effect of peat on mycorrhizal colonization and effectiveness of the arbuscular mycorrhizal fungus...

© 2007 Japanese Society of Soil Science and Plant Nutrition

Soil Science and Plant Nutrition (2007) 53, 744–752 doi: 10.1111/j.1747-0765.2007.00204.x

Blackwell Publishing LtdORIGINAL ARTICLEEffect of peat on mycorrhizal colonizationORIGINAL ARTICLE

Effect of peat on mycorrhizal colonization and effectiveness of the arbuscular mycorrhizal fungus Gigaspora margarita

Nan MA1, Kazuhira YOKOYAMA2 and Takuya MARUMOTO2

1College of Resources Science and Technology, Beijing Normal University, Beijing 100875, China and 2Department of Biological Science, Faculty of Agriculture, Yamaguchi University, Yamaguchi 753-8515, Japan

Abstract

The influence of the addition of Chinese peat and Canadian peat on arbuscular mycorrhizal colonization,mycorrhizal effectiveness and host-plant growth was investigated in a pot experiment. Chinese peat orCanadian peat was mixed with Masa soil (weathered granite soil) at different levels (0, 25, 50, 100, 150 or200 g kg–1) into which an arbuscular mycorrhizal fungus (AMF) Gigaspora margarita Becker & Hall wasinoculated, and seedlings of Miscanthus sinensis Anderess were planted. There was a significant increase inplant growth with increasing amounts of Chinese peat. The growth-promoting effect of the AMF on thehost was enhanced when the addition of Chinese peat was increased from 25 to 100 g kg–1. Root coloniza-tion and the number of spores proliferating increased with increases at low levels of Chinese peat (from25 to 100 g kg–1), and decreased gradually with higher Chinese peat increments. Although plant growth androot colonization with the addition of Canadian peat increased slightly, Canadian peat suppressed mycor-rhizal effectiveness. In contrast to Canadian peat, the addition of Chinese peat improved considerably thephysical and chemical properties of the soil, which might result in the promotion of AM formation andmycorrhizal effectiveness.

Key words: arbuscular mycorrhizal fungi, Gigaspora margarita, mycorrhizal colonization, mycorrhizal effectiveness, peat addition.

INTRODUCTION

Organic matter in soil clearly affects different soilorganisms and processes, but its impact on arbuscularmycorrhizal fungi (AMF) has not been studied toany great extent (Vestberg et al. 2005). Organic matterenhances soil fertility and stimulates the development ofAMF in soil (Hepper and Warner 1983; Joner andJakobsen 1995).

Peat is produced when incompletely decayed plants,including species of sedges, grasses and mosses, accumulateunder cool temperatures and conditions of decreasedoxygen and nutrient levels (Wang et al. 2001). Peat isoften the major component of potting mixes, and itsspecific characteristics exert a significant influence onAMF (Linderman and Davis 2003). There are studies

showing both negative (Wang et al. 1993) and positive(Biermann and Linderman 1983; Graham and Timmer1984) impacts of peat on AMF. Linderman and Davis(2003) indicated that the interactions of fungal isolates,peat types and levels of peat application to the mediumcould be specific. However, in studies related to theeffect of peat on AM formation, Canadian peat is thesubstrate most frequently used. The effect of Chinesepeat on AM formation is poorly documented. Chinahas abundant peat resources, amounting to more than46 hundred million tons (Wang et al. 2001). Studies onthe use of peat have been carried out for agriculture,horticulture and reforestation programs over a longperiod of time in China. Chinese peat is expected tobecome a valuable fertilizer in reforestation programs indegraded soil and maximize the value of arid land inChina. Peat properties vary considerably depending onthe location. Plant species, climatic conditions andhydrological factors affect the distinct characteristics ofpeat. Chinese peat is mostly composed of herbage, andarbor and moss account for 1%. In contrast, Canadianpeat is mostly composed of moss (Wang et al. 2001).Therefore, there are large differences in the physical and

Correspondence: K. YOKOYAMA, Department of BiologicalScience, Faculty of Agriculture, Yamaguchi University,1677-1, Yoshida, Yamaguchi 753-8515, Japan. Email:[email protected] 19 July 2007.Accepted for publication 21 August 2007.

