plant nitrogen capture from organic matter as affected by spatial dispersion, interspecific...

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Research

Blackwell Publishing, Ltd

Plant nitrogen capture from organic matter as affected by spatial dispersion, interspecific competition and

mycorrhizal colonization

Angela Hodge

Department of Biology, Area 2, The University of York, PO Box 373, York, YO10 5YW, UK

Summary

• The capture of nitrogen (N) by plants from N-rich complex organic material dif-fering in spatial (uniform dispersion or discrete patches) heterogeneity was meas-ured, as well as the subsequent impact on N capture of the addition of a mycorrhizalinoculum (

Glomus hoi

).• The organic material was dual-labelled with

15

N and

13

C to follow plant uptake ofN (as

15

N) and to determine the amounts of original

13

C and

15

N which remained inthe soil at harvest. The organic material was added to microcosm units containing

Lolium perenne

or

Plantago lanceolata

in intra or interspecific competition.• Plant N capture from the dispersed organic material was more than twice thatfrom the discrete patch (dispersed: 17%; discrete: 8%). There was no effect of spe-cies composition or the mycorrhizal inoculum on total plant N capture except whenin interspecific plant competition. Here, N capture was dependent on the root lengthproduced and was always higher when the mycorrhizal inoculum was present.• Mycorrhizal colonization increased N capture from the organic material when ininterspecific plant competition but not in monoculture.

Key words:

arbuscular mycorrhizal (AM) fungi, decomposition, organic material,spatial heterogeneity, intra- and interspecific competition.

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Author for correspondence:

Angela Hodge Tel: +44 11904 328562 Fax: +44 11904 432860 Email: [email protected].

Received:

6 August 2002

Accepted:

21 October 2002

Introduction

Within the soil environment organic matter inputs such asleaf litter, dead roots and soil animals, and their subsequentdecomposition, help to create temporal and spatial heter-ogeneity in nutrient availability. Thus, in order to optimizenutrient capture plant roots have to be physiologically and/ormorphologically plastic to enable them to respond to suchvariations in resource supply. It is well established that plantscan respond morphologically by the proliferation of rootswithin localized nutrient-rich zones or patches which isconsidered to be a foraging response to the heterogeneousnature of the soil environment (Robinson, 1994; Fitter

et al

.,2000). This root proliferation response however, was thoughtto have no benefit to plants in terms of nitrogen (N) capture,a key limiting nutrient in terrestrial ecosystems, as no relation-ship between root proliferation and N capture could bedemonstrated (van Vuuren

et al

., 1996; Fransen

et al

., 1998;

Hodge

et al

., 1998). However, when plants with differingcapabilities for proliferation are competing for the N releasedthen the speed and the extent of the root proliferation responsebecomes important and the ability to proliferate roots doesconfer an advantage (Cahill & Casper, 1999; Hodge

et al

.,1999a, 2000b; Robinson

et al

., 1999). Thus, if the conditionsunder which the proliferation response has evolved, such asplant-plant competition, are omitted then the advantage ofthe response may be obscured. Another factor that is oftenover looked in root responses to soil heterogeneity is thecolonization of the plant roots by mycorrhizal fungi.

Mycorrhizal associations are ubiquitous in the naturalenvironment, thus being mycorrhizal is the normal conditionfor most plant roots. Of the different types of mycorrhizalassociations that can form, the arbuscular mycorrhizal (AM)association is the most common, occurring in two thirds of allplant species (Smith & Read, 1997). As with plant roots,fungal hyphae can also proliferate in nutrient-rich zones as has


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been demonstrated for both free-living (Ritz

et al

., 1996) andsymbiont fungi in the ectomycorrhizal (Bending & Read,1995) and AM associations (Mosse, 1959; St John

et al

.,1983; Hodge

et al

., 2001). Proliferation of the mycorrhizalfungal symbiont either instead of, or in addition to, the hostroots could confer a number of advantages to the host plant.Firstly, as the fungal hyphae are thinner, proliferation of thefungal hyphae should represent a lower construction cost tothe host plant not only in terms of carbon (C; Fitter, 1991),but also N, as fine roots have elevated N concentrations(Pregitzer

et al

., 1997). Secondly, again by virtue of their size,AM hyphae should be better able to penetrate throughoutthe decomposing organic material and so increase N capture,either by being more effective at competing with the othersoil microbial community for the inorganic N released or byaccessing organic forms of N directly. Although the results ofsome studies suggest that AM fungi may be able to accesssimple forms of organic N intact (Cliquet

et al

., 1997; Näsholm

et al

., 1998), evidence for a direct role of AM hyphae inorganic N uptake and transfer is still lacking and remains amatter of some debate in the literature (Smith & Read, 1997).AM fungi can however, transfer NH

4+

to their associated hostplant (Ames

et al

., 1983; Mäder

et al

., 2000). Furthermore, ithas recently been demonstrated that an AM fungus,

Glomushoi

, both enhanced decomposition of, and transferred N to itsassociated host plant from, a complex organic patch in soil(Hodge

et al

., 2001).In addition to enhanced nutrient capture, mycorrhizal

fungi are known to confer a number of other benefits to theirhost plant including drought resistance and protection againstpathogens (Newsham

et al

., 1995a; Smith & Read, 1997).Newsham

et al

. (1995b) proposed a model whereby rootsystems with a poorly branched architecture rely on the AMmycorrhizal symbiont predominately for enhanced nutrientcapture, while those root systems that had a highly branchedarchitecture rely on the mycorrhizal symbiont for otherbenefits such as pathogen resistance. The aims of this studytherefore were to examine how two plant species (

Plantagolanceolata

L. and

Lolium perenne

L.) of varying root architec-ture captured N from complex organic material (

L. perenne

shoots) added to soil either as a discrete patch or moreuniformly dispersed. The influence of an AM inoculum uponN capture and the host plants’ root proliferation response wasalso followed.

Glomus hoi

was selected as the AM inoculum asit has previously been shown to enhance N capture from com-plex organic patches (Hodge

et al

., 2001).The following hypotheses were tested; (i) Root growth in

general would increase in the sections receiving the organicmaterial and would be greatest where the organic material wasmost concentrated. Specifically, root morphology would alter,resulting in longer thinner roots and so an increase in specificroot length (SRL); (ii) Root growth, particularly root length,would be reduced when the mycorrhizal inoculum waspresent and this would be most apparent in the sections

containing the organic material because hyphal, instead ofroot, proliferation would occur; (iii) Plants would capturemore N from organic material added as a patch from organicmaterial that which had been dispersed, and the species whichhad the most root length in the patch zone would capturethe most N overall. More N would be captured from the dis-persed organic material treatments when the AM mycorrhizalinoculum was present due to the spatial placement of thefungal hyphae; (iv) Overall

Lolium

monocultures, by virtueof their increased root length, would capture more N from theorganic material than

Plantago

monocultures; N capture bythe mixed culture would be intermediate between these two.However, the addition of a mycorrhizal inoculum wouldbenefit (in terms of N capture from the organic material)

Plantago

species more than

Lolium

.The organic material added was dual-labelled with

13

C and

15

N so that the dynamics of N capture by the plants and thedecomposition of the organic material could be followed.Although the organic material differed in its spatial distribu-tion each experimental unit was supplied with the same totalamount of N and C.

