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

Clay-hydrochar composites (CHCs) are of great significance in ammonium (NH4+) adsorption and have the potential to be applied to paddy fields to prevent ammonia (NH3) 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 NH3 volatilization and NH4+ loss in floodwater were monitored after three fertilization dates. The results showed that among the three CHCs, only the BTHC significantly reduced cumulative NH3 volatilization (by 41.8%), compared to that of the unamended control (without addition of hydrochar or clay-hydrochar-composite). In the unamended control, NH3 volatilization loss accounted for 31.4% of the applied N fertilizer; with the BTHC amendment, NH3 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 NH3 volatilization was mainly attributed to the increased soil NH4+ 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 NH4+ retention. This study, for the first time, screened BTHC as an excellent material for mitigating NH3 volatilization from paddy fields. The reduced NH3 volatilization loss might contribute to increased soil N retention and plant N use efficiency.