Bentonite hydrochar composites mitigate ammonia volatilization from paddy soil and improve nitrogen use efficiency

Clay-hydrochar composites (CHCs) are of great significance in ammonium (NH 4 + ) adsorption and have the potential to be applied to paddy fields to prevent ammonia (NH 3 ) volatilization. In this study, three CHCs were produced by infusing different clays to poplar-sawdust-derived hydrochar , including a bentonite hydrochar composite (BTHC), montmorillonite hydrochar composite (MTHC), and kaolinite hydrochar composite (KTHC). These three CHCs were applied to a paddy soil column system growing rice. The temporal variations in NH 3 volatilization and NH 4 + loss in floodwater were monitored after three fertilization dates. The results showed that among the three CHCs, only the BTHC significantly reduced cumulative NH 3 volatilization (by 41.8%), compared to that of the unamended control (without addition of hydrochar or clay-hydrochar-composite). In the unamended control, NH 3 volatilization loss accounted for 31.4% of the applied N fertilizer; with the BTHC amendment, NH 3 volatilization loss accounted for 17.4% of the applied N fertilizer. The reduced N loss via the BTHC amendment resulted in an increased N supply and further improved the N use efficiency and yield by 37.36% and 18.8% compared to that of the control, respectively. The inhibited NH 3 volatilization was mainly attributed to the increased soil NH 4 + retention as a result of BTHC's larger pore volume and specific surface area. In addition, the BTHC treatment significantly reduced the abundance of archaeal amoA genes ( AOA ), which possibly inhibited nitrification and increased soil NH 4 + retention. This study, for the first time, screened BTHC as an excellent material for mitigating NH 3 volatilization from paddy fields. The reduced NH 3 volatilization loss might contribute to increased soil N retention and plant N use efficiency.