Final Report Summary - QUASOM (Quantifying and modelling pathways of soil organic matter as affected by abiotic factors, microbial dynamics, and transport processes) Climate change and global carbon cycle at decadal to centennial time scale is influenced by the changes of the very large amount of carbon stored in soil organic matter (SOM). The dynamics of these changes so far have been treated in models in a very simplified manner. These simplified treatments have been shown to be inadequate to reproduce field and lab experiments on shorter time scales, and are suspected to be one of the largest sources of uncertainty in projections in Earth System Models. The QUASOM project had two aims: first to improve understanding of soil carbon and water dynamics affected by abiotic and biotic factors, and second to utilize the existing and new knowledge to enhance the representation of the soil in the Earth System models. Studies were done at several spatial and temporal scales, from detailed process-level experiments, up to assessing patterns of carbon stores and respiration at the global scale. A central component was developing models with increasing levels of simplification and integrating those models with data. Both a soil incubation experiment at controlled laboratory incubations and a greenhouse field experiment utilized labelling, i.e. marking, of carbon atoms by stable isotopes and tracing these labels through the system to elucidate the pathways and controls of new carbon entering the system by plant turnover. One main outcome was SOMPROF, a parsimonious model of vertical transport of SOM across the soil profile. The depth explicit description allows improved representation other processes, e.g. the thermal insulation of the soil by an organic layer or energy-limitation of microbes responsible for SOM stabilization in the subsoil. By implementing the SOMPROF into the global ecosystem model JSBACH these interactions can now be simulated and studied at a global scale and can be better benchmarked with globally distributed soil maps.Another finding was the strong but complicated control of living microbes on the dependence of SOM dynamics on environmental conditions. Therefore, detailed models describing microbial dynamics on small scales have been simplified to represent the most important effects on SOM decomposition. Usually the interactions between the fresh carbon from plants and the older soil carbon cannot be neglected, as is currently done in most models.Furthermore, patterns of carbon turnover have been compiled that help benchmarking and improving earth system models.Overall the QUASOM project gained and combined insight of SOM dynamics at different scales. These insights resulted in improved parsimonious descriptions and models that are useful to a broader scientific community especially in the climate modelling community.