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Keywords:

  • Paxillus involutus;
  • Suillus variegatus;
  • Ectomycorrhiza;
  • Biotite;
  • Microcline;
  • Apatite;
  • Bacterium;
  • Fatty acid;
  • Thymidine incorporation

Abstract

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

The influence of ectomycorrhizal fungi on the soil bacterial community was studied by growing pine seedlings in artificial soils consisting of a peat/sand mixture amended with microcline, biotite or apatite. In the microcline-amended and unamended soils both Suillus variegatus and Paxillus involutus reduced bacterial activity as measured by thymidine incorporation. S. variegatus grew best in the biotite soil, where it increased both bacterial activity and biomass as measured by microscopic counts and specific bacterial fatty acids. Further, the positive influence of S. variegatus on the bacteria in the biotite soil modified the bacterial community, as reflected in the bacteria-specific phospholipid fatty acid composition. The increases in bacterial biomass and activity and changes in the bacterial community induced by S. variegatus may be due to the production of organic substances by this fungus, as indicated by an 10-fold increase in soil-solution citric acid. Two isolates of S. variegatus and an unidentified ectomycorrhizal fungus all tended to stimulate bacterial activity in the apatite-amended soil in compartments isolated from roots by a mesh. We conclude that the same ectomycorrhizal fungus may stimulate bacterial growth under certain conditions and inhibit bacterial growth under other conditions.


1Introduction

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

One important function of the ectomycorrhizal symbiosis is to increase the amount of surface area that can be used for nutrient uptake by plants. In addition, the mycelium can access organic N and P [1] and minerals in rocks [2], which are not directly available for plant uptake. Ectomycorrhizal fungi may, for example, influence mineral weathering through the production of organic acids [3–6]. Such acids may reduce the soil pH, thereby increasing the solubility of the minerals [7]. Some organic acids form complexes with metals or calcium, which may release phosphate and other nutrients from minerals [2, 8]. Wallander et al. [9] demonstrated that the ectomycorrhizal fungus Suillus variegatus reduced the pH and increased the citric acid concentration of an apatite-amended soil, resulting in an increase in the release of phosphorus (P) from the apatite.

The activity of the ectomycorrhizal mycelium, which extends far beyond the rhizosphere, may influence the surrounding bacterial community as a result of the uptake of nutrients and exudation of organic compounds. Interactions with the bacterial community are of special interest since certain bacteria may influence the weathering of primary minerals [10, 11]. Although Söderström [12] suggested that the spread of the mycorrhizal mycelium might promote bacterial growth in the bulk soil as the mycelium degenerates, many studies indicate that ectomycorrhizal mycelium negatively affects saprophytic microorganisms [13–17]. For instance, Olsson et al. [17] found that the mycelium of P. involutus inhibited bacterial activity by an average of 30% in three experiments.

Allocation of carbon in plants to the root and mycorrhiza is greatly influenced by nutrient status of the plant. Applications of primary minerals such as biotite, microcline and apatite, as sources of limiting nutrients, may, for instance, stimulate growth of ectomycorrhizal fungi and the production of organic acids by the external mycelia [9, 18]. Biotite and microcline (a feldspar) are common as potassium (K) sources to plants in acid soils and apatite is common as P source, for instance in calcareous soils. The objective of this study was to determine how the activity and composition of the bacterial community was influenced by mycelia from different ectomycorrhizal fungi, and how this influence was related to fungal growth when different primary minerals were added to the soil.

2Materials and methods

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

In the first experiment ectomycorrhizal and non-mycorrhizal pine plants were grown under K-limitation and provided by the K-containing minerals microcline or biotite (the K-source experiment). In the second experiment the pines were grown under P-limitation and provided by the P-containing mineral apatite in soil compartments that could only be reached by the fungal mycelia (the P-source experiment). The K-source experiment included only root soil, while the P-source experiment included both root soil and root-free soil separated from the root by a mesh that allowed entrance of mycelium but not roots.

