Influence of fungi on N and C dynamics during organic matter decomposition in boreal forests

Soils in terrestrial ecosystems store more carbon (C) than plants and the atmosphere combined, and ecosystems’ C dynamics are strongly dependent of nitrogen (N) availability. Moreover, plant production in boreal ecosystems is often limited by low N availability, and N retention in soils is a major constraint on N recirculation to plants. Soil fungi strongly influence C and N interactions in boreal ecosystems. However, a better knowledge of their role in the C and N interactions and balances is required. In this work we address this question with modelling and experimental approaches and explore impacts of fungi on C and N balances during soil organic matter decomposition in boreal ecosystems. We developed a biogeochemical model of C and N flows between plants, soil organic matter (SOM), saprotrophs, ectomycorrhizal (ECM) fungi, and inorganic N stores to predict the effects of ECM fungi decomposition on plant production and soil C sequestration. Model-based qualitative investigations on plant-mycorrhizal symbiosis were performed to improve our understanding of the mutualistic-parasitic continuum of ECM fungi’s relationships with plants. Under controlled laboratory conditions microcosms were set up with saprotrophic fungi (Gymnopus androsaceus and Chalara longipes) to explore saprotrophic fungi’s influence on soil N retention and changes in microbial carbon use efficiency (CUE) during decomposition. The model-based analysis indicated that under low-N conditions, increased ECM decomposition promotes plant growth but decreases soil C storage. Further, the model analysis indicated that the mutualistic-parasitic continuum between plant and the mycorrhiza depended on the rates of C allocation from the plant to ECM fungi. The experimental observation showed that both G. androsaceus and C. longipes incorporated N into the non-hydrolysable fraction, but with a relatively higher N incorporation in the latter. Overall, this thesis highlights that soil C storage in boreal forests is regulated by the relationship between plant and ECM fungi and this relationship depends on the partitioning of decomposition between saprotrophs and ECM fungi. This work shows that differences in decomposing strategies between two saprotrophic fungi play an important role in N retention during litter decomposition. In addition, we propose that better methods to evaluate CUE could improve predictions of C and N dynamics in ecosystem models. Taken together, I suggest that enhanced knowledge about the functional properties of soil fungi and incorporating different fungal traits into ecosystem models could significantly improve predictions of ecosystem responses to environmental changes.