A dual-solution for coastal cities: global potential of Ulva prolifera biochar in macroalgal waste disposal and wastewater nitrogen removal enhancement.

Converting green-tide biomass into biochar could connect macroalgal waste management with wastewater nitrogen removal, but how feedstock and pyrolysis temperature influence competing nitrogen-transformation pathways remains unclear. Here, unpyrolyzed Ulva prolifera and wheat straw and their biochars produced at 300-700 °C were compared in 90-d activated-sludge batch microcosms. Material characterization, physicochemical monitoring, isotope tracing, functional-gene sequencing, and association-network analysis were integrated to examine nitrogen-transformation responses. Ulva prolifera biochar produced at 500 °C (UBC-500) exhibited the highest cumulative denitrification and anammox, representing increases of 334.3% and 220.4%, respectively, relative to the unamended control. By comparison, wheat straw biochar produced at 500 °C (WBC-500) increased denitrification by 75.1% but decreased anammox by 21.4%. Cumulative dissimilatory nitrate reduction to ammonium decreased by 55.8% in UBC-500 and 58.3% in WBC-500, indicating comparable suppression. UBC-500 shifted nitrogen conversion toward denitrification, increasing its contribution to 91.9% while decreasing the dissimilatory nitrate reduction to ammonium (DNRA) contribution to 2.4%. Enhanced denitrification was associated with near-neutral sludge pH, moderate TOC:TN, and earlier and more sustained Gammaproteobacteria enrichment. Considering both biochar yield and nitrogen-removal performance, 500 °C was the most favorable preparation temperature within the investigated range. Under the central scenario, the dry-matter-uncorrected upper-bound estimate of additional nitrogen removal was approximately 2.50 × 103 t N yr-1. These findings identify UBC-500 as a promising amendment and provide a quantitative basis for integrating green-tide biomass valorization with wastewater nitrogen removal.