Materials and Methods

Experimental design

Plants were grown in microcosm tubes made out of a sectionof PVC pipe (length 20 cm, I.D. 10 cm). At the top of eachtube, the top 2 cm section of a PVC funnel (I.D. 10 cm attop and 7 cm at base) was placed to direct the roots intothe middle section of the tube where the organic material wasto be inserted. Each microcosm tube was filled to a depth of5 cm with a 50 : 50 mixture of sand : soil (a medium loam asdescribed by Hodge

et al

., 1999b) and a smaller section ofPVC pipe (length 15 cm, I.D. 6.5 cm) placed centrally insidethe larger unit, to enable placement of the organic materialonce the seedlings had developed and to keep disturbance ofthe unit to a minimum. The area between the outer and innertube was filled with the sand : soil mixture containing themycorrhizal inoculum (fresh or autoclaved) and the microcosmunit was then ready for planting.

Eight microcosm tubes were contained within six large(60

×

40

×

30 cm) freely draining insulated boxes containinga mixed turf of

Trifolium repens

L. (white clover) and

Loliumperenne

L. cv. Fennema (perennial rye-grass) to buffer themicrocosm tubes against fluctuations in external temperatureand to produce a realistic microclimate around the tubes. Theboxes were maintained in a glasshouse and watered daily.

Mycorrhizal treatments received 150 g wet weight of

Glomus hoi

(Berch & Trappe) isolate UY 110 inoculum addedto the sand:soil medium. The non-mycorrhizal controlsreceived 150 g wet weight of the mycorrhizal inoculum whichhad been autoclaved (121

°

C; 30 min). The inoculum con-sisted of

Plantago lanceolata

L. (ribwort plantain) root medium

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colonized with

G. hoi

and included the sand and Terra-Green® (a calcined attapulgite clay soil conditioner, Turf-Pro Ltd, Staines, UK) growth medium. The inoculum waschecked to confirm the presence of both root colonization andspores before addition to the experimental microcosm units.In addition, all microcosm units received 10 ml of filteredwashings from the mycorrhizal inoculum, passed through a20-µm mesh twice to remove AM propagules, to preventinitial differences in microbial communities among micro-cosm units.

Plantago lanceolata

L. and

Lolium perenne

L. seeds suppliedby Emorsgate Seeds, Norfolk, UK, were planted into eachmicrocosm tube on 16 March 2000 (two seeds in each tube).The microcosm units contained three species combinations:either two

Plantago

or

Lolium

seedlings as monocultures (inintraspecific competition) or one seedling of each in a mixedculture (in interspecific competition). All seeds in the micro-cosm tubes had germinated after one week. Forty-two daysafter planting the organic material was added. The experimentran for 22 d between 3 May and 25 May 2001. The meantemperature over the duration of the experiment was 19.5

°

C(SE

±

0.08) with a mean daily maximum of 35.0

°

C (SE

±

1.19) and mean daily minimum temperature of 15.7

°

C(SE

±

0.49). Photosynthetically active radiation (PAR) fluxwas recorded weekly at noon and averaged 502 µmol m

−

2

s

−

1

at plant level.

Patch addition

The organic material added to the microcosm tubes was0.5 g oven-dried finely milled

L. perenne

shoot materiallabelled with both

15

N and

13

C produced as described inHodge

et al

. (1998). The material was placed in the spacecreated by removal of the inner PVC tube and was addedeither as a thin, concentrated layer (

c

. 6.5 cm diameter, 1 mmdepth) at 11 cm depth in the microcosm unit (‘patch’treatment) or dispersed uniformly with the backgroundsand : soil mix in a 10-cm band starting 3 cm below thesurface (‘dispersed’ treatment). The remainder of the spacewas filled with the sand: soil mix only and each microcosmunit contained 1600 g d. wt. of the sand : soil mix. Theorganic material added to the tubes contained 9 mg N(1.37 mg

15

N) and 202 mg C (3.55 mg

13

C) with a C : Nratio of 22 : 1. There were four replicate tubes for eachcombination of species, organic material placement andmycorrhizal treatment.

Plant and soil analysis

At harvest, each soil core was removed intact from its tubeand then cut into four sections – the top 2 cm and then a top,middle and bottom section, each of 6 cm thickness. Theshoots were oven-dried at 60

°

C, weighed and analysed asbelow. The roots extracted from the different sections were

washed thoroughly and the total root length from eachsection measured on a WinRHIZO (Régent Instruments Inc.,Québec, Canada) image analysis system (scanned at 300 dpi).As it was impossible to separate roots of

Lolium

from those of

Plantago

when grown in interspecific competition, the rootlength produced by each species was estimated by addingtogether the root lengths produced by

Lolium

and

Plantago

monocultures and obtaining an average value for each of thefour mycorrhiza/organic placement combinations separately;the proportion of roots which were

Plantago

and

Lolium

wasthen calculated from this average sum. The approximate rootlengths of each species in competition were then estimatedfrom these proportions as described in Hodge

et al

. (2000b).This method relies on two assumptions: the species thatyielded the most in monoculture was assumed to be the mostsuccessful in the mixture; and both intra- and inter-specificinteractions were the same. Root N capture from the organicmaterial by the plants grown in competition was similarlyestimated and summed with that captured by the shoots(which could be easily separated) to obtain a value of plant Ncapture for each species in competition. As > 70% of the Ncaptured from the organic material was detected in the shootsthis estimate was less prone to error.

