Modelling SOC and Nutrient Dynamics under Organic Farming Expansion across the EU

Food production contributes significantly to environmental degradation, accounting for an estimated 78% of global ocean and freshwater eutrophication (Poore & Nemecek, 2018), being the leading driver of biodiversity loss, and representing a major driver of soil health loss (EUSO, 2024). Organic farming is often proposed as a strategy to mitigate these impacts by enhancing biodiversity, reducing nutrient losses at large-scale adoption, and improving multiple soil quality parameters (Seufert & Ramankutty, 2017). Consequently, policy initiatives such as the European Green Deal’s Farm to Fork strategy aimed to expand organic farming across Europe (European Commission, 2020). However, the sustainability benefits of organic agriculture depend strongly on local conditions. For example, transitioning to organic management can risk decreasing soil organic carbon (SOC) stocks (Gaudaré et al., 2023), reducing yields, and potentially increasing greenhouse gas emissions per unit of product due to lower productivity (Meier et al., 2015). These outcomes depend on region-specific factors such as soil properties, climatic conditions, management practices, and nutrient availability.

This study evaluates how a transition to organic agriculture influences SOC and nutrient (N, P) dynamics across the EU by comparing a business-as-usual (BAU) scenario with a scenario in which 25% of agricultural land is managed organically by 2030. We employed the spatially explicit, process-based biogeochemical model DayCent at a 1 km² scale across the EU, which has been calibrated and tested for European conditions (Muntwyler et al., 2023), to simulate SOC turnover, nutrient cycling, and crop yields across diverse soil and climate gradients. The model integrates detailed representations of mineralization, stabilization, plant uptake, and nutrient losses, thus capturing key processes. To evaluate broader environmental consequences, model outputs were combined with a life cycle assessment (LCA) framework using regionalized characterization factors that quantify N- and P-related impacts on freshwater fish biodiversity (Zhou et al., 2024).

Achieving the 25% organic target showed potential to improve degraded soils (defined by nutrient surplus/excess), reduce reliance on mineral fertilizers, and maintain or lessen current eutrophication impacts on freshwater fish. The spatially explicit modelling framework enabled identification of hotspot regions where transitions to organic agriculture yield environmental benefits with minimal productivity losses. However, these benefits were accompanied by reduced average yields of grain and tuber crops, partly driven by increased fodder crop production in organic rotations. A complementary cover crop scenario highlighted the benefits of increased N fixation, improved yields, and mitigation of SOC decline, but also led to higher impacts on freshwater biodiversity due to increased N losses.

These results underscore the importance of jointly considering interconnected N, P, and C cycles, yield responses, and potential feedbacks when evaluating management transitions. The approach provides valuable insights into the synergies and trade-offs between agricultural practices and environmental consequences at high spatial resolution, supporting evidence-based decisions for sustainable land management and policy.