Mycorrhizal Dependence of Andropogon gerardii and ...

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Mycorrhizal Dependence of Andropogon gerardii and Schizachyrium scoparium in Two Prairie Soils Author(s): R. C. Anderson, B. A. D. Hetrick and G. W. T. Wilson Source: American Midland Naturalist, Vol. 132, No. 2 (Oct., 1994), pp. 366-376 Published by: The University of Notre Dame Stable URL: http://www.jstor.org/stable/2426592 . Accessed: 05/02/2014 09:49 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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Am. Midl. Nat. 132:366-376

Mycorrhizal Dependence of Andropogon gerardii and Schizachyrium scoparium in Two Prairie Soils R. C. ANDERSON DepartmentofBiology,IllinoisState University, Normal61790

B. A. D. HETRICK AND G. W. T. WILSON DepartmentofPlant Pathology, Kansas State University, Manhattan 66506 ABSTRACT.-Previous researchin tallgrassprairiein Kansas indicatedthatwarm-season,C4, grasses are obligate mycotrophsand do not grow normallyin the absence of mycorrhizal symbiosis.However,the degree to whichsuch grassesdepend on mycorrhizaein otherprairie soils has not been examined. Growthand mycorrhizalcolonizationof roots of Andropogon gerardiiand Schizachyrium scopariumwere compared in soil collected fromKonza Prairie Research Natural Area (KPRNA), RileyCounty,Kansas and fromSand Ridge State Forest (SRSF), Mason County,Illinois.Plantsof both species were grownin the twosoils and were inoculatedwithGlomusetunicatum sporesoriginallycollectedfromKPRNAor colonized root pieces fromS. scopariumplants collected fromSRSF. Glomusetunicatuminoculum resulted in significantly greater root colonization and biomass of both plant species in steamed KPRNA soil than did root piece inoculum. There was no benefitfrominoculationin nonsterilesoil whichcontainedindigenousmycorrhizal fungi.In SRSF soil,therewas no response to inoculationwithmycorrhizalfungifromeithersource. The lack of mycorrhizalresponse in SRSF soil is attributedto the greaterplant-availableP level of thissoil. For S. scoparium grownin SRSF soil, plantsgrownin steamed soil produced more biomass than plantsgrown in steamed soil amended withnonsterilesoil sievings(containingsoil organismsother than mycorrhizalfungi),or in nonsterilesoil. These differencescould be due to competitionfor inorganic nutrientsbetween soil microbes and the plant or antagonisticrelationshipsbetweenthe plant or the mycorrhizalassociationand the soil microbes.Thus, the mycorrhizal dependence of these plant species is related to both soil and inoculum typeor species. INTRODUCTION

The importance of vesicular-arbuscular mycorrhizae (VAM) to plant growth and survival has been demonstrated under varied conditions in both laboratory and field studies (Allen, 1991; Brundrett, 1991). The beneficial effects of VAM mycorrhizae to plant growth are usually related to enhanced availability of inorganic nutrients to plants (Allen, 1991; Brundrett, 1991). Uptake and translocation of water to host plants by mycorrhizal fungi has also been confirmed (Allen and Boosalis, 1983; Faber et al., 1991). The degree to which plants rely on mycorrhizal symbiosis for acquisition of inorganic nutrients or water is usually a function of the architecture of the plant species' root system. (Baylis, 1970; Warner and Mosse, 1982; Reinhardt and Miller, 1990) and the availability of inorganic nutrients in soil (Jasper et al., 1979; Hays et al., 1982; Anderson and Liberta, 1992). Specificity between a host plant and a mycorrhizal symbiont can also modify the degree to which a plant species will benefit from mycorrhizal symbiosis (Bevege and Bowen, 1975; Lambert et al., 1980; Wilson et al., 1988; Chanway et al., 1991). Thus, among mycotrophic plants there is a continuum of mycorrhizal dependency. Plants with extensively branched, fibrous root systems with abundant root hairs generally rely less on mycorrhizal symbiosis since they are able to obtain nutrients from soil largely without assistance (facultative mycotrophs). In contrast, many plant species with coarse root systems cannot complete their life cycle in nutrient-limitedsoils without the symbiosis and are referred to as obligate mycotrophs (Jan366