2.1Plant and fungi

Seeds of Pinus sylvestris L. were sown in vermiculite and watered with distilled water. Five weeks after germination the seedlings were inoculated with ectomycorrhizal fungi or left uninoculated. In the K-source experiment the fungi used were: Paxillus involutus (Fr.) Fr. (isolate K from Dr. A. Dahlberg, Uppsala, Sweden) and Suillus variegatus (Swartz: Fr.) O.K. (isolate 2-10 from Dr. A. Dahlberg, Uppsala, Sweden). Fungi used in the P-source experiment were Suillus variegatus (isolates 94 001 and 94 002, Lund, Sweden) and one unidentified isolate (isolate 94 003, Lund, Sweden, with ITS-RFLP pattern described by Wallander et al. [19]; this isolate originated from a pine root grown in a soil collected from a Haploic podzol on sandy moraine from a coniferous forest in southern Sweden).

2.2The K-source experiment

Seedlings were inoculated with the mycorrhizal fungi by placing them beside already colonised P. sylvestris seedlings on a thin peat layer in vertical plastic microcosms. The mycelium was then allowed to grow out and colonise the new seedling during a 4-week period. Once the seedlings had been colonised by the fungi they were replanted in plastic pots containing 250 g of soil. Microcline from a pegmatite from Varuträsk, Sweden was crushed with mineral-crushing equipment to a particle size of less than 160 μm, and it was applied in an amount large enough to reach a concentration of 25% (w/w). Biotite from a pegmatite in Moen, Norway was crushed in an electric coffee mill to a particle size of less than 250 μm, and enough was applied to reach a concentration of 3% (w/w) in the biotite soil. Control soils consisted of quartz sand and peat (3:1, v/v), which also was the soil type to which the minerals were applied. Before application of the minerals to the soils they were washed in distilled water to remove the most readily released mineral nutrients, including K+. In a separate experiment a complete Ingestad solution (including K+) was added [20], but this treatment was performed only with Paxillus involutus inoculated and non-mycorrhizal seedlings.

Plant seedlings were harvested 223 days after planting. No differences in pH, which varied between 4.8 and 5.2, were detected between fungal treatments.

2.3The P-source experiment

Ectomycorrhizal colonisation was achieved by growing a plant and fungus together in Petri dishes filled with a vermiculite/peat mixture (3:1, v/v) soaked in a modified Melin-Norkrans medium according to the method of Duddridge [21] as modified by Finlay et al. [22]. The plants were placed in mesh bags (mesh size 100 μm) containing 180 g soil, which allowed penetration of mycelium, but not roots, to create a root-free soil volume in the pot outside the mesh containing 360 g soil. The soil in the pots was a peat/sand (3:1, v/v) mixture. Apatite (ground to a fine powder with a particle size between 50 μm and 250 μm) was added as a P-source to reach a concentration of 1% (w/w) in the root-free soil. The apatite came from Siilinjärvi, Finland and contained 16% P (w/w).

The seedlings were harvested 240 days after planting. No differences in pH, which varied between 4.7 and 5.2, were detected between fungal treatments.

2.4Growth conditions

The pots (4 of each treatment) were placed in boxes, where a capillary mat soaked up the nutrient solution from a reservoir at a distance of 5 cm below the seedlings. The nutrient solution was balanced in accordance with Ingestad and Kähr [20] and concentrations were as described by Nylund and Wallander [23], except that in the K-source experiment K+ was omitted and replaced with Na+, and in the P-source experiment, PO2−4 was omitted and replaced with Cl. The seedlings were placed in growth cabinets at approximately 300 μmol m−2 s−1 PAR, with an 18-h/6-h 18°C/15°C day/night cycle.

