Sustainable microalgal cultivation at commercial scale requires nitrogen recycling. This study applied hydrothermal carbonization to recover N of hot-water extracted Arthrospira platensis biomass residue into aqueous phase ( AP) under different operation conditions and evaluated the N utilization, biomass yield and quality of A. platensis cultures using AP as the sole N source. With the increase of temperature at 190-210 degrees C or reaction time of 2-3 h, the N recovery rate decreased under nitrogen-repletion (+N) cultivation, while contrarily increased under nitrogen-limitation ( -N) cultivation. Under +N biomass accumulation in the cultures with AP under 190 degrees C was enhanced by 41-67% compared with that in NaNO3, and the highest protein content of 51.5% DW achieved under 200 degrees C-2 h was also 22% higher. Carbohydrate content of 71.4% DW under -N cultivation achieved under 210 degrees C-3 h was 14% higher than that in NaNO3. HTC-algal cultivation strategy under -N mode could save 60% of conventional N. (C) 2016 Elsevier Ltd. All rights reserved.