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os, 1980; Crowelland Boerner,1988). Because the plant host is the onlycarbon source for the VAM fungalsymbiont,thereis a metaboliccost to the plant associatedwithsupportof the symbiosis.Consequently,under conditionsof high nutrientavailabilityin soil, if the symbiosisis not suppressedby the host plant,biomass production,flowering,and survivorship of mycorrhizalplants may be lower than plants grown under the same conditions withoutmycorrhizae(Koide, 1991; Anderson and Liberta,1989, 1992). Since grasseshave fibrousroot systems,we anticipatedthat cultivatedC3 and nativeC4 grasses of the tallgrassprairie would have limited dependence on mycorrhizalsymbiosis (Hetrick et al., 1988). However,using native mycorrhizalfungi and soil fromthe Konza Prairie Research Natural Area (KPRNA) near Manhattan,Kansas, all C4 grassestested to date have been obligatemycotrophs(Hetricketal., 1986; Hetricketal., 1988; Wilson et al., 1988; Hetrick et al., 1990). Despite low plant-availableP (usually less than 10 mg kg-'), (Bentivengaand Hetrick,1991). tallgrassprairiesin thisregion have high productivity In contrast,no dependence on mycorrhizaehas been demonstratedfor (Andropogon scoparium(Michx.) Nash) grownwithVAM inoculum gerardiiVitm.) and (Schizachyrium and soil fromSand Ridge State Forest (SRSF), Mason County,Illinois (Bentivenga,1988; Seo et al., 1988; Andersonand Liberta,1989, 1992; Meredithand Anderson,1992). In fact, when these plant species were grownin steamed SRSF soil withoutmycorrhizae,theirbiomass generallyexceeded that of plants grown in nonsterilesoil where mycorrhizalfungi were present.This enhanced growthof nonmycorrhizalplants occurred even though the differencein availabilityof inorganicnutrientsbetween treatedsoils and nontreatedsoils was small (Seo et al., 1988; Anderson and Liberta, 1989, 1992; Meredith and Anderson, 1992). Plant-availablesoil P levelsof the SRSF werehigherthanthoseof the KPRNA,(Bentivenga and Hetrick,1991; Meredithand Anderson,1992). Therefore,it was unclear whetherthe mycorrhizaldependence of these plant species differedin the two soils due to differences in phosphorus availability, or whetherother microbialfactors(e.g., pathogens) were responsiblefor the reduced growthof these plant species in nonsterileSRSF soil. To clarify dependence of twotallgrassprairiegrasses theseapparentcontradictionsin the mycorrhizal when grownin SRSF and KPRNA soil, a studywas conducted which testedthe mycorrhizal dependency of A. gerardiiand S. scopariumin the two soils and withthe funginative to both sites. MATERLALS AND METHODS

Soil collectionand description.-Soil(0-15 cm deep) was collected fromthe rhizosphere of at least ten A. scopariumplantsat a singlelocation (20 X 20 m) in SRSF (Mason County, Illinois), in a nativesand prairiesite,and fromthe top 0-15 cm of a single location (20 X 20 m) of KPRNA (RileyCounty,Kansas), a nativetallgrassprairiesite dominatedbyAndronutans (L.) Nash. Soil fromeach sitewas thoroughlymixed pogongerardiiand Sorghastrum and transportedto a laboratoryat Kansas State University forprocessingand experimental set-up.Two-thirdsof the soil fromeach site was steamed for 2 h at 80 C and allowed to cool and equilibrate for 72 h. The SRSF and KPRNA soils have been analyzed for soil chemical and physicalpropertiesin several earlier studies (KPRNA-Bentivenga and Hetrick,1991 and SRSF-Anderson and Liberta,1992; Meredithand Anderson,1992). In this paper,we presentthe resultsof the chemical analysesof a single compositesample of each of the soils (P, K, Zn, F, N03-N, % organic matter,and pH) whichwere conducted by the Soil TestingLaboratorybeforeand aftersteaming.These resultsare Kansas StateUniversity similarto those reportedin the previousstudies.ExtractableP was determinedby the Bray I method (Olsen and Sommers,1982). Exchangeable K was extractedby 1 M NH4OAc and


