The dynamic character of forest soil organic carbon (SOC) stocks is demonstrated in the simulation model TransparC2U. The model has up to 10 layers, each with two pools with first order kinetics running for 40 years. The model is aimed at teaching soil science to a broad audience in academia, extension services, secondary schools and public life. The learning objective of the model is to bring about an understanding of SOC dynamics over time, uncertainty, and the complexity of the soil profile with its pedogenesis represented by soil horizons. The user works in an illustrated spreadsheet calculator with options for instant and interactive exploration of the multiple uncertainties and constraints on forest SOC stocks. The user divides the initial SOC stock in each horizon into two pools, Cnew with faster mineralization, and Cold with very slow mineralization, based on their knowledge of horizon designations and stabilization mechanisms.

Data from 23 soil Nordic and Baltic forest soil profiles with horizons characterized by chemical and physical properties are available. Site information to the user intended for reflections on choosing parameter values includes soil classification, tree species, climate, stand age, soil nutrient status and properties that govern physical or chemical protection of soil C (extractable phosphorus (P), total nitrogen (N), subsoil clay content and extractable iron (Fe)). Other changeable inputs are the average annual aboveground litter fall, root litter allocation to individual soil horizons as a fraction (%) of aboveground input, and decomposition rates of each pool. The game played aims for preservation of the initial C stock by manual calibration by the user. The sensitivity of the potential C change (dC ±) is tested by changing the parameter values and watching the effects on model results, i.e. C stocks in each horizon over time. The sensitivity analysis can involve the uncertainty of the input, the initial C content in each horizon, the fraction of Cold and the turnover rates that represent heterotrophic respiration.

The processes representing C input to deeper horizons (roots and exudates), bioturbation by macrofauna, as well as podzolisation provide optional transfer of C between the forest floor and mineral soil layers when judged appropriate by the user based on the site and stand information.

The user may interpret results of the simulation by examining the outputs of total C stock change over 40 years by inspecting total N and P requirements for new litter inputs against potentially available current and future N and P stocks in the soil and the capacity of C binding by subsoil clay and Fe-oxides. It highlights the physical and chemical soil properties required to justify the simulated fate of new and old SOC and the sensitivity of controlling parameters on a decadal timescale to the user.