2.5Bacterial enumeration

Numbers of culturable bacteria were determined as colony forming units (CFUs) using the dilution-plate technique as in an earlier study [24]. Colonies were counted after incubation for 8 days at 20°C. Acridine orange direct counting (AODC) of bacteria was performed according to Frostegård and Bååth [25] on the suspensions that were used for determining thymidine and leucine incorporation (see below).

2.6Thymidine and leucine incorporation

The procedures were in accordance with the methods described by Bååth [26, 27]. Ten grams of soil (wet weight) were homogenised in 40 ml distilled water with an Omnimixer at 16 000 rpm for 1 min. After centrifugation of the soil suspension (750×g, 10 min), the supernatant was filtered through glass wool. The bacterial suspension was incubated for 2 h in 200 nM methyl [3H]-thymidine and 775 nM [14C]-leucine (Amersham, 925 GBq mmol−1 and 11.9 GBq mmol−1, respectively). After adding 1 ml of 5% formalin, the suspension was filtered through a Whatman GF/F fiberglass filter and washed with 3×5 ml of ice-cold ethanol followed by 3×5 ml ice-cold trichloroacetic acid (TCA). The filter was placed in a scintillation vial, and after addition of 1 ml of 0.1 M NaOH, the vial was kept at 90°C for 2 h. Once the vial had cooled, 10 ml of scintillation cocktail was added, and radioactivity was counted in a liquid scintillation spectrometer.

2.7Analysis of organic acids

Concentrations of organic acids in the soil solution (centrifugation of soil samples for 1 h at 10 000 rpm) were estimated by ion chromatography according to Ström et al. [28].

2.8Analysis of phospholipid fatty acids

The method of lipid extraction followed Frostegård et al. [29], which is a modification of the Bligh and Dyer [30] procedure. Extracted lipids were fractionated into neutral lipids, glycolipids and polar lipids on silicic acid (100–200 mesh, Unisil) columns by eluting with chloroform, acetone and methanol, in that order. The methanol fraction (containing phospholipids) was subjected to a mild alkaline methanolysis [31], which transforms the fatty acids into free fatty acid methyl esters. These were analysed on a Hewlett Packard 5890 gas chromatograph with a flame ionisation detector and a 50 m HP5 capillary column, according to details given by Frostegård et al. [32].

The following phospholipid fatty acids (PLFAs) were considered to be of bacterial origin: i15:0, a15:0, 15:0, i16:0, 16:1ω5, 10Me16:0, i17:0, a17:0, cy17:0 and cy19:0 [33], and the sum of these was used as an estimate of bacterial biomass. Changes in bacterial community structure in the soils were examined by using principal component analysis (PCA) of the 10 bacteria-specific PLFAs (relative amounts). PLFA 18:2ω6,9 was used as an indicator of fungi and the difference between mycorrhizal and non-mycorrhizal treatments was supposed to indicate biomass of the external mycelium of the ectomycorrhizal fungi. This fatty acid is considered specific for eucaryotic organisms and is common in basidiomycetes [34]. The nomenclature of fatty acids follows Tunlid and White [35].

2.9Statistical methods

All results are given as means with standard error of the mean (S.E.). The effects of ectomycorrhizal treatment and roots were tested with ANOVA (F-ratio) and differences between means were tested by Tukeys’ HSD. Residuals did not differ from normality and the ANOVA was only performed when there were no significant differences between group variances (Bartlett's test). The bacterial PLFA composition was analysed by using principal components analysis (PCA) with the computer program SIRIUS [36].