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measured by atomic absorptionspectrophotometry (Thomas, 1982). Nitratewas extracted using 2 M KCl (1:1 soil/solution) and determinedcolorimetrically (Keeney and Nelson, 1982). Soil Zn and Fe were extractedusing DTPA and measured by atomic absorption spectrophotometry (Lindsay and Norvell, 1978). Soil organic matterwas determinedcolorimetrically oxidation in the absence of heat (Graham, 1948), and pH by H2SO4-K2Cr2O7 in 1:1 soil:watersuspensions(Rhoades, 1982). To determinemycorrhizalspecies compositionand abundance of the two sites,a single 250 g (dryweight) composite soil sample fromeach site was wet-sievedon a 38-jim sieve and decanted. Spores were recovered using sucrose densitycentrifugation(Daniels and Skipper,1982). The spores were counted and identifiedto species (Trappe, 1982). For both Illinois and Kansas soils,30 plasticpots (6-cm diam X 25-cmdeep) were filled with575 g (dryweight)of steamedsoil and 15 potswere each filledwith575 g (dryweight) of nonsterilesoil fromthe respectivesite.One-halfof the pots containingsteamedsoil from each site (15 pots/site)were amended with 75 ml per pot of nonsterilesoil sievate (i.e., the suspension that passes througha 38-jim sieve as described below). The remaining15 pots per site containingsteamed soil and all pots containingnonsterilesoil were amended with75 ml per pot of sterile(autoclaved 121 C for60 min) sievate. Soil sievatewas collected fromthe two soils separately,and each soil was amended with sievate collected fromnonsterilesoil of the same soil. The soil sievatewas prepared by blending 1400 g (dryweight)nonsterilesoil with7 literof steriledistilledwaterand passing the slurrythrougha 38-jimsieve.The relatively funlarge vesiculararbuscularmycorrhizal gal spores were trapped on the sieve (Hetrick et al., 1988). Two-thirds(4.7 liter) of the sievatewas thenautoclavedat 121 C for60 min.The numberof colony-forming units (cfu)/ ml of fungiand bacteria contained in the nonsterilesievatewas determinedby dilution platingthe sievatesuspensiononto plates of potato dextroseagar (Difco Laboratories,Detroit,Michigan) amended withstreptomycin (0.1 g/liter)and chloramphenicol(0.1 g/liter) or peptone yeastextractagar (Difco), respectively. After5 days at 22 C, colonies forming on the plates were counted, and the number of cfu/mlof sievatewas calculated. Sievate collected fromthe Illinoissoil contained approximately5.3 X 107 cfu/mlbacteriaand 4.0 X 104 cfu/mlfungi.Approximately 5.9 X 105 cfu/mlbacteria and 720 cfu/mlfungiwere presentin the Kansas sievate.There were no cfu in the autoclaved sievates. Inoculum and seedlingpreparation.-Each of the three soil treatmentswere subdivided into three inoculum regimes (n = 5): (1) inoculationwith GlomusetunicatumBecker & Gerd., (2) inoculationwithS. scopariumroot pieces or (3) noninoculatedcontrol. Glomus etunicatumspores were originallycollected fromKPRNA and propagated on S. scoparium for 12 mo before use. Spores were recoveredfrompot culture by wet-sieving, decanting in a 20-40-60%sucrose densitygradient (Daniels and Skipper,1982). and centrifugation Spores were suspended in steriledistilledwater,with400 spores per ml and deliveredin a 1 ml aliquots onto the roots of one-thirdof the seedlings at transplant.The remaining seedlingsreceived 1 ml autoclaved (121 C for 15 min) spore inoculum. Schizachyrium scopariumplantswere randomlyselectedfromthe Sand Ridge StateForest site at the timeof soil collectionand transportedto the Kansas State University Laboratory. Roots were subsampled and stained in trypanblue (Phillips and Hayman, 1970) and examined microscopicallyto ascertainif the rootswere colonized by mycorrhizalfungi.The roots contained an average of 12% mycorrhizalfungalcolonization. Root inoculumwas collected by severingS. scopariumroots into 2.5-mmsegments.Onethirdof the pots of each soil treatmentreceived 1 g nonsterileroot segments(wetweight), while the pots of the other twoinoculum regimesreceived 1 g autoclavedroot pieces (121 C for15 min). Seeds of A. gerardiiand S. scopariumwere germinatedin sterilevermiculite.