Effect of peat on mycorrhizal colonization 745


chemical properties between Chinese peat and Canadianpeat (Wang et al. 2001).

All peats are not equal in physical, chemical andbiological characteristics, so one cannot predict theoutcome of mycorrhizal inoculations into mediawith peat variation (Linderman and Davis 2003). Theimprovement by Chinese peat of soil physical andchemical properties, such as aeration, water retentionand nutrient status, suggest that Chinese peat has apositive effect on the infection of roots by AM fungiand the growth of the host plant (Ma et al. 2006a).Furthermore, it has been shown that an extract solutionof Chinese peat improved spore germination and hyphalgrowth, whereas an extract solution of Canadian peathad a negative effect (Ma et al. 2006b). Based on theresults of the effect of Chinese peat and Canadianpeat on presymbiotic growth (spore germination andmycelial development) (Ma et al. 2006b), the furtherstudies on the effects of different peat on symbioticgrowth and the key factor of the effects were expectedin our experiment.

The objective of the present study was to investigatethe effect of different peat additions on the establish-ment and performance of AM in soil and the growthof plants, and to further identify the key factor of theeffect by comparing Chinese and Canadian peat.

MATERIALS AND METHODS

Soil amendments

Masa soil, a weathered granite soil, was sampled fromthe lower layers of mountain soil in Fukuoka and iscommercially available in Japan (Kakeuma Sangyo,Fukuoka, Japan). Masa soil is distributed in manyareas in Japan. The soil is poor with respect to aeration,water-holding capacity and nutrients. It is necessary toadd organic matter to improve the properties of Masasoil for afforestation. Peat is a suitable material for theimprovement of the physical and chemical propertiesof Masa soil (Koshimizu 1986). Two types of peat,Chinese peat and Canadian peat, which are commerciallyavailable in Japan, were used in the present study. Onewas a peat sample purchased from Nibannsu, Japan(Nievance Co. Ltd., Fukuoka, Japan). The other wasa peat moss sample purchased in Japan (Origin:Canada). Chinese peat was referred to as peat andCanadian peat was referred to as peat moss in thepresent experiment. Masa soil was passed through a2-mm mesh sieve before use, autoclaved twice at 121°Cfor 30 min. Peat was autoclaved twice at 80°C for30 min.

Soil amendment consisted of six treatments, includingthe addition of 0, 25, 50, 100, 150 or 200 g kg–1 peat or

peat moss (0, 2.5, 5, 10, 15 or 20% hereafter). Theexperiment was carried out with three replicationsper treatment.

Inoculum of AM fungiThe AM fungi used in the present study were obtainedas spores of Gigaspora margarita MAFF520054 fromthe National Institute of Agrobiological Sciences,Tsukuba, Japan. Spores were multiplied in pot cultureson white clover for 3 months (Saito 2001). Soil sampleswith spores were collected and stored at 4°C untilrequired. Newly developed spores were extracted bywet sieving and decanting 5 days before inoculation(Gerdemann and Nicolson 1963). Spores were pickedup individually with pipettes, washed in sterile deionizedwater and stored.

PlantThe seeds of Miscanthus sinensis Anderess were purchasedfrom Kaneko (Hiroshima, Japan). The seeds weresterilized in a 10% NaClO solution for approximately40 min and rinsed thoroughly with sterile deionizedwater and then germinated on moist filter paper in ster-ilized Petri dishes at 25°C. The seeds germinated afterapproximately 2 weeks in the growth chamber. Thethree seedlings that were either inoculated with 10spores (inoculated plants) or not inoculated (non-inoculatedplants) were put into plastic pots with a volume of87.45 cm3. The inoculated seedlings in one pot wereplanted by putting the 10 spores near the roots ofthe three seedlings. Thereafter, the pots were placedin transparent Sunbags (44.0 cm × 20.5 cm) (Sigma,St Louis, MO, USA) in a growth chamber. Light(44.7 μmol m–2 s–1) was provided under a photoperiodof 12 h. Temperature was 25 ± 2°C (all day). Steriledeionized water was supplied every 6 days.

