Municipal sewage sludge treatment and soil pH conclusively affect the nitrogen dynamics of amended soils

• Soil type, sewage sludge type, and their interaction significantly affected soil N dynamics. • The low soil pH inhibited NH 4 + oxidation, while the high soil pH inhibited NO 3 - reduction. • The N 2 O emission factor was lower in sewage sludge amended soils than in those fertilized. • Amended soils lost up to 88% equivalent of their initial organic C as CO 2 emissions. • Sewage sludge-amended soils had higher mineralizable N content. Under the Circular Economy and EU Green Deal framework, sewage sludge represents an ideal soil amendment. Nonetheless, its agronomic use comes with an uncertainty of its capacity to release ample plant-available N. This study investigated the N dynamics of two contrasting soils (pH 5 and pH 8) amended with two types of stabilized sewage sludge, limed (LM) or air-dried (AD). A fertilized (F), and a non-amended treatment (C) were also included. During the mesocosm incubation of 90 days, soil type, sewage sludge type, and their interaction were significant ( p <0.05) factors affecting soil NO 3 - , NH 4 + , N 2 O, and CO 2 . It appears that in acidic soils receiving sewage sludge, the relatively low soil pH inhibits NH 4 + oxidation, whereas in alkaline soils the relatively high pH inhibits NO 3 - reduction. Cumulative CO 2 emissions were ∼1.3x greater in LM and AD than in F and C treatments, and cumulative N 2 O emissions were ∼1.5x greater in AD only than in F and C treatments for both soils. However, sewage sludge amendments reduced ∼ 2% the N 2 O emission factor compared to inorganic fertilization. Amended soils lost approx. 88 and 42% equivalent of the initial org. C as CO 2 , for the acidic and alkaline soil, respectively. The alkaline soil, LM and AD treatments had significantly ( p =0.01) more org. C than the C treatment, no significant differences were found for the acidic soil. Total N was on average 1.5 ±0.3 g Kg -1 for both soils. Sewage sludge amended soils had a greater potential to supply N in the longer term, as evidenced by enhanced N mineralization rates in both soils. Trace-metal concentration increased in all treatments, yet it remained below legislative critical levels. Our preliminary results show that stabilized sewage sludge has the potential to be a safe soil amendment under the framework of Circular Economy and EU Green Deal.