A subsample of root material was taken from the middlesection only for mycorrhizal assessment. The root materialfrom the different sections was then oven-dried at 60

°

C andweighed before being combined for milling. A subsample ofthe root, shoot and soil material from the middle section wasanalysed for total N, C,

15

N and

13

C by continuous-flow iso-tope ratio mass spectrometry (CF-IRMS). Subsamples of thesoil from the different sections of each tube were used forgravimetric moisture content determinations (105

°

C).For mycorrhizal assessment, roots were cleared in KOH

(90

°

C, 10 min), acidified in HCl (room temperature, 1 min)and stained with acid fuchsin (90

°

C, 20 min) (Kormanik &McGraw (1982) but without phenol). Mycorrhizal coloniza-tion was examined with a Nikon Optiphot-2 microscopeusing brightfield and epifluorescence (Merryweather & Fitter,1991) and

×

200 magnification. Mycorrhizal scoring, using100 intersections, was by the method of McGonigle

et al

.(1990). Numbers of arbuscules, vesicles and root length colon-ized (RLC; the percentage of total intercepts where hyphaeor other AM fungal structures were present) were recorded foreach intersection. External mycorrhizal hyphae were extractedfrom two 5 g (FW) samples from the middle soil section only(which contained the organic material added as a patch or halfof the organic material added in the dispersed treatment)using a modified membrane filter technique (Staddon

et al

.,1999). Assessment of hyphal length was carried out using thegridline intercept method (Miller & Jastrow, 1992) for aminimum 50 fields of view at

×

125 magnification (using a10

×

10 grid of 1 cm side lengths, obtained from GraticulesLtd, UK). The hyphal lengths were then converted to hyphaldensities (m hyphae g

−

1

soil : sand d. wt).


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Statistical analysis

Data were analysed using the General Linear Model(GLM) factorial design command or, for the root data inthe individual sections, the GLM repeated measurementscommand, in SPSS v. 10.0. For the root data monocultureswere directly compared with the mixed culture unlessotherwise stated. Data for

Plantago

and

Lolium

were analysedseparately except for the shoots where data for only oneseedling in each case (i.e.

Plantago

or

Lolium

) was used andthe data for the neighbouring seedling discarded. The shootdata in competition was then compared against the mean ofthe two seedlings grown in monoculture. The ‘seedling’ termrefers to the plant species examined whereas the ‘competitor’term refers to the other plant species present. Thus, when

Plantago

was the ‘seedling’ it was compared against

Plantagoas the ‘competitor’ in the monoculture tubes and Lolium inthe intraspecific competition tubes. Differences referred to inthe text were statistically significant with P < 0.05, unlessotherwise stated. All data were checked and transformedappropriately to normalize skewed distributions beforestatistical analysis. In all cases, a randomised block design wasused.

Results

Organic material decomposition

At harvest the soil from the middle section contained more13C and 15N than the controls, indicating that some N and Cfrom the organic material remained in the soil. As only themiddle soil section was analysed, which contained all of theorganic material added as a patch but only half that added inthe dispersed treatment, the values recovered from the dispersedtreatment were multiplied by 2 to obtain an estimate of thetotal mg 13C and 15N remaining in the tube.

The relationship between 13C and 15N loss was differentdepending on how the organic material had been added(Fig. 1) with more N retained in the soil per unit C when theorganic material had been added as a discrete patch comparedwith when it had been more uniformly dispersed. In an analysisof covariance with mg 13C as the covariate, organic material

placement, mycorrhiza inoculum and the interaction betweenorganic material placement and plant species were significant(Table 1).

The percentage of original 13C added in the organic mate-rial recovered in the soil at harvest was similar (c. 13%) regard-less of how the organic material had been placed in the tube.In contrast, the original 15N recovered was markedly affectedby the method of incorporation with c. 63% of the original15N recovered in the patch treatment but only c. 35% recoveredin the dispersed treatment (Table 2). When the dispersed andpatch data were analysed separately species composition wasa significant factor (P = 0.015) in the dispersed treatmentwhereas mycorrhiza inoculum was a significant factor (P =0.039) in the patch treatment. In both cases mg 13C was a sig-nificant covariate. A one-way ANOVA followed by Fisher’spairwise comparisons confirmed that the mass 13C : 15N ratioof the soil from Plantago monocultures was lower than that ofthe mixed culture soil in the dispersed organic material treat-ment. In the patch treatment there was a tendency for more15N to be retained in the soil when the mycorrhizal inoculumwas present (Table 2) but the differences found in the analysisof covariance not confirmed by a one-way ANOVA, suggest-ing that the differences due to the mycorrhizal inoculumwere slight.

Fig. 1 The relationship between mg 13C and 15N in the dispersed (open symbols) and patch (closed symbols). Data shown are raw values for Plantago monocultures (circles), Lolium monocultures (squares) and mixed culture (triangles). In an analysis of covariance, mg 13C was a significant covariate (P < 0.001) and O.M. placement a significant factor (P < 0.001).

Covariate d.f. F P

mg 13C Covariate 1 106.0 < 0.001Species composition 2 2.51 0.096O.M. placement 1 118.9 < 0.001Mycorrhiza inoculum 1 4.94 0.033Species composition × O.M. placement 2 5.24 0.010Species composition × Mycorrhiza inoculum 2 2.34 0.111O.M. placement × Mycorrhiza inoculum 1 0.40 0.529Species composition × O.M. placement × Mycorrhiza inoculum

2 2.16 0.131

Table 1 Analysis of covariance using species composition, organic material (O.M.) placement, mycorrhizal inoculum and their interaction as the factors for mg 15N remaining in the soil with mg 13C remaining in the soil as the covariate. All F-tests have 47 error degrees of freedom

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Mycorrhizal colonization and external hyphae production

The percentage of root length colonized (RLC) and the numbersof arbuscules were affected by the plant species present and inboth cases the species × mycorrhizal interaction was significant.In the presence of the mycorrhizal inoculum percentage RLCwas in the order Plantago monocultures (75 ± 2.7%) > Mixedculture (43 ± 5.4%) > Lolium monocultures (30 ± 1.9%),whereas in its absence the levels were lower and in the orderPlantago monocultures (19 ± 2.9%) > Mixed and Lolium mono-cultures (mean across these treatments = 7 ± 1.7%). Numberof arbuscules followed the same pattern as percentage RLC.

Although low levels (c. 0.15 m g−1 soil d. wt) of aseptatehyphae were observed in treatments which did not receivethe mycorrhizal inoculum, hyphal length densities weresignificantly higher (P < 0.001) in the mycorrhizal treat-ments (1.21 ± 0.105 m g−1 soil d. wt). Greater hyphal lengthdensities were recovered from Plantago monocultures(0.79 ± 0.189 m g−1 soil d. wt) compared with Lolium mono-cultures (0.57 ± 0.132 m g−1 soil d. wt). When both plantspecies were grown together hyphal length densities wereintermediate and not significantly different from either plantspecies grown in monoculture (0.69 ± 0.169 m g−1 soil d. wt).There were however, no differences in hyphal length densities,percentage RLC or arbuscules between ‘patch’ and ‘dispersed’organic matter treatments.

Root length, d. wt and specific root length (SRL)

Of the 7 possible 2-, 3- and 4-way interactions for each of rootlength, root d. wt and SRL in the different sections, 5 weresignificant (P < 0.05) for root length and d. wt and 4 weresignificant (P < 0.05) for SRL. Of these, the most biologicallyinteresting are presented below. There was never a significant4–way interactions between section × mycorrhiza × species ×organic material.

