Soil carbon sequestration is an important but hardly understood mechanism counterbalancing atmospheric CO2 emissions and thereby climate change. An important biotic determinant of soil carbon transformations is mycorrhiza, a plant-fungal symbiosis featured by nearly all vascular plants on Earth. Mycorrhizae have different forms, among which arbuscular and ectomycorrhiza (AM and EM) are the most wide-spread. Using thorough experimentation and modeling, this project fillss the principle knowledge gap with regard to soil carbon accumulation processes: to reveal if and how AM and EM fungi differently affect soil carbon cycling.