To study the role of mycorrhizal types in soil carbon (C) storage, the first two years of the project were largely devoted to the development and implementation of novel innovative technologies. To trace C flux from the atmosphere through plants to mycorrhizal mycelium and ultimately to the soil or back to the atmosphere, experimental setups ranging from microcosms (Petri dishes), pots, and mesocosms (lysimeters) to field conditions have been established.
In microcosms, we investigated the carbon use efficiency (CUE) of various fungal species, including both saprotrophs and ectomycorrhizal fungi, depending on substrate quality and nutrient availability. A system using isotope labeling coupled with cavity ring-down spectrometry to measure real-time fungal respiration was developed to estimate CUE. Additionally, methods to extract representative sample solutions from the medium in Petri dishes were tested and established for measuring hydrolytic and oxidative enzyme activities. This will help to elucidate the decomposition mechanisms of individual fungi depending on chemistry of the substrates provided.
Using 13C-CO2 pulse labeling of plants in pots combined with high-time resolution measurements of soil CO2 and its isotopic composition, we acquired data for modeling C allocation within the plant-soil system. The modeling exercise was conducted in collaboration with project partners at the TUM, Munich, Germany. A first version of the model is established and will further be refined and validated with data from laboratory experiments.
The model will later be used to predict C allocation in mini-forest experiments. The MYCO-SoilC mini-forests are established. They consist of ten tree species, half of which have arbuscular mycorrhizal associations and the other half have ectomycorrhizal associations. A newly developed system for 13C-CO2 pulse labeling of trees in the mini-forests allow us to trace C allocation from trees to fungal partners and soil over three consecutive years. The flux of 13C via mycorrhizal mycelium to soil organic matter fractions (particulate and mineral-associated organic matter) will further be studied, aiming to understand the contributions of different mycorrhizal types and fungal residues to soil C stabilization.
Together with partners from the Indiana University Bloomington, USA, we conducted research to examine how different mycorrhizal types contribute to soil carbon stabilization and the formation of mineral-associated organic matter in a temperate forest. To estimate the contribution of mycorrhizal fungal residues to particulate and mineral-associated organic matter, we informed Bayesian mixing models with 13C and 15N natural abundances of particulate and mineral-associated organic matter sources, i.e. leaves, roots, arbuscular mycorrhizal hyphae, ectomycorrhizal and saprotrophic sporocarps. Overall, our results suggest fungal, not plant residues, as the main source of mineral-associated organic matter in temperate forests and highlight the critical role of mycorrhizal types for soil C stabilization.
