Soil organic carbon dynamics under land-use and management scenarios in a metropolitan urban–rural territory

Metropolitan authorities, governing territorially contiguous areas where urban, peri-urban, and agricultural land uses coexist, play a key role in steering territorial trajectories through spatial planning and incentive-based policies. To effectively implement climate strategies, they require a clear understanding of how land-use patterns and management practices across agricultural, natural, and urban spaces may evolve and affect soil organic carbon (SOC) stocks and tree carbon storage. In line with climate commitments such as the European Green Deal and local climate–energy action plans, metropolitan authorities are required to develop strategies that combine greenhouse gas emission reductions with enhanced SOC sequestration. While increasing SOC and biomass carbon is widely recognized as a climate mitigation strategy, land-use change and management practices can also drive SOC losses and CO₂ emissions, depending on the trajectory adopted. Existing models that quantify SOC dynamics under different agricultural management practices, as well as methods estimating carbon stock changes based on land-use conversion factors. However, the combined effects of SOC fluxes, land-use change, and management in mixed urban–rural areas are rarely modelled within a single, spatially framework, particularly urban expansion’s impact on soils. Scenario-based SOC modelling is key to guiding decisions for achieving carbon-neutrality.

 

This study evaluates SOC and biomass carbon fluxes under four land-use and management scenarios co-developed with territorial planning stakeholders from the Rennes Metropolitan Area (north-western France). The study area covers 705 km² and includes agricultural and natural land (510 km²) and urban areas (195 km²). The first scenario follows current trends, with continued urban expansion and unchanged agricultural practices. The second assumes moderate improvements in farming practices and reduced conversion of agricultural soils to urban land. The third represents a transformative trajectory characterized by a strong increase in permanent grasslands at the expense of arable land, enhancing SOC storage. The fourth, maximalist scenario converts all agricultural land to forest to estimate the upper bound of SOC and biomass carbon sequestration. These scenarios were assessed relative to the current situation to simulate changes in soil and tree‑biomass carbon stocks by 2050. Models of varying complexity were used to address uncertainties associated with long‑term projections.

 

Initial results indicate that all scenarios increase SOC and biomass carbon stocks; however, the first two remain insufficient to offset projected metropolitan greenhouse gas emissions by 2050. Even under the most ambitious scenarios, results highlight those urban green spaces alone cannot achieve carbon neutrality. Rural soils constitute the main SOC sink at the territorial scale, but they are also associated with significant emissions, particularly from grazing systems, underscoring the need for ambitious soil management strategies and cross-territorial compensation mechanisms.