Ectomycorrhizal fungi in boreal forests: their role in mineral weathering and nitrogen mobilisation

Ectomycorrhizal (ECM) fungi play key roles in mineral weathering and nitrogen (N) mobilisation in boreal forests, yet knowledge gaps remain regarding how these processes interact and mechanistic explanations of mineral weathering are lacking. While progress has been made in understanding N mobilisation and transport gene regulation, the regulation of base cation transporter genes, uptake, and low molecular weight organic acid (LMWOA) production remains largely unexplored. This thesis investigates the relationships between mineral weathering, N mobilisation, and plant C allocation, focusing on ECM fungal communities, the evolution of base cation transporter gene families, and the regulation of base cation transport and LMWOA production. The results show that mineral weathering and base cation mobilisation are up-regulated in plants with greater organic matter availability, driven by increased nutrient demand and C allocation to the O and B soil horizons, where the ECM fungi Piloderma sphaerosporum and Suillus bovinus were most abundant. Base cation transporter gene families in Suillus evolved rapidly, with expansions linked to base cation uptake, especially in the MgtE gene family, which was significantly positively correlated with Mg uptake. Significant expansions were also observed in Piloderma. Transcriptional analysis of S. bovinusrevealed up-regulation of several base cation transporter genes in response to mineral treatment, with regulation occurring at both gene copy number and transcription levels. Additionally, Piloderma fallax in symbiosis with Pinus sylvestris showed significantly higher LMWOA production and exudation in the organic N treatment compared to the inorganic N treatment, with gene expression influenced by the N source. These findings highlight the interconnectedness of mineral weathering and N mobilisation and underscore the need for integrated approaches to understand these processes and their regulation, with implications for sustainable forestry and climate change mitigation.