Evaluating circular practices in agriculture: A farm-level nitrogen and greenhouse gas marginal abatement cost curve approach

Adoption of more circular farming systems that prioritise renewable and recycled resources could enhance nutrient cycling, reduce farm costs and lower greenhouse gas (GHG) emissions by decreasing reliance on external inputs. This study is the first to incorporate nitrogen (N) efficiency into a marginal abatement cost curve framework, bridging GHG mitigation with nutrient management. This integrated approach provides new insights into circular practices that can deliver win–win benefits and help avoid trade-offs between two of agriculture’s most pressing environmental challenges. Using regional farm data and mass flow models, we investigated eight circular practices across five representative UK farming systems. Alternative feed ingredients reduced GHG emissions by 4.7% and saved £200 Mg CO 2 e −1 yr −1 for the mixed farm. Solid-liquid slurry separation was the most cost-effective and N efficient practice for the dairy farm, reducing emissions by 4.8%, saving £129 Mg CO 2 e −1 yr −1 and reducing the N balance by 17 kg N Mg CO 2 e −1 yr −1 . Cover crops and multispecies leys were optimal for cost-effective and N-efficient emission reductions on arable and beef-sheep farms. While anaerobic digestion reduced emissions and improved N efficiency, it was not cost-effective. System-based practices, such as mixed crop-livestock integration, showed greater regional than individual farm-level mitigation potential. Future research could explore regional-scale and broader environmental, economic and social impacts of circular practices to better inform policy and increase their appeal. Our results reveal farm-specific strategies that deliver multiple, cost-effective environmental benefits, highlighting the value of linking climate and nutrient management in sustainable agriculture. • Circular practices were assessed in diverse UK farming systems. • A MACC approach considered GHG and N mitigation. • Cover crops cut arable emissions by 28% and N balances by 28 kg N Mg CO 2 e −1 yr −1 • Alternative feeds offer GHG and cost savings, while AD is not cost-effective. • Slurry separation is the most cost-effective and N-efficient practice for dairy.