Methane emissions sources and impact mechanisms altered by the shift from rice-wheat to rice-crayfish rotation

Methane (CH4) emissions from rice paddies constitute a significant source of greenhouse gases in the agricultural sector. However, the impact and mechanisms of changing traditional paddy-upland systems to rice-crayfish symbiotic systems on CH4 sources are not well understood. This study involved a two-year field experiment to investigate the sources and determinants of CH4 emissions in rice‒wheat rotation (RW) and rice-crayfish rotation (RC) by collecting CH4 emission fluxes from paddy fields, non-rhizosphere soil, and rice plants in conjunction with environmental and soil indicators. Our study shows that due to the increase in the amount and activity of carbon sources brought by crayfish farming, the activity of mcrA in RC increased by 156.17% before rice planting. The peak CH4 emissions from RC paddy fields occurred earlier and were higher, and the total emissions were 19.51%–37.17% higher than RW in the rice growing season. Moreover, the regreening and tillering stages were the peak periods of CH4 emissions, during which the CH4 emissions from non-rhizosphere soil in RW accounted for 44.86%–60.28%, and after conversion to RC, the contribution rate increased from 45.15% to 83.28%. However, the total CH4 emissions from rice plants were consistent in both RW and RC, with contributions between 46.09% and 64.06%. Rice roots were more conducive to CH4 production in the early stage and more conducive to CH4 oxidation in the late stage. Environmental indicators at a depth of 10–20 cm were more reflective of CH4 emission flux in RW, with moisture being the most important factor. After conversion to RC, the depth increased to 20–30 cm, and the most important environmental indicator became dissolved organic carbon (DOC) in surface water. These findings help to clarify the role of human activity management, including field water and carbon sources, on CH4 production under two rotation systems and provide a basis for mitigating the greenhouse effect.