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Eventheinteriorofweatherablemineralsmight

becomeexploited byectomycorhizalhyphae. Thin

sectionmicrographsoffeldsparsandhornblendes

fromconiferousforestsoilshowopentunnelswith

roundedends, curvedtracksandadiameterof

310 m(Refs 32,34). Thetunnelswithaconstantdiameterdiffermorphologicallyfromthe

crystallographicallyorientedetchpitsandsaw-tooth

cracksthatwouldresultfromchemicalweathering35

(Box 4). Thepresenceofhyphaeinsidethetunnels

indicatesthatsinglehyphaereachtheinteriorofthe

tunneledweatherableminerals32,34,36.Exudationof

organicacidsatthehyphaltipswouldfreeessential

cationssuchasK+, Ca2+ andMg2+ fromthemineral

interior32 andcould beresponsibleforformationof

thetunnels32,34,36. Subsequenttransportofdissolved

cationsthroughtheectomycorrhizalmycelium

towardsthehosttreerootscould benefittree

growth37. Ithas beenshownthatisotopicallylabeled

Mgcan betranslocatedoveradistanceofseveral

centimeterstoectomycorrhizalroots, improvingthe

Mgnutritionofthehostplant37. Somefeldspars

containapatiteinclusions32,38 andectomycorrhizal

hyphaemightaccesstheseenclosedP sources

throughtunnelgrowth, therebyexploitinga

mineralPsourceunavailabletoplantroots.

SelectivedissolutionofCa-richinclusionsinVOLCANIC GLASS has beenascribedtoacidexcretion

byinvadingplantsymbioticfungalhyphae36 and

bacteriaingroundwatersystemsselectively

colonizeandweatherP-richfeldsparswhenP is

inshortsupply38.

Implications of ectomycorrhizalweathe ring

Theimplicationsofectomycorrhizalfungalmineral

weatheringaremanifold.

First, itredefinesourtraditionalviewontherole

ofectomycorrhizalfungiinrootnutrientuptake. The

effectofmineralnutrientMOBILIZATION by

ectomycorrhizalfungiaddstotheestablished

quantitativeeffectsonuptakeandtranslocationof

dissolvedelementsfromthesoilsolutionandthe

TRENDSin Ecology & Evolution Vo l.16 No.5 May 2001

http://tre e.trends.com

251Review

As well as the quantit ati ve effects on p lan t

nutrien t uptake (increase in uptake surface and

exp loited soil vo lume),the ectomycorrh iza l

fungus inf luences the uptake of p lan t nutrientsin two qua lit ative ways:

(F ig. Ia) Via enzyme producti on ,the

ectomy corrhiza l fungus can utilize organic

nitrogen (N) and phosphoro us (P) forms,

which would otherwise remain large ly

unava ilab le to roots. Nutrien t mobiliza tion

from am ino acids , pep tides, proteins , am ino

sugars,chitin and nucleic acids has been

showna,toge ther with transfer of N and P

into the host plan ta,b. Direct hyphal absorption

of am ino acids and simple peptide s can

also occur a.

(F ig. Ib) The ectomycorrh iza l fungus canmobil ize P, potass ium (K),calci um (Ca) and

magnesium (Mg) from so lid m ineral substra tes

through organ ic acid excre tionc,d,e.In addit ion,

tunnels in weatherable m inerals enable

ectomycorrh iza l hyphae to reach the inter ior of

the m inerals and access P from a pa tit e

inclus ions . Essential nutrien ts be come

ava ilab le to the host plant via the

ectomycorrh iza l myceliumc. Ana logous to

the ir organ ic nutrien t mob ili zing capab ilities ,

the abilities of d ifferent ectomycorrh iza l fung i

to mobil ize inorganic nu trien ts m ight be

sp ecies speci fic f.

References

a Chalot, M. and Brun,A. (1998)Physiology of

organicnitrogenacquisition by ectomycorrhizal

fungiandectomycorrhizas. FEMSMicrobiol. Rev.

22, 2144

Box 3.Nutrientmob ilizationbyectomycorrhizalfungi

I

b Antibus, R.K. etal. (1997)Rootsurface

phosphataseactivitiesanduptakeof32P-

labelledinositolphosphateinfield-collected

gray birchandredmapleroots. Mycorrhiza

7, 3946c Wallander, H. (2000)UptakeofP from

apatite byPinussylvestrisseedlings

colonized bydifferentectomycorrhizal

fungi. PlantSoil218, 249256

d Wallander, H.andWickman, T. (1999)Biotite

andmicroclineaspotassiumsourcesin

ectomycorrhizalandnon-mycorrhizalPinus

sylvestrisseedlings. Mycorrhiza9, 2532

e Wallander,H. (2000)Use ofstrontium

isotopesandfoliarKcontentto estimate

weatheringofbiotite induced bypineseedlingscolonised by ectomycorrhizal

fungifromtwodifferentsoils.PlantSoil

222, 215229

f Lapeyrie,F. etal. (1991)Phosphate-

solubilizingactivityofectomycorrhizal

fungiinvitro. Can. J. Bot. 69, 342346


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morerecentlystudied qualitativeeffectsof

