Impacts of double-cropping rice variety combinations on greenhouse gas emissions and yield in paddy fields

This study aims to systematically evaluate the effects of the different combinations of double-cropping rice varieties on the greenhouse gas (GHG) emissions and yield in paddy fields in southeastern Hubei Province, China. Two early-season rice varieties (EZ18 and LY287) and three late-season rice varieties (JY38, TY398, and EJ403) were selected in this experiment. Six combinations (LY287-JY38, LY287-EJ403, EZ18-JY38, EZ18-EJ403, LY287-TY398 and EZ18-TY398) were set as well. A split-plot design was used to carry out a two-year field experiment under the mechanized planting mode. A systematic investigation was then made to determine the methane (CH₄) and nitrous oxide (N₂O) emissions, carbon footprint (Cf), yield, and net ecosystem economic benefit (NEEB) in paddy fields. The results showed that there were significant differences in yield among double-cropping rice combinations (P < 0.05). Specifically, the LY287-JY38 treatment achieved the highest yield in 2020, increasing by 2.8%-17.1% over the rest. The highest yield was observed in the LY287-TY398 treatment in 2021, increasing by 0.9%-7.6%. Additionally, the double-cropping combinations significantly dominated the cumulative GHG emissions (CH₄ and N₂O) (P < 0.05). The CH₄ emissions were ranked in the descending order of: EZ18-JY38, LY287-JY38, EZ18-EJ403, EZ18-TY398, LY287-EJ403, LY287-TY398, for the six combinations. The LY287-TY398 exhibited the lowest CH₄ emissions from 2020 to 2021, which were reduced by 2.6%-29.9% and 7.7%-44.6%, respectively. While the EZ18-EJ403 and LY287-EJ403 shared the lower N₂O emissions, which were reduced by 11.2%-20.5% and 10.9%-20.6%, respectively. However, there was no significant difference in the N₂O cumulative emissions between EZ18-EJ403 and LY287-EJ403. Furthermore, the NEEB ranged from 8 900 to 10 935 CNY/hm² in the different double-cropping rice combinations. According to the two-year data, LY287-JY38 achieved the highest NEEB, with 16 098.09 CNY/hm² in 2020 and 10 057.01 CNY/hm² in 2021. GHG emissions from agricultural inputs ranged from 2 478.9 to 2 631.4 kg/hm² for the early and late rice, and from 5 009.5 to 5 175.1 kg/hm² for different double-cropping rice combinations. In terms of double-cropping rice combinations, the LY287-JY38 and LY287-TY398 combinations shared the lowest GHG emissions from the agricultural inputs, compared with the rest. Among them, the main source of agricultural GHG emissions was the diesel consumption of mechanical operations, accounting for 52.4 %-55.3 % of the total. Moreover, the double-cropping rice combinations significantly dominated the Cf (P < 0.05) from 2020 to 2021. In conclusion, the LY287 combination with the TY398 and JY38 demonstrated the higher yields, higher NEEB, and lower carbon emissions, suitable for the mechanized, green, low-carbon, and high-yield cultivation of the double-cropping rice. These findings can also provide the theoretical support to reduce the GHG emissions for carbon neutrality in farmland. The scientific evidence can also be gained to optimize the variety combinations for high-yield, stable, and low-carbon rice production.