Determination of plant growth and root colonizationin each treatment were carried out at 40, 60, 90 and120 days after inoculation. Plants were harvested bycutting the shoots at the soil surface. All collected potswith soil samples at harvest time were stored at 4°Cuntil required. Shoot fresh weight and root fresh weightwere recorded immediately. Shoot dry weight wasrecorded after the plant was dried for 72 h in an oven at80°C. The relative mycorrhizal effectiveness (RME),that is, the mycorrhizal contribution to the growth ofthe mycorrhizal plant, was defined by the followingformula: RMEDW(%) = [(Ymyc+ – Ymyc–)/(Ymyc+)] × 100where Ymyc+ and Ymyc– are the shoot dry weights of themycorrhizal treatment with and without AM fungi,respectively (Vestberg et al. 2005).

The roots were rinsed and stained with trypan blue,and the percentage of root colonization was assessedusing the grid intersect method (Giovannetti and Mosse

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1980). The number of spores was counted using thewet sieving method (Gerdemann and Nicolson 1963).

Soil analysesSoils amended with peat were analyzed at the onset ofthe study for nutrient content. Maximum water-holdingcapacity was measured using the Hilgard method. Bulkdensity was measured using the core method (Black et al.1965). The pH was measured using a pH electrode meter(Dojyo hyojun bunseki sokutei hou iinkai 1986). Theelectrical conductivity was measured using an electricalconductivity meter (Dojyo hyojun bunseki sokutei houiinkai 1986). Total N content was determined using theKjeldahl method (Dojyo hyojun bunseki sokutei houiinkai 1986). Total organic C content was determinedusing the Tyurin method (Dojyo hyojun bunseki sokuteihou iinkai 1986). The concentration of available Pwas determined using the Truog method (Dojyo hyojunbunseki sokutei hou iinkai 1986). The physical andchemical properties of the soil media are listed in Table 1.

Statistical analysisAll treatments were arranged as follows: inoculatedtreatment, non-inoculated treatment and rate of peataddition. Each treatment was replicated three times.Differences between the treatments were determinedusing an anova and a Tukey test (P < 0.05). Statisticalcomparisons were considered significant at P < 0.05.

RESULTSPlant growth

There were significant differences in the shoot freshweight among the treatments at 90 and 120 days. In

the treatments with the addition of peat, there was asignificant increase in the shoot fresh weight of thenon-inoculated plants when peat addition increased(Fig. 1a,b). The value of the shoot fresh weight of thenon-inoculated plants with 15% of peat addition washighest at 120 days, but there were no significantdifferences between the values of 10 and 20% of peataddition (Fig. 1b).

With increasing levels of peat, the shoot fresh weightof the inoculated plants in the treatments with peatincreased significantly at 90 and 120 days (Fig. 1a,b).The highest value of the shoot fresh weight wasobserved with the addition of 10% peat during theculture period. There were no significant differencesbetween the values of 10 and 15% of peat addition.There was a slight decrease in shoot fresh weight whenthe peat addition was above 10%. The value of theshoot fresh weight with 20% of peat addition was similarto that with 5% of peat addition.

Meanwhile, in the treatments with the addition ofpeat moss, there was a slight increase in the shoot freshweight of the non-inoculated plants when peat mossaddition increased at 90 and 120 days (Fig. 1c,d). Thevalue of the shoot fresh weight of the non-inoculatedplants with 15% of peat moss addition was highest, butthere were no significant differences between the valuesof 10 and 15% of peat addition at 90 days.

With increasing levels of peat moss, the shoot freshweight of the inoculated plants in the treatments withthe addition of peat moss slightly increased (Fig. 1c,d).The plants in 15% exhibited the highest shoot freshweight. However, there were no significant differencesamong the values of 2.5, 5, 10, 15 and 20% of peatmoss addition.

