Abstract: The application of straw biochar in farmland in Dongting Lake area can realize the resource utilization of straw and reduce environmental risk. In 2020, a rice pot experiment was conducted to investigate the effects of different biochar application levels on ammonia volatilization rate, cumulative ammonia volatilization, pH value, and NH₄⁺-N concentration in surface water. Six levels of reed (Miscanthus lutarioriparius) biochar amount, i.e., 0%, 1%, 2%, 4%, 6% and 8% of soil weight of the 0-20 cm column, were applied in two typical paddy soils in sou-thern China, i.e., reddish clayey soil derived from quaternary red soil and granitic sandy soil derived from granite. Compound fertilizer was applied at a rate of 200 kg N·hm^-2. The results showed that biochar application resulted in significant differences in the rate and cumulative amount of ammonia volatilization between the two soils and among different biochar treatments. For the granitic sandy soil, peak ammonia volatilization under different biochar treatments appeared at the second day after fertilization, which was decreased by 23.6%-53.4%. For the reddish clayey soil, peak ammonia volatilization appeared between the 7th to 13th day after fertilization, which increased with biochar addition level. The rate of ammonia volatilization from the granitic sandy soil was generally higher than that from the reddish clayey soil. For the granitic sandy soil, addition of <4% biochar could inhibit the ammonia volatilization and cumulative volatilization amount, with the greatest reduction (46.9%) at the treatment with 2% biochar addition. The addition of biochar did not affect the pH value of surface water at the early stage of rice growth. For the reddish clayey soil, the pH value and NH₄⁺-N concentration in the surface water increased with biochar addition level, resulting in the increases of ammonia volatilization rate and cumulative volatilization amount by 1.3-10.5 times. Biochar addition level was the key factor affecting ammonia volatilization from the two soils. Elovich equation could well fit the variation trend of cumulative ammonia volatilization with time for the two soils, with the correlation reaching extremely significant level for each treatment. Overall, the application of reed biochar could suppress ammonia volatilization from the granitic sandy soil which was nearly neutral in acidity, while would promote ammonia volatilization via increasing pH value and NH₄⁺-N concentration of surface water for the acidic reddish clayey soil. Therefore, appropriate dosages of reed biochar should be selected for different types of soil in order to reduce nitrogen loss.

Key words: biochar, reed, rice, ammonia volatilization, pH