3Results

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

3.1The K-source experiment

Visual estimates at harvest confirmed that the seedlings were well colonised by both fungi in both the control and biotite soils, whereas in the microcline soil only P. involutus colonised well. However, few external hyphae were formed in the control soil, according to measurements of the fungal signature PLFA 18:2ω6,9 (Table 1). All soils with non-mycorrhizal pine seedlings had amounts of PLFA 18:2ω6,9 ranging from 0.24 to 0.28 nmol g−1. In the microcline soil, the presence of P. involutus mycelium increased the amount of PLFA 18:2ω6,9 by 150% compared with the control soil, whereas S. variegatus mycelium had no such effect. Both fungi formed extensive external mycelia in the biotite soil, as observed visually and as indicated by increased amounts of 18:2ω6,9, with a two-fold increase due to P. involutus and a 10-fold increase due to S. variegatus. The amount of PLFA 18:2ω6,9 per unit dry biomass in S. variegatus was 10-fold higher compared with that in mycelium of P. involutus, as determined in a pure culture study (P.A. Olsson and H. Wallander, unpublished). Using conversion factors from that study a biomass of 10 μg g−1 soil for both S. variegatus and P. involutus was estimated in the control soil (values from non-mycorrhizal treatments were subtracted), whereas in the microcline soil there was 190 μg g−1 of P. involutus, and no mycelium of S. variegatus detected. Estimated biomass was highest in the biotite soil, with 137 μg g−1 for S. variegatus and 340 μg g−1 for P. involutus. S. variegatus increased the concentration of citric acid in the biotite soil solution 10-fold, reaching a concentration of 24 μM, while no increase was found for P. involutus (see Wickman and Wallander [18] for further data).

Table 1.  Estimates of bacterial, fungal and pine-root biomass in a sand/peat mixture control soil and in soils amended with microcline or biotite
Fungal inoculumBacterial numbers (microscopical counts) (no.×107 g−1)Bacterial biomass (as bacterial PLFAs) (nmol g−1)Fungal biomass (as PLFA 18:2ω6,9) (nmol g−1)Root biomass (g dw/pot)
 Control soilMicrocline soilBiotite soilControl soilMicrocline soilBiotite soilControl soilMicrocline soilBiotite soilControl soilMicrocline soilBiotite soil
None15.4±0.5a8.4±1.49.1±0.7a3.7±0.22.4±0.62.0±0.3a0.28±0.030.25±0.030.24±0.07a0.65±0.040.36±0.14ab1.15±0.09
  1. The pine seedlings were non-mycorrhizal or formed mycorrhiza with Suillus variegatus or Paxillus involutus. Values represent means (n= 4)±S.E. Different letters indicate significant differences (P<0.05) between fungal treatments estimated with Tukeys’ HSD when the ANOVA indicated significant differences. The ANOVA was performed on log-transformed data for the bacterial biomass (biotite soil) and fungal biomass since the variances differed between treatments.

S. variegatus12.0±0.5b7.4±1.013.4±1.4b2.9±0.12.5±0.57.1±2.2b0.52±0.120.24±0.092.98±0.83b0.45±0.100.28±0.10a1.54±0.14
P. involutus13.2±0.7ab5.3±0.210.1±0.7ab3.1±0.32.3±0.43.8±0.5ab0.30±0.060.63±0.190.92±0.59ab0.66±0.120.84±0.18b0.89±0.19
             
ANOVA (P-value)0.02*0.580.005**0.070.840.005**0.100.070.02*0.250.04*0.06

Root biomass was highest in the biotite soil, with S. variegatus colonisation resulting in higher values compared with the non-mycorrhizal and P. involutus treatments (Table 1). In the microcline soil there was a significant increase in root biomass due to P. involutus, but S. variegatus showed poor root colonisation in this soil type and did not affect the root biomass. In the control soil ectomycorrhizal colonisation did not influence root biomass.

In the control and the microcline-amended soils both the amounts of the bacterial PLFAs and the numbers of microscopically counted bacteria were less in the fungal treatments in comparison with the non-mycorrhizal treatment (Table 1). By contrast, in the biotite soil S. variegatus caused significant increases in bacterial PLFAs (more than 3-fold) as well as in numbers of bacteria (47% increase). P. involutus also increased bacterial biomass, but the increase was only significant for the bacterial PLFAs.