Table 1 Physical and chemical properties of the soil medium

pHEC

ms cm–1

MWHCg kg–1

Bulk densityg cm–3

Available Pmg kg–1

Total Ng kg–1

Total Organic Cg kg–1

Masa soil 6.2 0.027 302.0 1.24 36.0 0.08 0.42.5% peat† 5.4 0.082 380.0 1.12 36.3 0.67 14.15% peat† 5.2 0.131 452.0 1.08 36.8 1.27 27.210% peat† 4.9 0.228 619.0 0.89 37.6 2.48 41.215% peat† 4.8 0.337 783.0 0.77 38.4 3.73 84.120% peat† 4.6 0.404 951.0 0.66 39.3 4.99 129.3peat 4.5 0.131‡ 4225.0 0.17 56.0 29.80 469.0peat moss 3.6 0.07§ 5650.2 0.09 44.0 6.40 329.12.5% peat moss† 4.7 0.035 328.4 1.06 36.0 0.11 7.15% peat moss† 4.3 0.052 567.4 0.85 36.2 0.23 14.310% peat moss† 4.0 0.088 812.1 0.65 36.3 0.46 25.215% peat moss† 3.9 0.117 952.7 0.53 36.8 0.75 39.220% peat moss† 3.8 0.157 1460.1 0.44 37.0 1.1 54.1

†These values correspond to 0, 25, 50, 100, 150 and 200 g kg–1 addition of peat or peat moss to Masa soil. ‡These values were measured using a water ratio of 1:10. §These values were measured using a water ratio of 1:20. EC, electrical conductivity; MWHC, mean water-holding capacity.



The shoot fresh weights of non-inoculated plantswith peat were similar to those with peat moss, but thefresh weights of plants in 15 and 20% of peat werehigher than those with peat moss. For the inoculatedplants, the values of the plants in each peat additionwere significantly higher than those in each peat mossaddition.

Mycorrhizal effectivenessComparable differences in shoot fresh weight betweenthe non-inoculated plants and the inoculated plantsclarified the mycorrhizal contribution to the growth ofthe mycorrhizal plant. The estimated mean for RMEwas positive in the treatments with the addition of peat(Fig. 2a). With increasing levels of peat addition from2.5 to 10%, the RME increased gradually. The highestvalue, which was more than 50%, was in the 10%addition during the culture period. The RME in the20% addition was lower than that in the 0% addition.

At the same time, in the treatments with the additionof peat moss, the values of the shoot fresh weight of theinoculated plants were lower than the values of the non-inoculated plants. The estimated mean for RME was nega-tive in the treatments with the addition of peat moss (Fig. 2b).With increasing levels of peat moss addition from 2.5to 15%, the RME decreased gradually. The lowestRME was observed in the 15% addition at 90 days,and in the 10% addition at 120 days. The decrease inAMF effectiveness in the treatments with peat mossresulted from a strong negative effect on RME (Fig. 2b).

In the Masa soil (0% addition), the shoot freshweight of the inoculated plants was slightly higher thanthe fresh weight of the non-inoculated plants, and theRME was approximately 15%.

Arbuscular mycorrhizal formationWith the addition of peat, the time-course of culturefrom the onset of the experiment to 60 days revealed an

Figure 1 Shoot fresh weight of plants with the addition of peat at (a) 90 days and (b) 120 days. Shoot fresh weight of plants withthe addition of peat moss at (a) 90 days and (b) 120 days. Bars represent standard deviations of the means of three replicates. Peator peat moss addition rate: 0, 2.5, 5, 10, 15 and 20% (w/w). Means followed by the same letter are not significantly different(P < 0.05) according to a Tukey test and a comparison between peat addition levels.

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increase in the root colonization of the plants in alltreatments (Fig. 3a). Thereafter, root colonizationremained stable in all treatments except for those inthe Masa soil (0% addition) from 60 days to 120 days.Root colonization of the plants in the peat additionranged from 40 to 80%, and the colonization of theplants in Masa soil (0% addition) ranged from 10to 25% from 60 to 120 days. The beneficial effectof peat addition on mycorrhizal colonization waspronounced, as evidenced by the high level of rootcolonization.

