The role of ectomycorrhizal fungi in the breakdown of soil organic matter

Mark Smits1, Francois Rineau1, Jan Colpaert1
1 Environmental Biology, Hasselt University

O 4.6 in Below ground turnover of C and nutrients in forest soils

14.07.2014, 15:35-15:55, H17

Most trees in temperate and boreal forests live in symbiosis with ectomycorrhizal (EcM) fungi. The receive up to 25% of the carbon fixed by the host tree, similar to plant investments in plant biomass. This flow of direct available energy into the belowground microbial community has a major impact on the belowground energy and carbon budget, and consequently belowground processes. Recent work has highlighted positive and negative effects on the soil organic carbon budget. Positive effects comprise (1) competition for space and nutrients with saprotrophic microorganisms (‘Gadgill effect’) and (2) the buildup of recalcitrant microbial biomass. Negative effects comprise the breakdown of energy consuming breakdown of complex organic matter in order to access nutrients (‘priming effect’). How these contrasting effects influence soil carbon dynamics on an ecosystem scale is largely unknown.

Here we present an overview of our work on EcM fungi in relation to soil organic matter (SOM), ranging from in situ experiments to field incubations and field data. In situ experiments show that EcM fungi have saprotrophic abilities similar to brown rot fungi. Both transcriptome profiling and detailed organic matter analysis reveal that they use  fenton chemistry (radical based oxidation) that can disrupt the organic matter-protein complexes. The saprotrophic actions are energy driven, and not affected by nitrogen demand.

These observations were supported by a field incubation experiment. In two experimental forest fertilization sites in Sweden, we studied the alteration of compost mixed with sand in mesh bags and incubated for 1, 2 or 3 years in the mineral soil (Wallander et al., 2011). At one site, lignin degradation was slower in the fertilized plots than in the non-fertilized plots, while at the other site there was no clear trend. But, taken both sites together, lignin degradation showed a positive linear relation with fungal ingrowth in the mesh bags.

A negative effect of N fertilization on the degradation rate of lignified SOM has been known for a long time. The leading hypothesis has been that high N availability represses the lignin-degradation activity of white-rot basidiomycetes. We propose that a reduction of the belowground allocation of plant energy into their fungal symbionts, due to N-fertilization, is the key factor in a reduced degradation rate of lignified SOM in the mineral soil. 

Wallander H. Ekblad A, Bergh J (2011) Growth and carbon sequestration by ectomycorrhizal fungi in intensively fertilized Norway spruce forests. Forest Ecology and Management 262: 999–1007.

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last modified 2014-06-19