Ectomycorrhizal (ECM) fungi play a key role in nutrient cycling in boreal forests. Especially plant growth-limiting nutrients such as nitrogen (N) and phosphorus (P) are known to be taken up efficiently by ECM fungi. These nutrients are subsequently transferred to the plant host in exchange for sugars assimilated though photosynthesis. However, most N and P in soils are not readily available for uptake by mycelia since they are mostly present in organic molecules such as amino acids, proteins, chitin etc. or they are enclosed in, bound to or embedded in intact cell walls, lignin, etc. Hence, in order for ECM fungi to gain access to these nutrients, a broad spectrum of more or less complex organic molecules must be broken down. Previous experiments on P. involutus indicated that this ECM fungus employs a mechanism similar to the one found in brown-rot fungi. This mechanism is based on Fenton-type reactions whereby hydroxyl radicals are produced. These hydroxyl radicals are subsequently able to attack and degrade the complex organic soil molecules making desired nutrients more readily available for uptake. To get a more detailed view on the SOM degradation mechanisms employed by ECM fungi, we now study radical reactions and the associated chemical modifications of carbon compounds in situ using Fourier-Transform Infrared and Raman spectromicroscopy. Furthermore, molecular nanoprobes monitoring changes in pH and reactive oxygen species activity are developed and used to further understand SOM modifications by ECM fungi.