Wastewater Sludge-Based Circular Fertilizer Regulates Nitrogen and Phosphorus Availability and Carbon Dynamics Under Saline-Affected Soils

Soil salinization impairs soil fertility and nutrient supply in reclaimed coastal lands. Wastewater sludge-based fertilizers are sometimes used to recycle nutrients. Synthetic biosolids consist of mineral-like particles engineered from activated sludge and were previously shown to release inorganic nutrients in response to root exudates, but their behavior under salt stress remains unclear. Dried activated sludge was prepared by oven-drying activated sludge from a municipal wastewater treatment plant at 85 °C for 2 days. We evaluated how synthetic biosolids, relative to a control and dried activated sludge, affect soil inorganic nitrogen, available phosphorus, and labile carbon fractions in a lettuce pot experiment at four salinity levels (electrical conductivity, EC, of 1, 3, 5, and 7 dS m−1). Synthetic biosolids maintained the highest available P2O5 at 3–7 dS m−1 (12–98% above the control). At 7 dS m−1, they maintained higher soil NH4+-N than the control and dried activated sludge (18.77 vs. 10.81 and 9.58 mg kg−1), lowered permanganate-oxidizable, cold-water-extractable, and hot-water-extractable C by 16.2–29.5%, and raised citric-acid-extractable C by 14.9% relative to the control. Lettuce biomass, relative chlorophyll content (SPAD), and root-to-shoot ratio did not differ significantly from the control. These findings suggest that synthetic biosolids can sustain phosphorus and ammonium availability under salinity while altering extractable carbon fractions, although the release mechanism and possible carbon stabilization were inferred rather than directly measured.