Root lengths followed the pattern predicted by hypothesis(i), being highest in the sections containing the organic materialcompared with those that had received no additions (Fig. 2a).Root d. wt followed a similar pattern (data not shown). Thepresence of organic material significantly altered specific rootlengths (SRL) but in the opposite direction predicted by hypo-thesis (i) as SRL was reduced in the patch where the organicmaterial was most concentrated (Fig. 2b), and in both treat-ments the highest SRL values were produced in the bottomsections which did not receive any organic material (Fig. 2b).

Although the mycorrhizal inoculum generally reduced rootgrowth, this reduction was not just confined to the sectionscontaining the organic material, thus only partially support-ing hypothesis (ii). Both root length and d. wt decreased in thetop section and increased in the bottom section when themycorrhizal inoculum was present. In the middle sectionroot length was decreased but d. wt was unaffected. Conse-quently, in the presence of AM inoculum, SRL was unaffectedin either the top or bottom sections but decreased in themiddle section (Fig. 3). Total specific root length (SRL) wasreduced (P = 0.021) by the mycorrhizal inoculum (withG. hoi inoculum: 162 ± 3.4 m g−1; without G. hoi inoculum173 ± 4.0 m g−1). Total root length and root d. wt from all thesections together were influenced only by the species compo-sition and were in the order Lolium monocultures > Mixedculture > Plantago monocultures (Table 3).

Shoot data

There was no difference between the shoot d. wt of Plantago(0.82 ± 0.036 g) or Lolium (0.85 ± 0.038 g) seedlings irrespec-tive of whether they were grown in competition or monoculture.However, the presence of a mycorrhizal inoculum, reducedshoot d. wt (with G. hoi inoculum: 0.79 ± 0.034 g; withoutG. hoi inoculum 0.88 ± 0.037 g; averaged across species). Shootd. wt was unaffected by the method of organic material addi-tion. Shoot N concentration was only affected by the placement

Table 2 Percentage of original organic material 15N and 13C and the mass ratio of mg 13C : mg 15N recovered in the soil at harvest across all treatments1 and for the factors2 shown to be significant after an analysis of covariance on each method of organic material (O.M.) incorporation separately. SE are in brackets

O.M. placementPatch 15N %

Patch 13C %

13C : 15N mass ratio

Patch1 Across all treatments 62.6 (3.58) 13.7 (0.77) 0.62 (0.062)Patch2 With mycorrhiza inoculum 67.3 (4.32) 14.1 (1.09) 0.54 (0.02)

Without mycorrhiza inoculum 57.9 (5.56) 13.3 (1.13) 0.70 (0.12)Dispersed1 Across all treatments 34.9 (3.20) 13.3 (1.00) 1.04 (0.070)Dispersed2 Plantago monocultures 39.8 (8.30) 11.9 (2.08) 0.80 (0.101)a

Lolium monocultures 32.7 (2.97) 13.6 (1.53) 1.11 (0.113)ab

Mix culture 32.0 (4.19) 14.3 (1.64) 1.20 (0.111)b

Differences in the dispersed treatment were confirmed by a one-way ANOVA. Different letters denote significant differences determined by a Fisher’s pairwise comparison. The significant differences observed in a ANCOVA due to the mycorrhizal inoculum in the patch treatment were not confirmed by a one-way ANOVA (see text for details).

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of the organic material, with higher concentrations when theorganic material was dispersed (17.4 ± 0.51 mg N g−1) thanwhen it was present as a discrete patch (15.9 ± 0.37 mg N g−1).

The quantity of 15N derived from the organic materialrecovered in the shoots was affected by the competitor and thetarget seedling present as shown by the significant (P = 0.008)

interaction. Least 15N was captured by Plantago when Loliumwas the competitor (Fig. 4). Organic material placement alsoaffected the amount of 15N detected in the shoots, with twicethe amount of 15N detected in the shoots of plants which had

Fig. 2 Differences in (a) root lengths and (b) specific root lengths in the top (shaded fill), middle (solid fill) and bottom (no fill) soil sections due to placement of the organic material in the tubes. Data shown are means with standard error bars (n = 24). Different letters denote significant differences between sections and organic material placement as determined by a Duncans Multiple Range test.

Species Root length (m) Root d. wt (g) SRL (m g−1)

Plantago monoculture 88 (4.8) a 0.52 (0.028) a 170 (7.0) aLolium monoculture 173 (9.8) c 1.01 (0.052) c 170 (3.2) aMixed culture 131 (5.1) b 0.81 (0.031) b 162 (2.9) a

Data were log10 transformed for statistical analysis. Different letters denote significant differences among species and sections at P < 0.05 as determined by a Duncans Multiple Range test.

Table 3 Total root length, d. wt and SRL produced by the different species combinations averaged over all other treatments. SE are in brackets

Fig. 3 Differences in specific root length in the top (shaded fill), middle (solid fill) and bottom (no fill) soil sections due to the presence or absence of the G. hoi mycorrhizal inoculum in the tubes. Data shown are means with standard error bars (n = 24). Different letters denote significant differences as determined by a Duncans Multiple Range test.

Fig. 4 Total N (14N + 15N) derived from the organic material in shoots of Plantago and Lolium seedlings grown in monoculture or mixture. Different letters denote significant differences among seedlings at P < 0.05 as determined by Fisher’s pairwise comparisons. Data shown are means (n = 16 for monoculture data and n = 8 for mixed culture) with standard errors.


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received the dispersed organic matter compared with thoseexposed to a discrete patch (dispersed: 0.08 ± 0.008 mg 15N;discrete patch: 0.04 ± 0.003 mg 15N). The presence of themycorrhizal inoculum did not affect the amount of 15Ndetected in the shoots.

Total d. wt, N content and N capture from the organic material

Total plant d. wt was affected only by the species present,being lower in Plantago monocultures (2.2 ± 0.06 g) comparedwith the mixture or Lolium monocultures which did not differ(mean across these treatments = 2.6 ± 0.08 g). On the otherhand total N content of the plants was affected only by themycorrhizal inoculum, being reduced when the inoculum waspresent (i.e. with G. hoi inoculum: 34 ± 0.90 mg N; withoutG. hoi inoculum: 38.5 ± 1.16 mg N).

Plants captured more than twice as much N from dispersedorganic material than from that added as a discrete patch (i.e.dispersed: 16.7%; discrete patch: 7.9% of the N originallyadded). Thus, hypothesis (iii) which predicted that plantswould capture more N form the organic material added as a patchthan that which had been dispersed was incorrect. Moreover,there was no significant interaction between the method oforganic material addition and the mycorrhizal treatmentsuggesting that mycorrhizal treatment did not enhance Ncapture from the dispersed treatment as had originally beenhypothesized. The N captured from the organic material, as apercentage of that added, did not differ between plant speciestreatments nor was it affected by the mycorrhizal inoculum(mean across treatments = 12.3%); however, the interactionbetween these two terms was significant (P = 0.021). BothLolium and Plantago monocultures captured more N fromthe organic material without mycorrhizal inoculum, but whengrown together in a mix culture N capture from the organicmaterial was greater with mycorrhizal inoculum (Fig. 5).