enzymaticreleaseofNandP fromorganicsources16

(Box 3). Inadditiontoproducingalargesurfacearea

incontactwiththesoil, ectomycorrhizalhyphaecan

activelydissolvetheirmineralsurroundingsand

releaseessentialplantnutrients(Box 3). The

selectivecolonizationandweathering byectomycorrhizalhyphaeofK-, Mg-orCa-rich

feldsparscouldspecificallyimproveK, MgorCa

nutritionoftrees.

Second, ectomycorrhizalhyphaecanaccess

enclosednutrientsourcesinmineralsthatare

unavailabletoroots.Apatiteinclusionsinfeldspars31

areunavailabletoroots butcould beexploited by

hyphae. Organicacidexcretioninthetunnelswill

acceleratefurtherdisintegrationofthemineralsand

resultinenlargedweatherablesurfaceareas,

accessibletorootsaswellasfungi. Rootswillpromote

mineralweathering, butjustastheireffectivityin

nutrientuptakeisrestricted bytheirlimiteddistributioncomparedtofungalmycelium, their

effectivityasweatheringagentswillalso be

restricted.

Third, excretionofoxalate byectomycorrhizal

fungicouldreduceeffectsofsoilacidificationonforest

productivity.Atpresent, acidificationofconiferous

forestsoils, becauseofanthropogenicatmosphericinputs, iswidespread39. Thisacidificationhas been

accompanied bychangesinthe basecations:Alratio,

resultinginhighconcentrationsofAl3+ inthe bulksoil

solution, whichcouldhamperrootuptakeofCa2+ and

Mg2+ andinhibitrootelongation. InresponsetoAl

stress, plantrootsmightsecreteoxalicacidand

detoxifyAl3+ byformationofAloxalatecomplexes40.

WhenexposedtoelevatedAl, mycorrhizalpine

seedlings, aswellasnonmycorrhizalpineseedlings,

showenhancedoxalicacidproduction29. Inadditionto

formationofAloxalatecomplexes, ectomycorrhizal

fungicouldalleviateAlstresstothetrees by

mobilizing basecations(Ca2+ andMg2+)frommicrositesinaccessibletothetreeroots32.

Afourthimplicationinvolvessoil-formingprocesses

andtheirbiogeochemicalconsequences. Takinginto

accountthatectomycorrhizalassociationsare

widespreadandwereestablishedatleast50 Mya41,

mobilizationofmineralelementsbyectomycorrhizal

hyphaeandectomycorrhizalroottipshasprobably

contributedtosoilformationandglobalelementcycling

viaeffectsonmineralweatheringandPODZOLIZATION32.

Prospects

Inmost borealforestsystems, Nisthegrowth-

limitingnutrientforthestandingtrees39,42,43.Nitrogenlimitationimpairsthesynthesisofamino

TRENDSin Ecology & Evolution Vo l.16 No.5 May 2001

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252 Review

Fig. 1. Hyphae linking

plan ts to m inerals. A

thin-section of (a) a

cross-sectioned

ectom ycorrh iza l ro ottip

illus tra tes how (h)

ectom ycorrh iza l hyphae,

emanating from (fm) the

fungal ma ntle aroun d (r)

a root, en close (m)

m ineral pa rticles from the

ad jacen t so il . The direct

con tact be tween the

hyphae and the m inera l

surface is revealed on (b)

a SEM p icture of

branching hyphae tha t

cover and penetra te a

m ineral pa rticle. Scale

bars = 50 m and 10 m

respecti ve ly.

Box 4. Formationofetchpits

Chem ical wea ther ing of m inera ls leads to formation

of regular ly arranged, and regular ly shaped, angu lar

cav ities , so-called etch p its. Etch pits form because

m inera ls disso lve preferentially at and along

crystallographically de ter m ined dislocations and

planes. Saw-tooth cracks eventua lly form when side-

by-side aligned etch pits coalescea (F ig. Ia, from lefttorigh t).

Th in-secti on m icrographs in cross-polar ized

ligh t show ing (Fig. Ib) a che m ically weathered

fe ldspar w ith partly coa lesced e tch pits. By con trast,

a thin-section m icrograph (Fig.Ic) of a tunne led

fe ldspar shows a more or less irregular pattern of

open,tub ular pores, 3-10 m in w idth tha tcriss-cross

the inter ior of the m ineral. Scale bars = 100 m for

Fig.Ib and Ic.

Reference

a Berner, R.A. andHoldren, G.R. (1979)Mechanismof

feldsparsweathering. II. Observationsoffeldsparsfromsoils.

Geochim. Cosmochim. Acta43, 11731185

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