Optimizing Fertilization through Compost and Mineral Nitrogen Synergies: Toward Sustainable Nutrient use and Soil Carbon Conservation

The nutritional and environmental performance of composts depends on their origin and processing methods, which strongly determine their biological stability. Biological stability, together with the C/N ratio, regulates nitrogen release and the persistence of soil organic carbon (SOC). This study evaluated the effects of compost origin and fertilization strategy on mineral nitrogen dynamics and on the mineralization of organic carbon added to the soil through composts. Three composts were compared: bio-waste compost (BWC), anaerobic digestate compost (DC), and sewage and agro-industrial sludge compost (SSC). Composts were applied either alone (100–0) or in combination with mineral nitrogen fertilizer supplying 50% of the nitrogen requirement (50–50) in a 112-days soil incubation experiment and a multi-harvest ryegrass pot trial. These treatments were compared with a mineral fertilized reference (Chem) and an unamended control (Ctrl). BWC, characterized by poor stability, in both fertilization strategies showed the highest mineralizable carbon (15–20%) and shortest half-lives (11–16 days), resulting in elevated CO2 emissions, nitrogen immobilization, and reduced plant nitrogen uptake. DC and SSC, highly stabilized, mineralized less carbon (7–16%), exhibited longer half-lives (≈ 70 days), and fully sustained plant growth when co-applied with mineral nitrogen. Amongst integrated treatments, DC50–50 and SC50–50 matched Chem in plant biomass and apparent N recovery, while enhancing soil organic carbon persistence. Our findings identify compost biological stability as a key predictor of nutrient release patterns and soil organic carbon turnover. Integrating stabilized compost with mineral fertilizer enhances nitrogen synchrony with plant demand, improves nutrient use efficiency, and promotes soil organic carbon persistence, supporting more effective and circular nutrient management strategies.