Greenhouse gases emissions from the transition to ratoon rice in typical single-season rice region: An observational study

Food security and global warming are major challenges facing humanity. Promoting ratoon rice cultivation in single-season rice regions where hydrothermal conditions are sufficient for one rice growing season but insufficient for two rice growing season can enhance yield and thereby safeguard national food security. However, changes in greenhouse gas emissions from paddy fields following the adoption of ratoon rice in these regions have not yet been reported. To address this gap, a year-long field observation was conducted in the Taihu Lake Basin using the static chamber-gas chromatography method to monitor GHG emissions and grain yield. Six cropping systems were examined: rice-wheat (RW), rice-rapeseed (RO), rice-cabbage (RC), rice-Astragalus. (RA), rice-fallow (RF), and rice-ratoon rice-vetch (Rr). The results showed that the annual CH₄ emissions of the single-season rice treatments (RW, RO, RC, RF, and RA) ranged from 43.8 to 134.8 kg(CH₄)·hm⁻², while that of the ratoon rice treatment (Rr) was 540.9 kg CH₄·hm⁻². Annual N₂O emissions ranged from 7.2 to 19.7 kg(N)·hm⁻² in the single-season systems and were 4.3 kg(N)·hm⁻² in the ratoon rice treatment. Correspondingly, annual yields ranged from 7.0 to 11.0 t·hm⁻² in the single-season treatments and reached 14.1 t·hm⁻² in the ratoon rice treatment. Compared with the single-season rice treatments, the ratoon rice system exhibited a 3.0 to 11.4 fold increase in annual CH₄ emissions, a 40.4% to 78.1% reduction in annual N₂O emissions, a 71.2% to 292.7% increase in total greenhouse gas emissions (TGHG), and a 33.4% to 101.6% increase in greenhouse gas emission intensity (GHGI). Specifically, compared to the rice-wheat rotation, the ratoon rice system achieved a 43.0% increase in annual yield. Compared to single-season rice treatments, the Rr significantly reduced N₂O emissions. However, the incorporation of straw from the first-season rice crop into the soil led to a marked increase in CH₄ emissions during the ratoon season, resulting in a substantially higher TGHG in Rr than in the five single-season rice treatments. In conclusion, ratoon rice cultivation in typical single-season rice regions can significantly enhance annual grain yield but also increases GHGI. Therefore, it is recommended to strengthen water management in ratoon rice fields and delay straw incorporation until after the ratoon season harvest to mitigate CH₄ emissions, thereby achieving the dual objectives of high yield and emission reduction.