Mycorrhizal Fungi Take over at Late Decomposition Stages

In our pine forest study, after 2-4 years of decomposition, when the litter began to lose its structural integrity and eventually became humus, the community of saprotrophic litter fungi was replaced by a totally different community dominated by mycorrhizal species (Figure 2). In well-decomposed litter and humus samples, ectomycorrhizal basidiomycetes, mainly within the genera Piloderma and Cortinarius, dominated the DNA pool together with ascomycetes, again from the Leotiomycetes and Dothideomycetes. Helotialean ascomycetes continued to play an important role at later stages of decomposition, but these taxa were different from the ones in less decomposed litter. Mycorrhizal symbiosis with conifers or ericaceous plants appears to be widespread throughout the Helotiales (Vralstad et al., 2002), and it is likely that the late helotialean colonisers were mycorrhizal. The Dothideomycetes recorded at later decomposition stages

Estimated age (Years) 0

10 16

Needles at abscission Litter 1

Litter 2

needles mosses

Fragmented litter Humus 1 Humus 2

E horizon Mineral soil

B horizon

Needles at abscission Litter 1

Fungal community composition C:N ratio

0 20 40 60 80100% 0 30 60 90 120150

c

□ "early" fungi ( H known saprotrophic fungi )

□ "late" fungi ( H known mycorrhizal fungi )

Figure 2 The graph shows a 28 mm diameter core taken through the forest floor and upper mineral soil of a swedish P. sylvestris forest. In a study by Lindahl et al. (2007), 27 such cores were collected and subdivided into vertical horizons. The fungal communities in the different horizons were analysed using ITS-PCR and TRFLP in combination with cloning and sequencing. The results indicate that, as decomposition progresses, an 'early' community (here defined as taxa more abundant in litter samples than in humus samples) is replaced by a 'late' community (here defined as taxa more abundant in humus samples than in litter samples). Age estimates represent time since abscission, as determined by analysis of the content of 14C (from thermonuclear bomb tests). Fungal community composition in each horizon is related to carbon:nitrogen ratios. During the early, presumably saprotrophic phase, C:N decreases progressively with litter age. During the later, presumably mycorrhizal phase, C:N increases with age of the organic matter. In the mineral soil C:N ratios are slightly lower. Different letters in the C:N ratio bar diagram Indicate significant differences (P<0.05). Reproduced with publisher's permission from Lindahl et al. (2007). (See Colour Section)

belonged to the genus Capronia, in which taxa have been identified as my-corrhizal symbionts of ericaceous plants (Allen et al., 2003). In addition, we recovered DNA from a group of recently discovered fungi, only distantly related to other ascomycetes, which hitherto have been described only in terms of their DNA sequence (Schadt et al., 2003). DNA sequences attributed to this intriguing group have also been obtained from mycorrhizal pine roots (Rosling et al., 2003), suggesting that they are root-associated. Thus, almost all the fungi detected in the litter-humus after a few years of saprotrophic decomposition were recognised as, or presumed to be, mycorrhizal. In soil microcosms, we demonstrated antagonistic interactions between a saprotrophic basidiomycete and certain ectomycor-rhizal fungi (Lindahl et al., 1999). The outcome of the combative interactions was strongly affected by the amount of resources available to the saprotroph (Lindahl et al., 2001). Together these observations suggest that litter saprotrophs, when their cellulose supply is depleted, are out-competed by mycorrhizal fungi that have direct access to plant-derived photo-assimilates. The spatial separation of the two functional groups of fungi may simply reflect the fact that saprotrophs obtain their carbon predominantly from litter, deposited from above, whereas mycorrhizal fungi obtain their carbon from roots that enter the forest floor from below.

Particularly noteworthy is the absence of sequences from typical 'soil fungi', such as Trichoderma and Penicillium, in the clone-libraries. According to conventional isolation-based studies, these genera usually dominate at later stages of litter decomposition (e.g. Widden and Parkinson, 1973; Soderstrom and Baath, 1978; Frankland, 1998; Virzo de Santo et al., 2002), and their absence in the DNA pool highlights the practical problems involved in isolating representative fungi from litter and soil. Most mycorrhizal fungi grow very slowly on agar plates, and many do not grow at all. When the dominant fungi in a sample fail to grow out on agar, opportunistic fungi, that initially constitute a very small fraction of the biomass in the sample or are present only as spores, may rapidly take over the agar plates. For example, Virzo de Santo et al. (2002) studied fungal colonisation of coniferous litter and observed dominance of basidiomycete mycelium on the litter from after 6 months and further throughout the entire 4-year experiment. However, when litter fragments were placed on malt agar plates, basidiomycetes were isolated only during the first 2 years, and were subsequently replaced by Trichoderma, Penicillium and Mucor species. These apparently conflicting observations are consistent with colonisation of well-decomposed litter primarily by slower growing mycorrhizal basidiomycetes, which escape detection using conventional isolation methods.

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