Fungal C translocation precludes N mineralization

Background/Question/Methods In traditional nitrogen (N) cycling models, N mineralization is the driving process where organically bound N is converted to inorganic N, carried out by the activity of soil decomposer organisms. Decomposition of a substrate with a lower C:N ratio than the organism requires, results in net N mineralization as the C availability becomes limiting. This concept assumes that all decomposer organisms are unicellular, exclusively dependent on the resources in their immediate surroundings. However, in forest soils fungi generally dominate decomposer communities. Litter degrading fungi often form large mycelia and may have a well developed capacity to translocate resources within their mycelia. The mycelium may then act as a single entity where the nutritional status of one part of the mycelium affects distant parts. We hypothesize that the ability of fungi to translocate carbohydrates within their mycelia will prevent local C limitation in low C:N ratio substrates, thereby reducing N mineralization. Furthermore, N translocation towards high C:N ratio substrates could reduce N-limitation, thereby increasing fungal activity and decomposition. Two litter decomposing fungi, Marasmius androsaceus and Mycena epipterygia, were grown in axenic laboratory microcosms containing spatially separated substrates: pine needles (C:N ratio of 135) and ¹³C¹⁵N-labelled glycine (C:N ratio of 2).

Results/Conclusions When connecting the two substrates, both fungi were able to overcome local C-deficiency on the glycine medium by translocating carbohydrates from the needles. In the presence of needles, N mineralization from glycine was negligible, even though the glycine was utilised by the fungi. The results support the hypothesis that translocation of carbohydrates within fungal mycelia may preclude N mineralization. Even though pine needle decomposition and fungal growth previously have been stimulated by additions of external N, only trace amounts of N were translocated to the needles in this study. These results suggests that N cycling models, where the C:N ratio of the substrate determines the fate of the N, may be over simplified and even inaccurate if applied to forest ecosystems were fungi dominate the decomposer community. The results highlight the importance to acknowledge the fundamental difference in physiology of different groups of microorganisms.