At each harvest time, root colonization increasedslightly with increasing peat moss addition (Fig. 3b).Root colonization of the plants in the peat moss addi-tion ranged from 10 to 40% during the culture period.However, there were no significant differences in rootcolonization among the 5, 10, 15 or 20% treatments ofpeat moss addition at 60, 90 and 120 days.

Similarly, there was a significant increase in the totalroot length of the mycorrhizal plants when the peataddition increased from 2.5 to 10% (Fig. 4a). And thecolonized root length of the plants in all treatments ofpeat addition increased as the culture time increased

(Fig. 4b). The high root colonization with the additionof peat most likely resulted from the increasing colo-nized root length when total root length increasedfrom 60 days to 120 days (Fig. 4a,b). In contrast, thedecrease in root colonization with the addition of peatmoss was probably because there was no change in thecolonized root length when total root length increasedfrom 60 days to 90 days (Fig. 4c,d).

The number of proliferating spores in the treatmentsconsisting of 0–20% peat addition increased until theend of the culture (Fig. 5a). The number of spores in thetreatments with 10% of peat addition was the highestat each harvest time. The lowest number was recordedin the Masa soil at each harvest time. From 90 to 120days, the spore number in the peat addition rangedfrom 190 to 680 pot–1.

In contrast to the peat addition, the number of pro-liferating spores in the treatments consisting of 0–20%of peat moss slightly increased at each harvest time(Fig. 5b). The number of spores in the plants with 15%of peat addition was the highest at each harvest time.From 90 to 120 days, the spore number in the peatmoss addition ranged from 100 to 200 pot–1.

Figure 2 (a) Effect of peat on the relative mycorrhizaleffectiveness in terms of mean shoot dry weight with theaddition of peat. (b) Effect of peat moss on the relativemycorrhizal effectiveness in terms of mean shoot fresh weightwith the addition of peat moss. The relative mycorrhizaleffectiveness (RME) was defined by the formula:RMEDW(%) = [(Ymyc+ – Ymyc–)/(Ymyc+)] × 100 (Ymyc+ and Ymyc– arethe shoot dry weights of the mycorrhizal treatment with andwithout AM fungi, respectively).

Figure 3 Root colonization for plants with the addition of(a) peat and (b) peat moss at each harvest time. Bars representstandard deviations of the means of three replicates. Thedifferences among the levels of peat or peat moss addition areindicated by anova. Means followed by the same letter are notsignificantly different (P < 0.05) according to a Tukey test anda comparison between peat addition levels on each samplingday.



DISCUSSION

In the present experiment, the observations confirmedthe overall beneficial effect of peat on plant growth.With an increase in the addition of Chinese peat,a stimulatory effect on shoot growth was observed.Furthermore, Chinese peat exhibited a strong positiveeffect on RME, and a stimulation peak was observedat the 10% addition (Fig. 2a). At the same time, therewas a significant increase in the root colonization per-centage and spore number with increasing Chinese peataddition. Thus, it is suggested that the incorporation ofChinese peat exerts a beneficial effect on AM formationand mycorrhizal effectiveness.

The beneficial effect of Canadian peat on plantgrowth was observed in the non-mycorrhizal plants.With increased additions of Canadian peat, the negativeeffect on mycorrhizal effectiveness was strengthened,although there was a slight increase in the root colo-nization and spore number with increasing Canadianpeat at each harvest time.

In the present study, the effects of Chinese peat andCanadian peat on AM formation and mycorrhizaleffectiveness were quite different. First, the Chinese peat

Figure 4 Total root length of the mycorrhizal plants with the addition of (a) peat and (c) peat moss at each harvest time. Colonizedroot length of the plants with the addition of (b) peat and (d) peat moss at each harvest time. Bars represent standard deviations ofthe means of three replicates.

