Soil organic matter (SOM) is important to the environment. Its carbon content is the largest reservoir in the global terrestrial ecosystem and decomposition of it emits greenhouse gases including CO2, CH4 and N2O. The current status of GB SOM is under debate because recent observation programmes suggested different findings. Independent and parallel computer simulations of SOM dynamics could provide useful information for the science debate and that is the objective of this thesis. A newly coupled land surface -- SOM dynamics model, JULES-ECOSSE, was applied for the computer simulations in this Ph.D study. The details of this coupled model are described in Chap. 2. Before the simulation studies in Chap. 6, three other studies were done to (1) derive a new algebraic method to spin up the content of SOM pools such that their equilibrium values can be found efficiently; (2) evaluate the model's capability in simulating various gas fluxes using the observed data from the NitroEurope project. This evaluation study provided important information about how well the model works in different aspects; (3) find better performing parameter sets for GB vegetation using a factorial experiment as sensitivity analysis followed by a multi-objective calibration scheme. After the aforementioned studies, the model was applied in Chap. 6 to answer three environmental questions. The simulations suggest that climate change over 1978-2007 had minor impacts on GB SOM, however the future impacts of climate change could potentially be big. The exact magnitudes vary between ecosystems and will also depend on the representative concentration pathway that the world will follow. Inclusion of other observed environmental changes (i.e. changing age structure and composition in woodlands, nitrogen eutrophication effect, changing soil pH and its impact on DOC mobility, soil erosion, non-equilibrium status of SOM) could better match observed changes with simulated changes. This suggests that they could be the candidates explaining the recent observed trends in GB SOM.