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DEPENDENCE

TABLE 1.-Results of chemical analysesof SRSF and KPRNA soils KPRNA Steamed

Nutrient(mg kg-') Phosphorus Potassium Nitrogen(NO3-N) Iron Zinc (mg kg-') Organic Matter(%) pH

8 185 1.4 5 300 2.7 7.1

SRSF

Nonsteamed

5 220 1.6 5 150 2.5 7.5

Steamed

38 40 1.0 22 1150 1.4 6.1

Nonsteamed

40 30 1.0 31 950 1.4 5.6

Seeds of S. gerardiiwere providedby the Soil ConservationService Plant MaterialsCenter, Manhattan,Kansas, and S. scopariumseeds were purchased from the Stock Seed Farm, Murdock,Nebraska.Sixteen-day-old seedlingswere transplanted,one per pot, into the soil treatments previouslydescribed (steamed,steamedamended withsievateor nonsterilesoil). Biomass of 16-day-oldseedlingsused of either plant species was less than 0.02 g and was consistentacross all treatment. Experimentaldesignand maintenance.-Potswere arranged in a randomized complete block design withfivereplicationsper treatment.The plants were maintainedin a 21-25 C greenhouse,watereddaily,and fertilizedeveryother week with0.08 g Peter's No-Phos Special FertilizerSolution (25:0:25) (Robert B. Peters Co., Inc., Allentown,Pennsylvania) dissolvedin 25 ml H20. Approximately35 mg kg-' N and 35 mg kg-' K were added biweekly.After14 weeks, plants were harvestedand roots washed free of soil. Plants were dried in a 90 C oven for three days,and root and shoot dryweightswere determinedto the nearestmilligram.Subsamplesof dried rootswere stainedin trypanblue (Phillipsand to assesspercentroot colonizationusing Hayman,1970) and wereexamined microscopically a petridish scored in 1-mmsquares (Daniels et al., 1981). of variancepriorto analysisusing Statisticalanalysis.-Data were testedforhomogeneity. Levene's testforhomogeneityofvariance(MillikenandJohnson,1984). Analysisofvariance (ANOVA,P ' 0.05) was performedon shoot, root and total dryweights,root/shootratio and root colonizationforeach plant species using the SAS statisticalpackage (SAS Institute Inc., 1988). Shoot and root dryweightwere each highlycorrelatedwithdryweight.Thus, for simplificationof data presentation,only total dryweightsat the end of the 14 week experimentalperiod are presented.Root/shootratioswerealso assessed,but no clear trends were observed and these data are not shown. RESULTS

Soil analysis.-The resultsof the soil analysisfor steamed and nonsteamed soil were similarforSRSF and KPRNA soils (Table 1). Compared withthe SRSF soil, KPRNA soil had higherlevelsof availablepotassiumand nitrate,percentorganicmatter,pH, but lowerlevels of plant-availableP. VAM fungal compositionand abundance and root colonization.-A varietyof Glomales species was collectedfromthe soil of the twosites (Table 2). KPRNA prairiesoil (100 g dry weight) contained an average of 349 VAM fungalspores of six species. The mostabundant Smith& Schenck, G. aggregatum Schneck & Smith,and species were Glomusambisporum G. constrictum Trappe. The SRSF soil contained390 sporesper 100 g of drysoil,represented


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TABLE 2.-Spores of VAM fungiisolated fromSRSF and KPRNA soils (No./100 g dryweightof soil) Species

KPRNA

Glomusambisporum Smith& Schenck G. aggregatum Schenck & Smith G. constrictum Trappe G. etunicatumBecker & Gerde. Entrosporainfrequens (Hall) Ames and Schneider G. mortonii Bentivengaand Hetrick G. geosporum(Nicol. & Gerde.) Walker G. deserticola Trappe, Bloss & Menge G. mosseae(Nicol. & Gerd.) Gerdemann& Trappe G. gigantea(Nicol. & Gerde.) Gerdemann & Trappe Total