The PCA ordination of the bacterial PLFA pattern in the control soil revealed a separation between all three fungal treatments along the first component (P<0.001, Fig. 1A). In the control soil the P. involutus treatment changed the bacterial PLFA pattern more compared with the non-mycorrhizal treatment. This was mainly due to differences in the proportion of PLFA 16:1ω5, which was significantly higher in the non-mycorrhizal treatment compared with the ectomycorrhizal treatments. No significant differences in the bacterial PLFA pattern between mycorrhizal and non-mycorrhizal treatments was found in the microcline soil (Fig. 1B), whereas in the biotite soil the PCA ordination revealed a difference between the S. variegatus treatment and the non-mycorrhizal treatment (P= 0.014, Fig. 1C). Increases in 16:1ω5 and cy19:0 in the S. variegatus treatment explained most of this difference.

image

Figure 1. PCA ordination of the bacterial PLFA patterns in the K-source experiment. Loadings of the individual PLFAs and the scores of the different fungal treatments (n= 4, ±S.E.) are indicated where P.i denotes Paxillus involutus, S.v Suillus variegatus and NM non-mycorrhizal (the variation explained by each component is indicated). The PCA was performed separately for the control soil (A), the microcline soil (B) and the biotite soil (C).

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In the control soil significant reduction in bacterial activity, as measured by thymidine incorporation, was observed in the treatments with S. variegatus (62%) and P. involutus (45%) compared with the non-mycorrhizal treatment (Table 2). In the microcline soil the corresponding reduction was around 30% (not significant). By contrast in the biotite soil S. variegatus more than doubled the bacterial activity, while P. involutus reduced it also in this soil. Moreover, in the experiment with the control soil amended with K, bacterial activity was reduced by about half by P. involutus compared with the non-mycorrhizal treatment. Leucine and thymidine incorporation was affected in similar ways by the fungi as the thymidine incorporation, except for P. involutus in the microcline soil (Table 2).

Table 2.  Bacterial activity, as measured by thymidine and leucine incorporation, in a sand/peat mixture control soil and in the same soil amended with microcline, biotite or K
Fungal inoculumThymidine incorporation (mol×10−14 h−1 ml−1)Leucine incorporation (mol×10−12 h−1 ml−1)
 Control soilMicrocline soilBiotite soilK soilControl soilMicrocline soilBiotite soilK soil
  1. The K-amended soil was from a separate experiment. Pine seedlings were non-mycorrhizal or formed mycorrhiza with Suillus variegatus or Paxillus involutus. Figures represent means (n= 4)±S.E. Different letters indicate significant differences (P<0.05) between fungal treatments estimated with Tukeys’ HSD when the ANOVA indicated significant differences.

None43.7±6.1a27.6±7.024.8±1.4a21.4±4.714.6±2.98.7±2.2ab8.4±1.2a4.2±0.5
S. variegatus21.8±1.0a19.6±2.560.1±4.6b 7.6±0.26.7±1.6a17.4±2.1b 
P. involutus24.1±6.6a19.5±2.717.4±0.7a10.6±1.69.0±2.415.4±2.3b6.7±0.9a1.6±0.2
         
ANOVA (P-value)0.05*0.39<0.001***0.070.160.04*0.002**0.003**

3.2The P-source experiment

Ectomycorrhizal colonisation was visually observed with all three fungi. S. variegatus (both isolates) raised the content of PLFA 18:2ω6,9 in the root bag from 0.60 nmol g−1 in the non-mycorrhizal treatment to 1.49 nmol g−1 (P= 0.04). No significant increase in PLFA 18:2ω6,9 was detected in the root-free soil due to the external mycelium. Still, the amount of PLFA 18:2ω6,9 tended to be higher compared with the non-mycorrhizal control for all three fungi (Fig. 2A).

