right (Glo,Glomus; Acau, Acaulospora; Scut, Scutellospora), along with the number of clones of each type found inwoodland (green numbers) and arable land (red numbers). Previously unpublished sequences have beendeposited in GenBank under accession numbers AF074340–AF074373.

Total

Geosiphon

S. pellucida

1000

0.01

AGl. etunicatum

997

AZ

GH

AGTe

Z1000

RUYG108 (Strain

from woodland)

990

ThP

999

Gl.mosseaeZPGl.mosseae

993

1000

AH

1000

Gl.vesiculiferumGl.intraradices

GG

977

PGl.versiforme1000

S.dipapillosa

Gi.giganteaGi.margarita

936

S.dipurpur.H

H

999

1000

GH

1000

HH

1000

Entrophospora sp. A.laevis

A.spinosaA A.rugosa

992

HHRA.scrobiculata

A959

995

932

Acau1

Glo6

Glo4

Glo2

Glo7

Glo1

Glo8

Glo3

Glo9

Glo5

Acau2

Acau4

Acau3

Scut1

Glo10

Wo

od

A

rable

11

1

18

3

1

92

16

5

2

15 1

3

4

1

89 1

154 100