Figure 5 Spore number of the plants with the addition of(a) peat and (b) peat moss at each harvest time. Bars representstandard deviations of the means of three replicates. Meansfollowed by the same letter are not significantly different(P < 0.05) according to a Tukey test and a comparisonbetween peat addition levels on each sampling day.

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had a stronger positive effect on AM formation thanCanadian peat. From 60 to 120 days, colonization ofthe plants in all Chinese peat additions ranged from 40to 80%, whereas the value in all Canadian peat addi-tions ranged from 10 to 40%. The spore numbers in2.5, 5, 10, 15 and 20% of Chinese peat addition were3.6, 4.3, 5.2, 3.0 and 4.1-fold higher than those afterthe addition of Canadian peat at 120 days, respectively.Second, Chinese peat exerted a promotive effect onmycorrhizal effectiveness, whereas Canadian peat hadan inhibitory impact. The shoot fresh weights of theinoculated plants in Chinese peat additions weresignificantly higher than those in Canadian peatadditions.

Peat has been reported to both stimulate (Wang et al.1993) and suppress (Biermann and Linderman 1983;Graham and Timmer 1984) root colonization. In thisexperiment, the effects of two types of peat on AM for-mation and mycorrhizal effectiveness were different.This finding is in agreement with the conclusion thatthe spread and efficiency of AMF as well as host-plantgrowth could be affected by the nature of the peat used(Linderman and Davis 2003; Ponton et al. 1990). Thepositive effect of Chinese peat on mycorrhizal varia-bles observed in this study differs from that seen inother studies, in which AM fungi have shown a negativeresponse to the incorporation of other peat in soil(Biermann and Linderman 1983; Calvet et al. 1992;Vestberg et al. 2000). Although AMF root colonizationwas significantly higher in peat-added soil than in non-added soil, the negative impact of peat on mycorrhizaleffectiveness may be because of the microbiologicalproperties of peat (Vestberg et al. 2005). The mechanismof the negative effect of peat on mycorrhizal formation andmycorrhizal effectiveness remains unknown (Lindermanand Davis 2003).

Soil factors, such as available nutrients, pH value,soil temperature, soil aeration and soil moisture, influ-ence AM formation (Liu and Li 2000). Chinese peat hasa physical and chemical function in that it promotesgood soil structure, improves aeration and moisture andincreases nutrient status, which are related to the devel-opment of AM symbiosis. Thus, it is very difficult toclarify the key factor at this early stage of research.Combined with the results of the effect of Chinese peat(Ma et al. 2006a), it was feasible to investigate the roleof bulk density and N and P content, which are themajor factors affecting the colonization of AMF. It hasoften been reported that soil aeration is closely relatedto the colonization of AMF because the AM fungi areaerobic (Liu and Li 2000). The availability of nutrientsin soils, in particular the P content and N content, hasbeen widely shown to affect the colonization of AMF.In our experiment, we focused on the soil physical

properties and soil nutrient conditions, such as P, N andorganic matter contents, by analyzing the differences inthese properties between Chinese peat and Canadianpeat (Table 1).

In the case of the soil physical properties, it is likelythat the ability of the AM fungus to spread and form ahyphal network in the substrates was influenced bytheir different physical properties, such as compactionand water retension (Gaur and Adholeya 2000). In ourexperiment, both the addition of Chinese peat andCanadian peat sharply reduced the bulk density of soil.Enhanced root colonization in the treatment of organicamendments may be related to better soil aeration. Theapplication of inorganic matter reduced soil bulkdensity and led to an increase in soil porosity, resultingin the elongation of the hyphae of the AM fungi and theelongation of the roots (Ezawa et al. 2002).

In contrast, bulk density of the potting media wasdecreased by the addition of peat, irrespective of theorigin of the peat. It appeared that the compaction ofthe media did not inhibit the growth of roots andfungal hyphae. Thus, the differences for mycorrhizalcolonization and plant growth between the two peatswould mainly result from differences in their chemicalproperties.

