Nitrogen dynamics of two contrasting soils amended with two types of municipal sewage sludge.

Under the framework of Cyclic Economy and EU Green Deal, sewage sludge represents an ideal soil amendment with a potential to increase soil OM, provide nutrients and reduce chemical fertilization, which otherwise would be disposed in landfills. Nonetheless, its agronomic use comes with an uncertainty of its potential to release ample plant-available N and trace-metals in a wide range of soils.

This study investigated the N dynamics of municipal sewage sludge applied in two contrasting soils; an acidic (pH 5) and an alkaline (pH 8). Stabilized sewage sludge, limed (LM) or air-dried (AD), was applied (2% dw) in soil mesocosms (1500 g) that were incubated for 90 days (25^oC; 12% soil moisture). A fertilized treatment (F: 100 mg/Kg NH₄NO₃), and a non-amended treatment (control) were also included. During the incubation soil NO₃^-, NH₄⁺, N₂O and CO₂ were regularly monitored. Anaerobic mineralizable N (AMN) was determined at 15 days. At the end of the incubation, trace-metals, organic C and total Kjeldahl N were determined using standard methods.

The acidic soil receiving LM and AD sewage sludge had 4x and 5x greater (p=0.004) AMN rates than the control. Whereas the alkaline soil receiving air-dried sewage sludge had 2x greater (p=0.01) AMN rates than the control. Soil organic C was on average(±SE) 10.4±0.6 g/Kg and no significant differences were found in the acidic soil. In the alkaline soil, organic C was on average 16.1±0.4 g/Kg, and LM and AD treatments had significantly more org. C than the control (p=0.01). Total N was on average 1.5±0.3 g/Kg and no significant differences were found in both soils. During the incubation, soil NH₄+ decreased in LM and AD treatments, and slightly increased in F and C treatments in the acidic soil. Soil NH₄+ in the alkaline soil slightly increased for all treatments. A sharp increase in soil NO₃- in the acidic soil was observed in all treatments except the control at approx. 60 d. In the alkaline soil, soil NO₃- remained at similar levels as initially. It appears that in acidic soils receiving sewage sludge, the relative low soil pH inhibits NH₄+ oxidation, whereas in alkaline soils the relative high pH inhibits NO₃- reduction. Cumulative CO₂ emissions were ~1.3x greater in LM and AD than F and control treatments, and cumulative N₂O emissions were ~1.5x greater in AD only than F and control treatments for both soils. Interestingly, N₂O emissions for LM were at similar levels to the control treatment for both soils. At the end of the incubation, trace-metal concentration increased in all treatments, yet, it remained below legislative critical levels. The above effects varied slightly between LM and AD sewage sludge, therefore further experimentation is required to understand the effects of sewage sludge type and quality on soil fertility and crop productivity. Our preliminary results show that stabilized sewage sludge has the potential to be a safe soil conditioner and fertilizer under the framework of Cyclic Economy and EU Green Deal.

Funding: The research work was supported in part by the Hellenic Foundation for Research and Innovation