Interactions between the external mycelium of the mycorrhizal fungus Glomus intraradices and other soil microorganisms as affected by organic matter
Introduction
AM fungi are associated with the roots of most herbaceous plants. The external hyphal network of AM fungi plays an important role in plant nutrition, as the inflow of mineral nutrients to the host plant from beyond the root zone, is facilitated by the spread of AM mycelium. In exchange of translocating mineral nutrients from the soil to the host plant, AM fungi receive carbon from their host plant (Smith and Read, 1997).
External mycelium of AM fungi has been suggested to preferably associate with organic matter (St. John et al., 1983). AM fungi have been shown to colonize decomposing leaves developing arbuscules and vesicles in dead leaf cells (Aristizábal et al., 2004) and AM fungi also colonize sphagnum leaves (Warner, 1984). This may be important for the recycling of mineral nutrients released during mineralization of dead organic matter (Aristizábal et al., 2004). Growth enhancement of AM fungal external mycelium has mainly been observed with complex organic matter applied both to the rhizosphere (Joner and Jakobsen, 1995) and the hyphosphere (Green et al., 1999), but also with more simple substrates like bovine serum albumin applied to the hyphosphere (Ravnskov et al., 1999). In contrast, cellulose amendment has been shown to reduce growth of the external mycelium (Ravnskov et al., 1999), and glycine amendment did not affect AM mycelial growth (Hodge, 2001). Knowledge on the mechanisms behind the interactions between AM fungi and organic matter is scarce, but Gavito and Olsson (2003) showed that carbon in AM mycelium proliferating in organic matter as expected originates from plant photosyntate, as indicated by incorporation of 13C in AM signature fatty acids, which had been pulse labelled to the plant as 13CO2. However, mycelial growth of AM fungi may be limited for other nutrients such as N, which can be obtained from decomposed organic matter as suggested by Ravnskov et al. (1999). Indeed, AM mycelial N (Ames et al., 1983, Hodge et al., 2001) and P (Joner and Jakobsen, 1995) uptake from various sources of organic matter has been described.
AM fungi interacts with various groups of soil bacteria (Paulitz and Linderman, 1991), and mycorrhiza can change communities of rhizosphere microorganisms (e.g. Andrade et al., 1997, Wamberg et al., 2003), but AM mycelia mediated changes in soil microbial communities has been less studied (Olsson et al., 1996, Mansfeld-Giese et al., 2002). The bacterium Burkholderia cepacia is commonly found in the mycorrhizosphere, but not from a corresponding rhizosphere of non-AM plants (Andrade et al., 1997). However, in another study B. cepacia was not specifically associated with AM fungi (Mansfeld-Giese et al., 2002).
Burkholderia cepacia has competitive saprotrophic abilities, and is also known as a plant growth promotor and as a possible biocontrol agent (Roberts et al., 1997, Larsen et al., 2003). Ravnskov et al. (2002) examined the influence of five different strains of B. cepacia on soil mycelial growth of the AM fungus G. intraradices and found all possible strain specific interactions. On the other hand, the presence of mycelium of G. intaradices in root-free soil decreased the biomass of three out of five strains of B. cepacia as measured using cyclic fatty acids as biomarkers for B. cepacia (Ravnskov et al., 2002).
The objective of the present experiment was to examine the influence of other soil microorganisms, on the growth enhancing effect of organic matter, in terms of ground barley leaves, on growth of the external mycelium of G. intraradices. Our main hypothesis was that bacterial inoculation of sterile organic matter would increase the proliferation of mycelium of G. intraradices in the organic matter.
Section snippets
Experimental design
Compartmented pots with two root-free compartments were used as experimental units (Fig. 1). The experiment had a factorial design with 12 treatments with three main factors: (1) G. intraradices (with and without), (2) organic matter (with and without) and (3) microbial inoculations (water, soil filtrate, B. cepacia). Each treatment had four replicates. Treatments in the main units (called root compartment) consisted of non-AM plants or of plants colonized with G. intraradices. Two matching
Hyphal length density
Mycorrhiza and organic matter both increased the hyphal length density in root-free compartments and in treatments with both mycorrhiza and organic matter the hyphal length density was further increased as compared to the non-AM control without organic matter (Fig. 2). In the absence of organic matter, B. cepacia increased the hyphal length density in treatments with mycorrhiza, whereas the soil filtrate had no effect (Fig. 2). With organic matter, neither B. cepacia nor soil filtrate affected
Discussion
In the present experiment, addition of organic matter to root-free sand compartments, as expected markedly increased all measured parameters of both saprotrophic microorganisms and the biotrophic AM fungus G. intraradices. However, our main hypothesis, that proliferation of external mycelium of G. intraradices would increase if the sand with the organic matter were inoculated with soil microorganisms, was not confirmed in the present experiment. Since the sand in the zone of interaction in the
Acknowledgements
We thank Tina Tønnersen and Keld Mansfeld (Danish Institute of Agricultural Sciences) for valuable support and Dr Daniel Roberts (United States Department of Agriculture) for providing us with the strain of B. cepacia.
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