From the view of chemical properties, the largestdifference between Chinese peat and Canadian peat wasin the content of N. Although it has been suggested thatN suppresses root colonization by AMF (Buwalda andGoh 1982; Johnson et al. 1984), N addition has alsobeen reported to stimulate root colonization (Brownet al. 1981; Hepper 1983). In the present experiment, asthe addition of Chinese peat increased, the total Ncontent of the soil markedly increased (Table 1). Thus,increased N level might be one of the key factors in thestimulatory effect on root colonization and plantgrowth. Combined with the results of spore germina-tion (Ma et al. 2006b), this result could be explained, inpart, by the result that the content of N in the peatextract solution had a positive effect on the germinationand hyphal growth of spores, which would increasethe chances of encounters between roots and infectivehyphae.

The P content in soil is one of the major factorsaffecting the colonization of AMF (Smith and Read1997). It is generally recognized that P fertilizer oftenadversely affects root colonization of many host speciesby AMF (Abbott et al. 1984; Miranda and Harris 1994).In our experiment, as the addition of Chinese peatincreased, the P content of soil slightly increased inthe soil media (Table 1). Compared with the treatmentof Masa soil, root colonization of plants with the addi-tion of Chinese peat or Canadian peat increased. Thus,it is reasonable to assume that the low level of P in the



organic matter did not depress the colonization of thesefungi in our experiment.

The amount of organic matter in the media might beanother factor to consider. Organic matter has beenfound to stimulate hyphal growth (Joner and Jakobsen1995). Some evidence has indicated that plants inorganic manure-amended soil often have increasedAM colonization levels (Douds et al. 1997; Ryan et al.1994). Chemical exudates released from organic matterwere considered to be one of the factors in the effect oforganic manure amendments on AM fungi (Ryan et al.1994). The effects of organic compounds on the growthof AM fungi in soil vary according to the chemicalcomposition of the substrate (Ravnskov et al. 1999). It ispossible that the different compositions of Chinesepeat and Canadian peat led to different effects onAM formation.

In conclusion, the present study indicated that theapplication of Chinese peat to soil might promote plantgrowth and stimulate mycorrhizal colonization andenhance mycorrhizal effectiveness. In contrast, Canadianpeat had a negative effect on mycorrhizal effectiveness.The effect of these two types of peat on AM formationand mycorrhizal effectiveness were different because oftheir different physical and chemical properties. Thepossible influence of peat on mycorrhizal developmentmay be explained, in part, by the direct effect of peaton the germination of the spores of AM fungi. Com-bined with the results of the stimulatory effect of peaton spore germination and hyphal growth (Ma et al.2006b), Chinese peat improved spore germination andhyphal extension, which increased the chances of rootpenetration, leading to rapid colonization. In contrast,Canadian peat suppressed spore growth and the slowerhyphal extension decreased the speed of colonization.Symbiotic efficiency is affected by plant and fungal spe-cies as well as environmental conditions (Declerck et al.1995). The different affect of these two types of peat onmycorrhizal effectiveness may be related to certainqualities of the peat, which modified the physicochemi-cal properties of the soil and affected the role of AMF.However, the mechanism of the effect of peat onmycorrhizal effectiveness remains unknown. Furtherstudies will be required to clarify the components that arerelated to mycorrhizal colonization and mycorrhizaleffectiveness. The key factors of the effect on AM for-mation and mycorrhizal effectiveness may shed light onhow to select suitable peat for reforestation programs.Moreover, mycorrhization with suitable species of AMfungi and the application of peat in small doses canhave cumulative positive effects on plant growth. Thecombined use of a suitable peat and AMF could becomea useful revegetation technique for successful reforesta-tion programs of degraded soil in the future.

ACKNOWLEDGMENTS

We thank Dr Takahiro Tateishi, Iwate University, forvaluable comments on the study. We also thank MrKimio Nakamura, Masanori Yumura and TakumiMitsuno for their assistance with the work and for helpfulsuggestions for the study.

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effect of peat on mycorrhizal colonization and effectiveness of the arbuscular mycorrhizal fungus...

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