135 97 80 25 7 5

SRSF

337 38 9 6 390

349

by four species of VAM fungi. No species of VAM fungi was common to the two soils. The majority of the spores isolated from the SRSF soil were of G. geosporum (Nicol & Gerd.) Walker. Without added mycorrhizal fungal inoculum, A. gerardii and S. scoparium were not colonized by VAM fungi when they were grown in steamed soil or steamed soil with added

TABLE 3.-The effectof soil treatmentand inoculum typeon percentroot colonizationof two tallgrassprairiegrasses (N = 5) A. gerardii Soil treatment/inoculum

KPRNA

S. scoparium

SRSF

KPRNA

SRSF

Steamed soil G. etunicatum

Root inoculum No inoculum Steamed soil withsievings G. etunicatum Root inoculum No inoculum

78.6a*

50.6a

42.2bc

24.4b

oc

Oc

Oc

Oe 67.0ab

15.5bc

36.Ocd

13.Obc

57.Oa

22.Ob

Oc

23.3ab 14.4abc

Oc 24.8a

3.2c Oc

Oe

Oc

13.Ode

12.2bc

8.5c

9.Obc

12.2de

15.3bc

11.5bc

13.Oabc

10.2e

12.3bc

8.5c

11.3abc

Oc

Nonsterilesoil G. etunicatum

Root inoculum No inoculum

ANOVA Results (F-values)

Main effects Soil treatments Inoculum type Two-wayinteraction

11.25*** 9.06*** 24.75*** 5.62***

8.04*** 6.64** 11.84*** 5.83***

20.55*** 7.09** 37.74*** 17.58***

*

2.66 0.26 5.73** 2.67**

Withina column,means withthe same letterare not significantly different(P < 0.05), according to the least significantdifferencetest ** P < 0.01, *** P < 0.001


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TABLE I-Effect of soil treatment and inoculumtypeon mean totalbiomassproduced (g dryweight) of twotallgrassprairiegrasses (N = 5)

A. gerardii Soil treatment/inoculum

Steamed soil G. etunicatum Root inoculum

No inoculum

KPRNA

S. scoparium SRSF

KPRNA

SRSF

1.53a

1.19

0.80a

1.90

0.80b 0.05c

2.00 1.17

0.15b 0.03b

1.66 1.64

Mean

1.72a

Steamed soil withsievings G. etunicatum Root inoculum No inoculum

1.06ab

0.92ab 0.02c

1.65 1.99

2.21

0.76a 0.05b 0.03b

Mean

1.12 1.27 1.01 1.14b

Nonsterilesoil G. etunicatum Root inoculum

No inoculum

1.03ab

1.60

0.78a

1.24

1.01ab

1.28

0.68a

1.32

1.00ab

1.67

0.85a

Mean

1.12 1.22b

ANOVA Results (F-values) Main effects Soil treatment Inoculum type Two-wayinteractions

5.18*** 1.89 12.26*** 3.23*

0.95 1.48 0.38 1.00

15.29*** 22.81** 22.86*** 6.06***

1.13 4.01* 0.33 0.15

a Withina column, means withthe same letterare not significantly different(P < 0.05), according to the least significantdifferencetest * P < 0.05, ** P < 0.01, 88 P < 0.001

nonsterilesoil sievates (Table 3). There was no differencein the colonization of either species when theywere grownin nonsterileSRSF or KPRNA soil regardlessof whetheror not inoculum (A. scopariumroot pieces or spores) had been added to the soil. For steamedKPRNA soil,withor withoutsievates,and steamedSRSF soil withoutsievates, inoculation of A. gerardiiwith G. etunicatumspores resultedin significantly higher levels of colonization than when colonized root pieces fromSRSF were used as inoculum. The highestlevelsof colonizationof A. gerardiiin SRSF soil occurred in steamed soil when G. etunicatumspores were used as inoculum. In steamed soil withadded sievatesand in nonsterilesoil, therewere no differencesin colonizationresultingfromthe twoinoculants. No colonizationoccurredwhen root inoculumfromSRSF was used and S. scopariumwas grownin steamed KPRNA soil,withor withoutnonsterilesoil sievates.In contrast,G. etunicatumspore inoculum resultedin colonization of plants grownin steamed KPRNA soil withor withoutsievates.Plants receivingspore inoculum in steamed KPRNA soil had sigWhen S. sconificantly higherlevels of colonizationthan all other KPRNA soil treatments. pariumwas grownin SRSF soil,however,percentagerootcolonizationwas similarin steamed soil regardlessof inoculum type,but in steamed soil withsievatesG. etunicatuminoculum resultedin greatercolonizationthan the root inoculum. Biomassproduction.-In the KPRNA soil, biomass productionof both plant species was


