Abstract: Biochar has been recognized as a new exogenous organic substrate and is often used as an important organic resource for carbon reduction because of its stability, adsorption and carbon nutrient richness. The study was conducted in a typical indica rice area of Nanning from 2021 to 2022, against the background of having high early indica rice yields, simultaneous rain and heat, and unique cropping system. In this paper, authors set three treatments: Control treatment (CK): no fertilizer. Inorganic N input (T1, chemical fertilizer) treatment: chemical fertilizer application at conventional fertilizer level, compound fertilizer 800kg·ha⁻¹, urea 260.87kg·ha⁻¹, potassium 193.55kg·ha⁻¹. Inorganic N with organic N (T2, biochar + chemical fertilizer) treatment: biochar 4000kg·ha⁻¹, compound fertilizer 738.67kg·ha⁻¹, urea 146.09kg·ha⁻¹, potassium 34.19kg·ha⁻¹. The cumulative greenhouse gas emissions, emission equivalents, rice yield traits and the effect of isonitrogen substitution of biochar application on greenhouse gas emissions and rice yield in early southern rice fields were analyzed by regular monitoring of soil greenhouse gas emissions in rice fields during the rice reproductive period using a split static box-meteorological chromatography method 5d after rice transplanting, this study provide a basis for optimizing intensive early rice low-carbon cultivation and reduce fertilizer and increase efficiency. The results showed that: (1) biochar can reduce CH₄ and CO₂ emissions from paddy soils, and reduce the combined emission equivalent by slowing down CH₄ emissions. The application of fertilizer with biochar can mitigate the increase of greenhouse gas carbon emissions caused by fertilizer application alone, and its delayed effect of mitigating CO₂ emissions is more obvious. In biochar treatment (T2), compared with the chemical fertilizer treatment (T1), the maximum CH₄ emission flux in 2021 was reduced by 41.38% and the cumulative emission was reduced by 31.25%, and the maximum emission flux in 2022 was reduced by 50.50% and the cumulative emission was significantly reduced by 50%, and the combined emission equivalents of 2 years were significantly lower than those of the T1 treatment. The maximum CO₂ emission flux and cumulative emission in 2021 were reduced by 57.38% and 37.68%, respectively, compared with the T1 treatment, and the corresponding reduction in 2022 was 16.06% and 35.52% compared to the T1 treatment. (2) Biochar can suppress N₂O emissions, significantly reduce cumulative emissions, and reduce nitrogen source emission equivalents. Compared to the T1 treatment, the maximum N₂O emission flux was reduced by 5.43% and the cumulative emission was significantly reduced by 33.53% in 2021 in T2 treatment; the maximum emission flux was reduced by 73.75% and the cumulative emission was significantly reduced by 54.33% in 2022, and there was no significant change with the CK treatment. (3) Biochar facilitates the optimization of intensive early indica rice cultivation structure and enhances the productivity of early indica rice. After biochar was put into the paddy field for 2 years, the effect of increasing yield became more and more obvious, and the theoretical yield of T2 treatment was 1.02−1.33 times that of T1 treatment, while the actual yield was 1.06−1.32 times that of T1 treatment. Fertilizer with biochar reduced greenhouse gas emissions and increased rice yield in early indica rice fields, which can be used as an optimization model for low-carbon production of intensive early indica rice in the south.

Key words: Greenhouse gas, Biochar, Low carbon optimization, Intensive rice field, Early indica rice