[Effects of Coconut Chaff Biochar Amendment on Methane and Nitrous Oxide Emissions from Paddy Fields in Hot Areas]

Based on the rice-vegetable crop rotation model, in-situ measurements of nitrous oxide (N₂O) and methane (CH₄) emissions were conducted in double-cropping rice fields in Hainan to determine the impact of coconut chaff biochar on greenhouse gas emissions. The experiment involved four treatments:conventional farming fertilization (CON), nitrogen fertilizer combined with 20 t ·hm^-2 biochar (B1), nitrogen fertilizer combined with 40 t ·hm^-2 biochar (B2), and no nitrogen fertilizer, as the control (CK). The N₂O and CH₄ emissions were measured using static chamber-gas chromatography during the two paddy seasons, and the global warming potential (GWP) and greenhouse gas intensity (GHGI) were also estimated. The results show that N₂O emission dynamics during the early rice season are closely related to the mineral nitrogen content of the soil. The N₂O is emitted at the rice seedling and tillering stages after fertilization. The cumulative N₂O emission during the early rice season was 0.18-0.76 kg ·hm^-2. Compared with the CON treatment, the biochar treatments reduced N₂O by 18%-43%, and the B2 treatment resulted in a significant reduction. The addition of biochar may promote the reduction of N₂O at the early rice seedling stage and increase N₂O emissions by improving the soil NO₃^--N content at the early rice tillering stage. During the late rice season, N₂O is emitted during the heading and maturity stages, and the cumulative N₂O emission was 0.17-0.34 kg ·hm^-2. The B1 treatment reduced emissions by 37%, and B2 increased emission by only 3%, which is not a significant difference. The peak of CH₄ emissions from rice fields appeared in the late phase of the early rice season and prophase of the late rice season. The cumulative emission of CH₄ in the early rice season was 3.11-14.87 kg ·hm^-2. Compared with CON, the CK treatment increased emission by 39%. The biochar treatment may increase soil aeration and limit the ability of CH₄ production in the early rice season, as B1 and B2 treatments reduced CH₄ emissions by 28% and 71%. The cumulative CH₄ emission in late rice season was 53.1-146.3 kg ·hm^-2, and the emission dynamics were significantly positively correlated with NH₄⁺-N content. CK and B1 treatments increased CH₄ emissions by 52% and 99%, respectively compared with CON, and the B2 treatment significantly increased CH₄ emissions by 176%. Compared with CON, the B1 and B2 treatments increased the yield by 12.0% and 14.3% when applied in the early rice season and by 7.6% and 0.4% when applied in the late rice season, respectively. Due to the increased methane emissions in the late rice season, biochar amendment increased the GWP of the double-cropping rice field, in which the high amount of biochar reached a significant level; different amounts of biochar had no significant effect on the GHGI of the double-cropping rice field. Thus, the application of coconut chaff biochar for the reduction of greenhouse gas emission, from rice fields in hot areas, requires further research.