Sewage sludge-derived hydrochar that inhibits ammonia volatilization, improves soil nitrogen retention and rice nitrogen utilization

Hydrothermal carbonization (HTC) is a promising technique for treating sewage sludge. In this study, three sewage sludge-derived hydrochars produced with water (SSHW), 1 wt% magnesium citrate (SSHM) solution, and 1 wt% magnesium citrate mixed with 1 wt% sulfuric acid (SSHMS) solution were applied to columns of packed paddy soil. We evaluated the effects of these differently modified sewage sludge-hydrochars on ammonia volatilization, soil nitrogen (N) retention and rice growth. Results showed that compared to the control, SSHMS reduced the cumulative ammonia volatilization determined after three split application of N-fertilizer. SSHM and SSHMS both reduced the yield-scale ammonia volatilization by 20.3% and 41.2% respectively. Moreover, the addition of three sewage sludge-derived hydrochars increased soil ammonium-N retention after the first supplementary fertilization; however, after the second supplementary fertilization, only SSHMS addition significantly increased soil ammonium-N retention. Of the three hydrochars tested, SSHMS has the strongest effects on soil ammonium-N retention and inhibition of ammonium-N loss in floodwater. This was attributed to increased ammonium sorption driven by SSHMS's lower surface pH and porous diameter, larger adsorption porous volume and higher abundance of carboxyl functional groups. Additionally, the increased soil N retention increased grain N content and yield. Our results provide a novel method to valorize sewage sludge into a valuable fertilizer that if applied to paddy soil it can inhibit ammonia volatilization, N loss in floodwater, and promote N use efficiency by rice, with positive implications for sustainable rice production.