In an analysis of covariance of total N capture (14N + 15N)from the organic material, root length in the sections to whichthe organic material had been added was a significant covari-ate and species composition was a significant factor (Fig. 6a,Table 4a). A Bonferroni means comparison test showed thatPlantago differed significantly from both the mixed andLolium monocultures. Thus, although Plantago monoculturesproduced absolutely less root length than either Loliummonocultures or the mixed culture, they were more efficientat N capture per unit of root produced (Fig. 6a). In contrasthypothesis (iv) predicted that Lolium monocultures, due totheir greater root length, would capture more N from the organicmaterial than Plantago monocultures. In addition, plants captured

Fig. 5 Total N (14N + 15N) captured from the organic material in the absence (open bars) or presence (closed bars) of the mycorrhizal inoculum. Different letters denote significant differences among treatments as determined by a Duncans Multiple Range test. Data are means (n = 8) with standard error bars.

Fig. 6 Relationship between plant N capture from the organic material and root length. (a) N captured by plants from the organic material in Plantago monocultures (white symbols), Lolium monocultures (black symbols) and mix culture (grey symbols) against root length. (b) Estimated plant N captured from the organic material against root length in Lolium (black symbols) and Plantago (white symbols) grown in competition only. (c) Same as (b) except showing the influence of the presence of the mycorrhizal inoculum (black symbols) compared to its absence (white symbols). Data are means (n = 4). Analysis of covariance results are given in Table 4.


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more N from the dispersed organic material than that addedas a patch irrespective of root length produced (Table 4a).

There was partial support for hypothesis (iv) from the plantinterspecific competition data. When Plantago and Loliumwere grown together in competition neither species or organicmaterial placement affected N uptake (Fig. 6b, Table 4b).However root length was a significant covariate, demonstrat-ing that the N captured from the added organic material wasdependent on the root length produced and not on the mannerin which the organic material had been incorporated, noron the plant species present. Thus, by virtue of its increasedroot length, Lolium did capture more N from the organicmaterial than Plantago but only when it was grown in directcompetition with Plantago.

In interspecific competition, mycorrhizal inoculum was asignificant factor and root length was a significant covariate asN capture from the organic material was higher when mycor-rhizal inoculum was present (Fig. 6c). Thus, while mycorrhizalinoculum enhanced N capture from the organic material thisonly occurred when the plants were in interspecific competi-tion and the benefit was to both plant species, not just to Plan-tago as predicted by hypothesis (iv). However, in an analysis ofcovariance using plant N capture from the organic material asthe variate, neither percentage RLC, number of arbusculesnor external hyphal length density were significant covariates(data not shown). Thus, plant N capture from the organicmaterial was not related to either internal or external mycor-rhizal parameters.

Discussion

Root proliferation

Although total root length and d. wt were only affected byspecies composition, localized increased root growth occurred

in the sections containing the organic material in agreementwith hypothesis (i). Such localized proliferation of roots innutrient-rich zones is commonly observed and, as occurredin the discrete patch treatment in this study, is oftencompensated for by a reduction in root growth outside thenutrient-rich area (Fig. 2a; Drew, 1975; Gersani & Sachs,1992; Hodge et al., 1998). Within nutrient-rich zones achange in root morphology to longer thinner roots is alsofrequently reported resulting in an increase in SRL (Robinson& Rorison, 1983; Eissenstat & Caldwell, 1988). However,in this study root d. wt increased more than length in thesections containing the organic material, thus SRL decreasedwhere the organic material was most concentrated (Fig. 2b),rather than the increase in SRL which had been predicted byhypothesis (i). AM fungi have previously been demonstratedto both reduce (Cui & Caldwell, 1996) and enhance (Hodgeet al., 2000a) root proliferation in nutrient-rich zones.However, in this study, although G. hoi did reduce rootlength in the top and middle sections, this reduction wasnot just confined to the sections which contained theorganic material. Thus, there was only partial support forthe hypothesis that root growth, particularly root length,would decrease when the mycorrhizal inoculum was present,particularly in the sections containing the organic material.

Although all species combinations responded to theorganic material by increased root growth, Plantago mono-cultures captured more N per unit of root length produced(Fig. 6a). Thus, there was no support for the fourth hypothesisthat Lolium monocultures due to their increased root lengthwould capture more N from the organic material than Plan-tago monocultures. No 13C enrichment was detected in theplant tissue, indicating that the N from the organic materialwas being captured in inorganic N form after microbialdecomposition of the organic residue had occurred. It haspreviously been demonstrated that when plants are grownas individuals (van Vuuren et al., 1996; Fransen et al., 1998;Hodge et al., 1998) or in monoculture (Hodge et al., 2000b)root proliferation in, and N capture from, localized N-richpatches are unrelated. Plants grown in monoculture maycapture similar amounts of N from a decomposing organicpatch irrespective of their proliferation response as nitratesare soluble in water, and are highly mobile in soil (Tinker &Nye, 2000). Thus, a small amount of root can absorb all theNO3

– available in a matter of a few days. Consequently,increased root growth will not benefit the plant. However,when grown in interspecific competition for a commonorganic patch then proliferation of roots does confer a com-petitive advantage; the species which proliferates the mostcaptures more N (Hodge et al., 1999a, 2000b; Robinsonet al., 1999). Similarly, in this study the species which pro-liferated the most in monoculture (Lolium), captured the mostN from the organic material when in intraspecific competi-tion, presumably as a direct result of the increased root lengthproduced. Thus although there was no support for hypothesis

Table 4 Analysis of covariance using total (14N + 15N) N (mg) captured by the plants as the variate, species, organic material (O.M.) placement or mycorrhiza as the factor and root length in the sections containing O.M. as the covariate

Comparison d.f. F1,8 P

(a) Comparison of inter and intraspecific competition

Covariate 1 42.52 < 0.001Species 2 16.78 0.001Covariate 1 2.02 0.189O.M. Placement 1 42.65 < 0.001Covariate 1 2.79 0.129Mycorrhiza Inoculum 1 0.015 0.905

(b) Comparison of species in interspecific competition only

Covariate 1 14.61 0.012Species 1 0.001 0.972Covariate 1 19.70 0.007O.M. Placement 1 0.005 0.949Covariate 1 84.88 < 0.001Mycorrhiza Inoculum 1 7.61 0.040


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(iv) from the monoculture data, the increased root lengthapparently produced by Lolium did confer an advantagewhen in interspecific competition.