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directlyrelated to the presence or absence of mycorrhizalsymbiosis,with noncolonized plants growingverypoorly or not at all (Table 4). In steamed KPRNA soil, mycorrhizal fungusinoculationincreased growthof A. gerardii,regardlessof inoculum type.However, the biomass produced by mycorrhizalplantsin steamed soil was not significantly different fromthatobtained in nonsterilesoil whetheror not additionalinoculumwas added to the nonsterilesoil. In the steamed KPRNA soil, A. gerardiiplants colonized by G. etunicatum produced significantly more biomass than those colonized by SRSF root inoculum. The effectof mycorrhizalsymbiosison biomass produced by S. scopariumplants grown in KPRNA soil was similarto that of A. gerardii.Noncolonized plants,includingthose inless biomass than plants colonized by oculated withroot inoculum,produced significantly mycorrhizalfungi,regardlessof the type of inoculum (i.e., G. etunicatumspores or the nativeinoculum contained in the nonsterilesoil). There were no differencesin biomass of A. gerardiigrown in SRSF soil regardlessof inoculum typeor soil treatment.For S. scopariumgrownin the SRSF soil, therewere sigin biomassproductionowingto soil treatment withthebiomassof plants nificantdifferences grownin steamed soil exceeding thatproduced by plants grownin the steamed soil with sievatesor in the nonsterilesoil. DISCUSSION

The resultsof thisstudyconfirmthe strongdependencyof both plant species on mycorrhizalfungalcolonizationin the KPRNA soil, thathas been previouslyreported (Hetricket al., 1988, 1990). In contrast,plantsgrownin the SRSF soil showedno mycorrhizal responses as noted in previous studies (Bentavinga,1988; Seo et al., 1988; Anderson and Liberta, 1989, 1992; Meredith and Anderson, 1992). The only significantdifferencesin biomass production among plants grown in SRSF soil was the greaterbiomass production of S. scopariumplants grownin steamed SRSF soil than in the steamed soil with sievatesand nonsterilesoil. These differencesmay be due to competitionfor inorganic nutrientsbetweenplants and soil microbes (Hetrick et al., 1988; Meredithand Anderson,1992). Anderson and Liberta (1992) have shown that SRSF soil is limitedin bases (available Ca = 325 vs. 225 mg kg-' and Mg = 35 vs. 25 mg kg-' forautoclaved and nonautoclavedSRSF soil, Meredithand Anderson, 1992) and plants grownin SRSF soil have positivegrowth status(Anresponsesto supplementaladditionsof thesenutrientsregardlessof mycorrhizal derson and Liberta,1992). Consequently,competitionbetweenplantsand soil microbesfor Ca and Mg mayhave reduced plant growthin the steamedplus sievatesand nonsterilesoil. Soil microbesmay have had an antagonisticeffecton plant growthor the mycorrhizal associationin waysother than throughcompetitionforinorganicnutrients,such as being pathogenic to the host plant or the fungalsymbiont.However,evidence for the presence of a pathogen in the SRSF forestsoil is absent. If a pathogen is presentin the SRSF soil, it would have to be species specific,because growthof A. gerardiidid not decline in the nonsterileand sterilesoil withsievatescompared to itsgrowthin the steamedsoil. Moreover, the pathogen but there it is likelythe S. scopariumroot inoculum would have transferred was no effectof inoculum typeon the growthof eitherA. gerardiior S. scopariumin SRSF soil. Responses to mycorrhizaeoccurred in KPRNA soil but not SRSF soil. These soil differences are most likelyrelated to the greateravailabilityof phosphorusin SRSF soil (40 mg the kg-') than in the KPRNA soil (5 mg kg-1). Under conditionsof high P availability, benefitsof mycorrhizalsymbiosismay be lost (Allen, 1991; Brundrett,1991; Koide, 1991), explainingthe lack of a mycorrhizalresponse in SRSF soil. Sylviaet al. (1993), in a recent studyof sorghumand soybeanmycorrhizalrelationshipsin a numberof soilswitha variety


