Effect of carbon availability, phosphorus, and water soil content on GHG emissions: insights from a soil incubation study

Bio-Based Fertilisers (BBFs) derived from nutrient recovery are increasingly promoted as sustainable alternatives to synthetic inputs. However, their effects on greenhouse gas (GHG) emissions remain poorly understood, especially how carbon quality interacts with phosphorus (P) availability and soil moisture to influence CO₂, N₂O, and CH₄ fluxes. A 28-day incubation experiment was conducted using a low-phosphorus agricultural sandy loam soil amended with two pig slurry–derived BBFs that differ in carbon lability: a labile liquid fraction (MFR) and a more recalcitrant solid fraction (BIO). Treatments were applied with or without mineral phosphorus addition and incubated at 55%, 70%, and 85% water-holding capacity. The results showed that carbon quality was the main factor affecting microbial respiration. Labile amendments boosted cumulative CO₂ emissions four- to fivefold compared to recalcitrant or unamended soils, reaching up to 18 g C–CO₂ m^-² under intermediate moisture with P addition. In contrast, several treatments with recalcitrant or no added carbon acted as net N₂O sinks at high moisture levels, with cumulative fluxes ranging from −400 to −900 mg N₂O–N m^-². Methane emissions remained consistently positive and showed no significant response to carbon type or P addition, indicating that methanogenic activity was not strongly promoted under the experimental conditions. These findings demonstrate that GHG emissions from BBFs are primarily controlled by carbon availability and soil moisture. Incorporating carbon-quality considerations into nutrient recovery strategies is therefore essential to support climate-smart fertilisation and help close nutrient loops in circular agricultural systems.