As part of the fourth hypothesis it was also predicted thatthe addition of the G. hoi inoculum would benefit N capturefrom the organic material by Plantago more than Lolium.Plantago monocultures did have higher AM hyphal lengthdensities and internal mycorrhizal colonization levels thanLolium monocultures, suggesting that Plantago had a greaterdependency upon the mycorrhizal partner for nutrient acqui-sition than Lolium, as expected from their differing root archi-tectures (Newsham et al., 1995b). However, the presence ofthe G. hoi inoculum affected both plant species in the samemanner: reducing N capture from the organic material whenin monoculture but increasing N capture by both specieswhen in interspecific competition (Fig. 5). Thus, it is unlikelythat the influence by G. hoi was a result of the reduction inroot length as, for the reasons discussed previously, this wouldhave been expected to reduce N capture when in interspecificcompetition rather than increase it. Nor was plant N capturefrom the organic material related to either internal or externalmycorrhizal parameters. Thus, the mechanism by whichG. hoi exerted this differing effect upon N capture by plantsin monoculture and in interspecific competition is currentlyunknown and requires further investigation.

Mycorrhizal parameters

The second hypothesis also predicted that in the sectionscontaining the organic material the reduction in root lengthdue to the presence of the mycorrhizal inoculum would becompensated for by AM hyphal proliferation. However,neither internal mycorrhizal colonization nor external AMhyphal length densities responded to the spatial placement ofthe organic material. AM fungi have been observed to con-centrate hyphae in organic materials (Nicolson, 1959; Hepper& Warner, 1983) and preferential growth of G. hoi hyphaeinto a compartment containing an organic patch ratherthan into one containing a potential new host has also beendemonstrated (Hodge et al., 2001). Thus, it was surprisingthat hyphal length densities of G. hoi were unresponsive to thespatial distribution of organic material in this study. St Johnet al. (1983) found that although AM fungi proliferatedhyphae in sterilized organic matter, this proliferation wasinhibited by the addition of a soil microbial filtrate. In thepresent study neither the growth medium or the organic patchmaterial were sterilized as the influence of the G. hoi inoculumin modifying N capture by its host plant when in competitionwith a native soil microbial community was of interest. It isunlikely however, that the lack of proliferation by the AMfungi in this study was solely due to the presence of the nativesoil microorganisms, as proliferation of AM hyphae in organicmatter has been observed in field-collected samples (Mosse,1959; Nicolson, 1959) while even in sterilized growth media,

three different AM fungi failed to proliferate within a simpleorganic patch (Hodge, 2001).

Although the AM external mycelium is the phase which is incontact with the soil, a reduction in internal mycorrhizal col-onization is often observed in fertile soils due to the enhancednutritional status of the plant (Sanders, 1975; Thomson et al.,1986; Braunberger et al., 1991). Duke et al. (1994) also reporteda local reduction in arbuscule frequency in field-grown Agro-pyron desertorum roots present in nutrient-rich patches con-taining KH2PO4 and NH4NO3. In this study, mycorrhizalcolonization in the middle section was not influenced by thespatial distribution of the organic material even though the pres-ence of the G. hoi inoculum reduced root growth in this sectionand total plant N content overall. Similarly, colonization ofP. lanceolata roots by three different AM fungi had differingeffects on the N content of their host but internal colonizationwas no different in roots experiencing a glycine patch, comparedto a water control patch even though overall colonization levelsdiffered among the AM fungi tested (Hodge, 2001). Theseresults imply that internal mycorrhizal colonization levels ofroots are generally unresponsive to N-rich patches in soil.

Decomposition of, and plant N capture from, the organic material

The organic material released more N when dispersed thanwhen added as a discrete patch. Of the N originally addedonly 35% was recovered from the soil receiving the dispersedmaterial compared with nearly twice as much (i.e. 63%)recovered from the patch soil, although levels of 13C recoverywere similar. The 15N and 13C recovered from the tubesreceiving the dispersed treatment were multiplied by 2 as onlythe middle section was analysed. This method assumed thatdecomposition of the organic material in the top and middlesoil section was similar and also did not allow for movementof N throughout the soil profile. At harvest, 71% (discretepatch) and 52% (dispersed) of the 15N originally added wasdetected in the soil-plant system, suggesting that some of theoriginal N may have been lost via denitrification and leaching.As previously discussed microbial release of N, predominantlyas NO3

–, from the organic material was probably importantin this study, supported by the fact that only 15N and no 13Cenrichment was detected in the plant tissue. Therefore, themore rapid release of N from the dispersed organic materialwas presumably due to the larger surface area accessible formicrobial decomposition. As NO3

– is highly mobile in soil itis likely that there was movement of this released N throughthe soil profile, and any NO3

– ions not captured by the plantswould be leached and lost from the soil-plant system. Thedispersion of the organic material may also have favouredmore rapid microbial turnover as once the smaller fragmentsof the organic material had been utilized the local increase inmicrobial growth would no longer be supported. In the patchzone due to the greater concentration of nutrients microbial


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population densities would be expected to be higher andsustained for longer, thus delaying inorganic N release intothe soil-plant system. It was surprising that the presence of theG. hoi inoculum did not influence N or C release from theorganic material as, by virtue of their size, AM hyphae shouldhave been able to penetrate to the sites of inorganic N releaseand compete with the other soil microorganisms for thereleased N. Furthermore, the presence of G. hoi has previouslybeen shown to enhance decomposition of an organic patch(Hodge et al., 2001). However, in that study the AM hyphaeproliferated extensively throughout the organic material.

In close agreement with the decomposition data, but incontrast to the predictions of hypothesis (iii), plant N capturefrom the dispersed organic material was more than twice thatfrom the discrete patch. Bonkowski et al. (2000) found thatL. perenne plants captured more N from organic materialadded as a discrete patch than that which had been moreevenly dispersed throughout the soil, whereas Hodge et al.(2000b) found no difference in plant N capture by eitherL. perenne or Poa pratensis regardless of the spatial distributionof organic material. Similarly Cui & Caldwell (1996) foundno difference in plant 15NO3

– acquisition due to N distribu-tion patterns. These contrasting results may be related to awide range of experimental conditions including the degree ofcompetition between plants and microorganisms, the C : Nratio of the material added, the amount of N added relative tothe background concentrations and experimental time scales.Plants captured more N from organic patches of varyingphysical and chemical complexity when the patches had a lowC : N ratio; the patches with a higher C : N ratio were stillbeing decomposed by the microbial community (Hodge et al.,2000c,d). The contribution that this captured N makes tototal plant N, rather than the amount captured per se, willdetermine the response of the plant and the sensitivity of rootsto the spatial scale of the organic material placement. In thestudy by Bonkowski et al. (2000) N capture from the organicmaterial was only 9% of the N originally added but this con-situted 15% of the total plant N. In contrast, in the study byHodge et al. (2000b) plant N capture from the organic mate-rial was higher (i.e. 26%) but represented only 1% of the totalplant N, which may also explain why root growth did notrespond to the spatial placement of organic material as levelswere insufficiently different to background to be detected asN-rich zones. In the present study, of the N originally addedin the organic material, 8% was captured from the patch treat-ment compared to 17% capture from the dispersed treatmentwhich corresponded to 2.0% and 4.1% of the total plant N,respectively. Although these values appear low, particularlyin the case of the patch treatment, they were apparentlysufficient to evoke enhanced root growth in response to theorganic material (Fig. 2a) and this was related to N capturefrom the added material (Fig. 6).

