[Effect of Applying Hydrochar for Reduction of Ammonia Volatilization and Mechanisms in Paddy Soil]

Hydrochar, as a product of the hydrothermal carbonization of biomass, has good application prospects for the NH₃ volatilization reduction in rice fields due to its rich pore structure and functional surface. In this study, hydrochar was applied as a soil conditioner to paddy soil. A soil column experiment was conducted to investigate the effect of hydrochar on NH₃ volatilization throughout the growth period of rice. The experiment was conducted with three treatments:CKU (control without hydrochar); SHC (sawdust hydrochar); and W-SHC (water-washed sawdust hydrochar). The application rate of SHC and W-SHC was 0.5% (w/w). The study investigated the effects of different hydrochars on the pH and concentrations of NH₄⁺-N in floodwater, the flux and accumulation of NH₃ volatilization, and the yield-scale cumulative emission of NH₃ volatilization. Results show that the SHC treatment significantly reduces cumulative emissions of NH₃ volatilization and the yield-scale cumulative emissions of NH₃ volatilization (P<0.05), which were 32.42% and 47.61% lower than CKU, respectively. The effect of W-SHC on ammonia volatilization reduction was slightly weaker, as the cumulative emissions of NH₃ volatilization and the yield-scale cumulative emissions of NH₃ volatilization decreased by 10.14% and 27.71%, respectively, compared with CKU. The NH₃ volatilization reduction was possibly related to the disturbance of pH and the decrease in NH₄⁺-N concentrations in the floodwater because of the application of hydrochar. Compared with CKU, both SHC and W-SHC treatments reduced the pH and NH₄⁺-N concentration in the floodwater. The impacts were more obvious in the rice base fertilizer period (BF) and the first supplemental fertilizer period (SF1) than in the second supplemental fertilizer period (SF2). The soil urease activity was significantly inhibited by hydrochar (P<0.05), and the abundance of soil ammonia-oxidizing gene (AOA, AOB) also significantly increased after application of SHC (P<0.05). This resulted in the enhanced efficiency of ammonia-oxidizing, which had an effect on the reduction of the NH₄⁺-N concentrations in the floodwater. This study provides theoretical and experimental data support for the application of hydrochar in agro-environments with regard to ammonia volatilization reduction in paddy fields.