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of endophytes,observed that mycorrhizalresponseswere most apparent in soils withless than 10 mg kg-' available P (Melech extractable).WorkingwithAgropyron smithiiRydb., Milleret al. (1987) demonstrateda positivemycorrhizal response at the lowerend of a soilphosphorusresourcegradient(2 mg kg-'), but at the higherend of the P-gradient(20 mg kg-') therewas no differencein biomass productionbetweenmycorrhizaland nonmycorrhizal host plants.Thus, it is not surprisingmycorrhizalresponseswere not evidentin the SRSF soil. The high levels of available soil P associated withthe SRSF soil are atypicalof soil conditionsin most prairie sites.Like the KPRNA site, available soil P for the central Illinoisremnanttallgrassprairiesoccurringon siltloam and loam soilsvariesfrom2-9 mg kg-' (Anderson et al., 1984; Dickman et al., 1984). Several workershave reported a general lack of specificity betweenVAM fungal endophytesand host plants (see reviewsbyBrundrett,1991; Chanwayet al., 1991). Nevertheless, there is considerable evidence supportingspecificity between host plants and VAM fungi resultingin variedlevelsof colonizationor host performanceowingto environmentalconditionsand/or host response to specificVAM endophytes(e.g., Rabatin, 1979; Allen and Boosalis, 1983; Stahl and Smith,1984; Boerner, 1990; McGonigle and Fitter,1990; Allen, 1991; Brundrett,1991; Chanwayet al., 1991; Allen et al., 1992; Dhillion, 1992). The lack of colonization of S. scopariumin the KPRNA soil when root inoculum from S. scoparium obtained fromthe SRSF soil was used, suggeststhere is a strongeffectof soil typeon host plant-fungal endophyteinteraction.In contrast,rootinoculumproduced colonizationlevels in A. gerardiiin KPRNA soil thatwere not significantly fromthose resultingwhen different plantswere colonized by the nativeVAM fungiin nonsterilesoil; although these colonization levels were significantly lower than those obtained when G. etunicatumwas used as inoculum in steamed soil with or withoutsievates.Clearly,inoculum type (spores versus colonized root pieces) and fungalspecies mayalso affectplant benefitfromthe symbiosis. these resultssuggestthatthe response of these twonativeprairiegrassesvaries Collectively, depending upon soil fertility and plant host-fungal endophytespecificity. It is likelythatin naturalsystemsthese factorsproduce an arrayof habitatsin which the dependency of plant species may varyfromobligate to nondependency (Boerner, 1986; Miller,1987; Sanders and Fitter,1992). It is also clear thata single plant species mayshow strongmycorrhizaldependencyunder one set of conditionsand a lack of dependencyin another.An interestingquestion raised by our workis whydo plants commonlyformmycorrhizalassociationsunder conditionswhere apparentlynutrientavailability(especiallyP) is such thata mycorrhizalassociationis not likelyto be beneficial?This is a topic requiring futureinvestigationand may relate to temporalvariationin P availabilityor variationin plant need forP duringits lifecycle (Fitter,1991; Sanders and Fitter,1992). researchwas partiallysupportedby the National Science Long Term EcologAcknowledgment.-This ical Research Program(grantBSR-8514327). LITERATURECITED M. 1991. The ecologyof mycorrhizae.CambridgeUniversity Press,New York.184 p. M. G. BoosALIs. 1983. Effectsof twospecies ofVA mycorrhizal fungion droughttolerance of winterwheat. New Phytol.,93:67-76. - , S. D. CLOUSE, S. L. WEINBAUM, C. F. FRIESE AND E. B. ALLEN. 1992. Mycorrhizaeand the integrationof scales: molecules to ecosystems,p. 488-515. In: M. Allen (ed.). Mycorrhizal functioningan integrativeplant-fungal process. Chapman and Hall, New York. ANDERSON, R. C., A. E. LIBERTA AND L. A. DicKMAN.1984. Interactionof vascularplantsand vesiculararbuscularmycorrhizalfungiacross a soil moisture-nutrient gradient.Oecologia,64:111-117.

ALLEN,

AND


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6 JULY 1993

ACCEPTED


23 MARCH 1994