Hypothesis (iii) also predicted that more N would be cap-tured from the dispersed organic material treatment when the

AM inoculum was present due to the spatial placement of thefungal hyphae. It is well established that AM fungi benefittheir host plant by capturing immobile phosphate sourcesby exploiting a larger volume of soil outwith the nutrientdepletion zone around the root surface (Smith & Read, 1997).In this study however, neither AM colonization nor externalhyphal length densities influenced N capture from the organicmaterial regardless of its spatial placement. As shown by thedecomposition data the dispersed organic material wasdecomposing rapidly and releasing N, presumably as NO3

–.As NO3

– is much more mobile in soil it would readily be avail-able for plant N capture regardless of its initial placement inthe tube. Similarly, Cui & Caldwell (1996) observed that AMcolonization did not influence plant NO3

– acquisition despitedifferences in N distribution patterns.

Conclusions

Few of the initial hypotheses tested were supported. Rootgrowth did increase in the sections containing the organicmaterial (hypothesis (i)) and although this was reduced in thepresence of the G. hoi inoculum it was not replaced by AMhyphal proliferation (hypothesis (ii)). Contrary to hypothesis(iii) plants captured more than double the amount of N fromthe dispersed organic material than from that added as adiscrete patch, and the presence of G. hoi did not enhance Ncapture from the dispersed organic material. The final hypo-thesis was also not supported, as while Lolium monoculturesdid produce greater root lengths overall, it was only whengrown in interspecific competition that increased root lengthconferred a competitive advantage. The influence of theneighbouring plant upon N capture from the organic materialwas clearly seen in the shoot data (Fig. 4) with Plantagocapturing less N when grown with Lolium. In addition, it wasparticularly striking that despite the differences in AMinternal colonization and external hyphal length densitiesproduced, and the differences between Lolium and Plantagoin root architecture, G. hoi had the same effect on both plantspecies: it decreased N capture from the organic materialwhen plants were grown in monoculture and increased Ncapture when in interspecific plant competition. Themechanism by which G. hoi exerted this differing influenceon plant N capture from the organic material however, isunclear. Tibbett (2000) suggested that root proliferationresponses to soil heterogeneity had been overestimated as theirassociated mycorrhizal partner would proliferate hyphae instead.In this study roots responded to the spatial distribution of theorganic material added but AM hyphae did not. Furthermore,the response of the roots was demonstrably more important inN capture from the organic material than that of the fungalpartner (Fig. 6). In a previous study, three different AM fungidid not respond to a glycine patch by the proliferation ofhyphae but the roots of their host did (Hodge, 2001). In thestudy by Hodge et al. (2001) where the fungus did benefit its


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host in terms of N capture from the organic material, only theAM hyphae were allowed access to the organic material. Thus,when both the fungus and its associated host are experiencingan N-rich patch the response of the plant appears moreimportant, whereas when roots are excluded the response ofthe AM fungus may be more important. In the soilenvironment both situations may arise.

Acknowledgements

A. H. is funded by a BBSRC David Phillips Fellowship. Ithank C. Scrimgeour and W. Stein (Scottish Crop ResearchInstitute, Invergowrie, Dundee UK) for conducting the massspectrometry analysis and A. H. Fitter, I. J. Alexander, K. S.Pregitzer and an anonymous referee for their perceptivecomments on the manuscript.

References

Ames RN, Reid CPP, Porter L, Cambardella C. 1983. Hyphal uptake and transport of nitrogen from two 15N-labelled sources by Glomus mosseae, a vesicular-arbuscular mycorrhizal fungus. New Phytologist 95: 381–396.

Bending GD, Read DJ. 1995. The structure and function of the vegetative mycelium of ectomycorrhizal plants. V. Foraging behaviour and translocation of nutrients from exploited litter. New Phytologist 130: 401–409.

Bonkowski M, Griffiths BS, Scrimgeour C. 2000. Substrate heterogeneity and microfauna in soil organic hotspots as determinants of nitrogen capture and growth of rye-grass. Applied Soil Ecology 14: 37–53.

Braunberger PG, Miller MH, Peterson RL. 1991. Effect of phosphorus nutrition on morphological characteristics on vesicular–arbuscular mycorrhizal colonization of maize. New Phytologist 119: 107–113.

Cahill JF, Casper BB. 1999. Growth consequences of soil heterogeneity for two old-field herbs, Ambrosia artemisiifolia and Phytolacca americana, grown individually and in combination. Annals of Botany 83: 471–478.

Cliquet JB, Murray PJ, Boucaud J. 1997. Effect of the arbuscular mycorrhizal fungus Glomus fasciculatum on the uptake of amino nitrogen by Lolium perenne. New Phytologist 137: 345–349.

Cui M, Caldwell MM. 1996. Facilitation of plant phosphate acquisition by arbuscular mycorrhizas from enriched soil patches. I. Roots and hyphae exploiting the same soil volume. New Phytologist 133: 453–460.

Drew MC. 1975. Comparison of the effects of a localized supply of phosphate, nitrate, ammonium and potassium on the growth of the seminal root system, and the shoot, in barley. New Phytologist 75: 479–490.

Duke SE, Jackson RB, Caldwell MM. 1994. Local reduction of mycorrhizal arbuscule frequency in enriched soil microsites. Canadian Journal of Botany 72: 998–1001.

Eissenstat DM, Caldwell JP. 1988. Seasonal timing of root growth in favourable microsites. Ecology 69: 870–873.

Fitter AH. 1991. Costs and benefits of mycorrhizas: implications for functioning under natural conditions. Experientia 47: 350–355.

Fitter AH, Hodge A, Robinson D. 2000. Plant response to patchy soils. In: Hutchings MJ, John EA, Stewart AJA, eds. The ecological consequences of environmental heterogeneity. Oxford, UK: Blackwell Science Ltd, 71–90.

Fransen B, de Kroon H, Berendse F. 1998. Root morphological plasticity and nutrient acquisition of perennial grass species from habitats of different availability. Oecologia 115: 351–358.

