Effect and Mechanism of Root Characteristics of Different Rice Varieties on Methane Emissions

Methane (CH₄), which is an important component of the greenhouse gases from paddy ecosystems, is a major contributor to climate change. CH₄ emissions from paddy ecosystems are closely related to the rice root system; however, how the rice root system affects CH₄ emissions remains unclear. We conducted a field experiment in 2023 at the Heping Irrigation District Rice Irrigation Experiment Station in Qing’an County, Heilongjiang Province. The field experiment used five local rice varieties with similar fertility periods to observe rice root morphology and physiology indexes, CH₄ emission fluxes, and cumulative CH₄ emissions. A structural equation model (SEM) was established to investigate the effects of root characteristics on the CH₄ emissions from rice and understand the potential mechanisms of these effects. The results showed that the seasonal patterns of CH₄ emission fluxes were similar in different rice varieties, and that, during the tillering to heading–flowering stages, the cumulative CH₄ emissions accounted for 89.8–92.6% of the total cumulative CH₄ emissions of rice. Significant negative correlations were observed between CH₄ emission fluxes and root volume, root dry weight, root oxidation activity (ROA), and root radial oxygen loss (ROL) (r = −0.839, −0.885, −0.401 and −0.934, <i>p</i> < 0.05), while there were significant positive correlations between root diameter; malic acid, citric acid, and succinic acid contents; and CH₄ emission fluxes (r = 0.407, 0.753, 0.797, and 0.685, <i>p</i> < 0.05). The SEM showed that CH₄ emission fluxes were directly influenced by ROL and organic acid contents, while the other root indicators had indirect effects by modulating ROL and organic acid contents. ROL and root volume had the largest total effect, indicating that ROL and root volume were the most significant root physiological and morphological indicators affecting CH₄ emission fluxes. This study provides theoretical support and reference data for achieving sustainable agricultural development in the black soil region of Northeast China.