imageimageimage

Figure 2. Measurements of the P-source experiment where pine seedlings were grown either non-mycorrhizal or mycorrhizal with either of two Suillus variegatus isolates or an unidentified fungal isolate. Measurements were made in a sand/peat mixture amended with apatite to root-free soil compartments which was separated from the root soil by a nylon mesh (100 μm). Fungal biomass was estimated as amount of PLFA 18:2ω6,9 (A). Oxalic acid was estimated in the soil solution (B). Bacterial activity was measured as thymidine (C) and leucine (D) incorporation and bacterial biomass as microscopical counts (E), bacterial PLFAs (F) and viable counts (G). The bars represent means of four replicates (±S.E.) and significant differences between mycorrhizal and non-mycorrhizal treatments are indicated by * for P<0.05 (Tukeys’ HSD). The ANOVA was performed on log-transformed data except on the data related to (E) and (F).

S. variegatus tended to increase the oxalic acid concentration in the soil solution in the root bag from 3.0±0.2 to 6.8±2.5 μM (P= 0.07), and all three fungal isolates gave higher oxalic acid concentration in the root-free soil, however, only significantly so for one of the isolates (Fig. 2B). Citric acid was only detected in one replicate, which was in the S. variegatus (isolate 1) treatment, where its concentration was 1.6 μM.

In the root-free soil, none of the fungi gave significant effects on bacterial activity, measured as thymidine and leucine incorporation, but all three isolates tended to increase thymidine (Fig. 2C,D). The ectomycorrhizal mycelia had no effect on bacterial biomass (Fig. 2E–G), although total bacterial biomass tended to be lower due to the fungal treatments. In the root soil, where no apatite was added, S. variegatus had no effect on bacterial activity. Bacterial activity was higher in the root soil, ranging from 64×10−14 to 69×10−14 mol thymidine h−1 ml−1, compared with the root-free soil, where it ranged from 8×10−14 to 35×10−14 mol thymidine h−1 ml−1. Estimates of bacterial numbers (Fig. 2E) and bacterial PLFAs (Fig. 2F) did not differ between root soil and root-free soil.

4Discussion

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

This study demonstrates that mycelia of ectomycorrhizal fungi can influence the activity and composition of the bacterial community in the soil near the roots (the K-source experiment). Olsson et al. [17] showed that the external ectomycorrhizal mycelium can negatively affect bacterial activity. Here, we demonstrate that under certain conditions external ectomycorrhizal mycelia can have a stimulatory effect in the soil near the ectomycorrhizal roots and such effects were also indicated at a distance from the roots (the P-source experiment). In the biotite soil the higher root biomass with S. variegatus may explain some of the increase in bacterial activity; however, despite high root biomass in the microcline and control soils, no increase in bacterial activity was found in those soils. This study showed that under certain conditions the presence of S. variegatus could lead to an increase of bacterial growth. In this respect the effect of this fungus differed from that of P. involutus. Our results were supported by a study by Nurmiaho-Lassila et al. [37], which demonstrated by electron microscopy that mycorrhizae of S. variegatus were more heavily colonised by bacteria than those formed by P. involutus.

After three months of seedling growth Olsson et al. [17] found that although bacterial activity was reduced in the presence of ectomycorrhizal mycelia, there was no reduction in bacterial biomass. In the present study, reduction of bacterial activity was in some cases accompanied by a reduction in bacterial biomass, probably because of the longer duration of the experiments (8 months) and because the estimated biomass of P. involutus was much lower in the study of Olsson et al. [17] than in the K-source experiment described here. In cases where bacterial growth is influenced by some external factor, changes in thymidine incorporation are expected to precede changes in the number of microscopically counted bacteria [38].

