[CH₄ Fluxes and Their Comprehensive Greenhouse Effects with CO₂ Fluxes in Direct-seeded Rice in Poyang Lake Plain]

Paddy fields are complex ecosystems that both emit CH₄ and absorb CO₂, which plays an important role in the global water-carbon cycle and carbon budget. In this study, the CH₄ fluxes and CO₂ fluxes of double-cropping direct-seeded rice fields in 2020 in the Poyang Lake Plain were obtained using the eddy covariance method, and the variation characteristics, accumulation in the whole growth period, and comprehensive greenhouse effects of two greenhouse gases were quantitatively revealed. The results showed that, the double-cropping direct-seeded rice field in Poyang Lake Plain was the source of CH₄ emission, and the emission during the whole growth period was 52.6 g·m^-2, with an average daily emission of 0.208 g·(m²·d)^-1. CH₄ emission and daily average emission in the early rice season were 20.7 g·m^-2 and 0.188 g·(m²·d)^-1, respectively, which were lower than the emissions of 31.9 g·m^-2 and 0.255 g·(m²·d)^-1 in the late rice season. CH₄ flux had significant seasonal variation characteristics. The strong emission period (emission peak) of CH₄ was concentrated in the middle growth stage of early rice and the early growth stage of late rice. A total of 85.5% of CH₄ in the early rice season and 92.1% of CH₄ in the late rice season were released during the strong emission periods, and seasonal peak values were 0.638 g·(m²·d)^-1 and 1.282 g·(m²·d)^-1, respectively. The diurnal variation characteristics of CH₄ flux showed three types:obvious unimodal type, non-obvious unimodal type, and irregular type. The strong emission period was mainly the unimodal type, and the peak values of 0.453 μmol·(m²·s)^-1 in the early rice season and 0.977 μmol·(m²·s)^-1 in the late rice season appeared at 14:00-15:00 and maintained a high emission rate at 12:30-16:00. The CO₂ accumulation in the whole growth period of early rice and late rice was -990.4 g·m^-2 and -1156.6 g·m^-2, respectively, and the total was -2147.0 g·m^-2. The comprehensive greenhouse effect of CH₄ emission and CO₂ exchange in the double-cropping paddy field was -673.6 g·m^-2 (calculated using the CO₂ equivalent), which showed a cooling effect. Excluding CH₄ emissions when evaluating the greenhouse effect of the paddy field, the CO₂ equivalent emission of 1473.4 g·m^-2 would be underestimated, accounting for 68.6% of the net CO₂ absorption. Considering CH₄ emissions, CO₂ exchanges, and carbon emissions caused by rice harvest, the two-season direct seeding paddy field in Poyang Lake Plain was the source of greenhouse gas emissions.