Gersani M, Sachs T. 1992. Developmental correlations between roots in heterogeneous environments. Plant, Cell & Environment 15: 463–469.

Hepper CM, Warner A. 1983. Role of organic matter in growth of a

vesicular–arbuscular mycorrhizal fungus in soil. Transactions of the British Mycological Society 81: 155–156.

Hodge A. 2001. Arbuscular mycorrhizal fungi influence decomposition of, but not plant nutrient capture from, glycine patches in soil. New Phytologist 151: 725–734.

Hodge A, Campbell CD, Fitter AH. 2001. An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material. Nature 413: 297–299.

Hodge A, Robinson D, Fitter AH. 2000a. An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient-rich patches in soil. New Phytologist 145: 575–584.

Hodge A, Robinson D, Griffiths BS, Fitter AH. 1999a. Why plants bother: root proliferation results in increased nitrogen capture from an organic patch when two grasses compete. Plant, Cell & Environment 22: 811–820.

Hodge A, Robinson D, Griffiths BS, Fitter AH. 1999b. Nitrogen capture by plants grown in N-rich organic patches of contrasting size and strength. Journal of Experimental Botany 50: 1243–1252.

Hodge A, Stewart J, Robinson D, Griffiths BS, Fitter AH. 1998. Root proliferation, soil fauna and plant nitrogen capture from nutrient-rich patches in soil. New Phytologist 139: 479–494.

Hodge A, Stewart J, Robinson D, Griffiths BS, Fitter AH. 2000b. Spatial and physical heterogeneity of N supply from soil does not influence N capture by two grass species. Functional Ecology 14: 645–653.

Hodge A, Stewart J, Robinson D, Griffiths BS, Fitter AH. 2000c. Competition between roots and soil micro-organisms for nutrients from nitrogen-rich patches of varying complexity. Journal of Ecology 88: 150–164.

Hodge A, Stewart J, Robinson D, Griffiths BS, Fitter AH. 2000d. Plant N capture and microfaunal dynamics from decomposing grass and earthworm residues in soil. Soil Biology and Biochemistry 32: 1763–1772.

Kormanik PP, McGraw A-C. 1982. Quantification of vesicular–arbuscular mycorrhizae in plant roots. In: Schenck NC, ed. Methods and principles of mycorrhizal research. St Paul, MN, USA: American Phytopathological Society. 37–46.

Mäder P, Vierheilig H, Streitwolf-Engel R, Boller T, Frey B, Christie P, Wiemken A. 2000. Transport of 15N from a soil compartment separated by a polytetrafluoroethylene membrane to plant roots via the hyphae of arbuscular mycorrhizal fungi. New Phytologist 146: 155–161.

McGonigle TP, Miller MH, Evans DG, Rairchild GL, Swan JA. 1990. A new method which gives an objective measure of colonization of roots by vesicular–arbscular mycorrhizal fungi. New Phytologist 15: 495–501.

Merryweather JW, Fitter AH. 1991. A modified method for elucidating the structure of the fungal partner in vesicular–arbuscular mycorrhiza. Mycological Research 95: 1435–1437.

Miller RM, Jastrow JD. 1992. Extraradical hyphal development of vesicular–arbuscular mycorrhizal fungi in a chronosequence of prairie restoration. In: Read DJ, Lewis DH, Fitter AH, Alexander IJ, eds. Mycorrhizas in ecosystems. Wallingford, UK: CAB International, 171–176.

Mosse B. 1959. Observations on the extramatrical mycelium of a vesicular–arbuscular endophyte. Transactions of the British Mycological Society 42: 439–448.

Näsholm T, Ekblad A, Nordin A, Giesler R, Högberg M, Högberg P. 1998. Boreal forest plants take up organic nitrogen. Nature 392: 914–916.

Newsham KK, Fitter AH, Watkinson AR. 1995a. Arbuscular mycorrhiza protect an annual grass from root pathogenic fungi in the field. Journal of Ecology 83: 991–1000.

Newsham KK, Fitter AH, Watkinson AR. 1995b. Multi-functionality and biodiversity in arbuscular mycorrhizas. Trends in Ecology and Evolution 10: 407–411.

Nicolson TH. 1959. Mycorrhiza in the Gramineae. I. Vesicular-arbuscular endophytes, with special reference to the external phase. Transactions of the British Mycological Society 42: 421–438.

Pregitzer KS, Kubiske ME, Yu CK, Hendrick RL. 1997. Relationships among root branch order, carbon, and nitrogen in four temperate species. Oecologia 111: 302–308.


Research314

Ritz K, Millar SM, Crawford JW. 1996. Detailed visualization of hyphal distribution in fungal mycelia growing in heterogeneous nutritional environments. Journal of Microbiological Methods 25: 23–28.

Robinson D. 1994. The responses of plants to non-uniform supplies of nutrients. New Phytologist 127: 635–674.

Robinson D, Hodge A, Griffiths BS, Fitter AH. 1999. Plant root proliferation in nitrogen-rich patches confers competitive advantage. Proceedings of the Royal Society (London) Series B 266: 431–435.

Robinson D, Rorison IH. 1983. A comparison of the responses of Lolium perenne L., Holcus lanatus L. & Deschampsia flexuosa (L.) Trin. to a localized supply of nitrogen. New Phytologist 94: 263–273.

Sanders FE. 1975. The effect of foliar-applied phosphate on the mycorrhizal infections of onion roots. In: Sanders FE, Mosse B, Tinker PB, eds. Endomycorrhizas, London, UK: Academic Press, 261–276.

Smith SE, Read DJ. 1997. Mycorrhizal symbiosis. London, UK: Academic Press Ltd.

St John TV, Coleman DC, Reid CPP. 1983. Association of

vesicular–arbuscular mycorrhizal hyphae with soil organic particles. Ecology 64: 957–959.

Staddon PL, Fitter AH, Graves JD. 1999. Effect of elevated atmospheric CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal fungus Glomus mosseae. Global Change Biology 5: 347–358.

Thomson BD, Robson AD, Abbott LK. 1986. Effects of phosphorus on the formation of mycorrhizas by Gigaspora calospora and Glomus fasciculatum in relation to root carbohydrates. New Phytologist 103: 751–765.

Tibbett M. 2000. Roots, foraging and the exploitation of soil nutrient patches: the role of mycorrhizal symbiosis. Functional Ecology 14: 397–399.

Tinker PB, Nye PH. 2000. Solute movement in the rhizosphere. Oxford, UK: Oxford University Press.

van Vuuren MMI, Robinson D, Griffiths BS. 1996. Nutrient inflow and root proliferation during the exploitation of a temporally and spatially discrete source of nitrogen in soil. Plant and Soil 178: 185–192.

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