Implications of mycorrhiza for biogeochemical cycling and coexistance of soil microbiota- a modelling study

Ectomycorrhizae are widespread symbionts of higher plants. However, their benefits for plant productivity and growth have not been well demonstrated since many studies do not suggest any improvement of plant growth or of plant nutrition for mycorrhizal plants. We use mechanistic modelling based on the population dynamics of decomposers to simulate the coexistence of mycorrhizal and non-mycorrhizal plants as well as the development of the soil decomposer community. The model assumed a fixed stoichiometry of each decomposer functional type. Decomposer growth depended on its carbon and nitrogen uptake. For mycorrhiza a part of the carbon is modelled to be supplied from the plant while a fixed proportion of the mycorrhizal nitrogen uptake is translocated to the plant. Carbon nitrogen ratios of decomposers were adjusted mineralization of nitrogen or overflow respiration of carbon. The results suggest that mycorrhizal plants do often outcompete non-mycorrhizal plants at no or little improvement of plant productivity. The main mechanism of mycorrhizal dominance is a reduction of the soil inorganic nitrogen pool and a rerouting of the nitrogen uptake of plants to the transfer nitrogen transfer from mycorrhizae to plants. On the other hand carbon subsidies from the trees allow to expand the niche of mycorrhizal fungi and to outcompete saprohytic fungi under a wide range of physiological and environmental parameters. This leads to dominance of mycorrhizal plants under a broad range of conditions and parameters including low transfer rates of nitrogen from the mycorrhiza to the plant, and low allocation of the plants to mycorrhiza. Significance statementThe paper uses mechanistic, population based modelling to explain the dominance of ectomycorrhizal plants in northern ecosystems while there is limited evidence that these increase plant productivity. We demonstrate that rerouting of the nitrogen cycle towards provision of organic nitrogen to the plant allows mycorrhizal plants to outcompete non-mycorrhizal competitors. Simultaneously, mycorrhizae benefit from carbon subsidies in their competition with saprophytic fungi and bacteria.