Increased bacterial activity in the present study was found only in soils amended with certain primary minerals. Since the calculations based on mycelia grown in pure culture indicated that the biomass of S. variegatus was lower than that of P. involutus in the biotite soil, differences in exudation and effects on the bacterial community between the two fungi could not have been due to a higher biomass of S. variegatus. The higher citric acid production by S. variegatus suggests that the positive influence of this fungus on bacterial biomass and activity may be due to a release of organic products from the mycelium. In the P-source experiment the increase in oxalic acid concentration due to one of the fungi indicated that it exuded organic substances. Only S. variegatus caused detectable amounts of citric acid, and only in one replicate. However, this does not necessarily mean that no citric acid was produced during the experiment since citric acid, as well as oxalic acid, disappears rapidly from the soil solution once production stops [39].

Reduced pH due to exudation of organic acids has been shown to be part of the antibiosis of P. involutus against a pathogenic fungus [40, 41], but in this study the fungi did not reduce the pH of the soil. Further, concentrations of citric acid and oxalic acid in the soil solution were at micromolar levels, and even millimolar levels only have weak antibiotic effects on bacteria [42]. Even though organic acid concentrations may be much higher in the hyphosphere, they may have no significant effect on the bacterial community as a whole. The organic acids produced by fungi may instead serve as substrate for saprophytic growth by bacteria [30].

Ectomycorrhizal fungal mycelia may increase the amount of dissolved organic carbon in the soil solution and organic acid exudation may be stimulated by nutrient limitation [3, 5, 6], as previously demonstrated for citric acid exudation by S. variegatus[9]. Such input of easily available carbon to the soil may increase bacterial growth, since the saprophytic microorganisms in soil are generally carbon limited [43]. The rhizosphere effect is an example of how root exudates can stimulate bacterial growth [44]. Although the plants were subjected to K-limitation in one of the experiments in the present study, the bacteria were probably not limited by K, as indicated by the observation that K-addition did not give higher bacterial activity and did not reduce the negative effect of P. involutus (Table 2). Thus, the negative effects of the ectomycorrhizal fungi on bacterial growth were probably not due to competition for the plant growth limiting nutrient.

A continual input of a specific organic substrate may result in changes in the bacterial community [45]. Changes in the bacterial PLFA profile indicated that such a shift did occur in community structure, possibly resulting in a bacterial community better adapted to using mycelial products or better able to tolerate the negative effects of the ectomycorrhizal fungi. The PCA ordination revealed that the S. variegatus induced increase in bacterial PLFAs in the biotite soil was not due to a similar increase in all bacterial PLFAs. Instead, cy19:0 and 16:1ω5 increased relatively more compared with the other PLFAs due to S. variegatus in the biotite soil. Bååth et al. [46] and Frostegård et al. [47] reported that in a forest soil liming and prescribed burning strongly influenced the microbial community. One of the effects was an increase in the PLFA 16:1ω5. This indicates that this PLFA is typical of bacteria that have a rapid growth response to changed conditions. Also the PLFA cy19:0, which is a signature of Gram-negative bacteria [35], was associated with the S. variegatus treatment. The signatures of Gram-positive bacteria (iso- and anteiso-branched PLFAs) were increased the least.

This study shows that the same ectomycorrhizal fungus may stimulate bacterial growth under some circumstances, such as with biotite addition, while it may inhibit bacterial growth under other circumstances, such as with no mineral added. This is most likely an effect of different exudation of the mycelia in response to different minerals. This study also shows that different ectomycorrhizal fungi respond differently to minerals and also in the way they influence the surrounding bacterial community. A change in bacterial biomass or changes in bacterial community composition due to an ectomycorrhizal mycelium may influence weathering rates as well as other soil properties.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References

The financial support provided by the Swedish Council for Forestry and Agricultural Research and the Swedish Natural Science Research Council is gratefully acknowledged. We thank Anna Fossum and Anne-Mari Fransson for technical assistance and Erland Bååth, Lotta Persmark, Bengt Söderström and Tonie Wickman for valuable suggestions as well as comments on the manuscript.

References

  1. Top of page
  2. Abstract
  3. 1Introduction
  4. 2Materials and methods
  5. 3Results
  6. 4Discussion
  7. Acknowledgements
  8. References
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