Increase in CH4 emission due to weeds incorporation prior to rice transplanting in a rice-wheat rotation system

• Methanogenic pathway (Fac) and fraction of CH4 oxidation (Fox) were estimated.
• Fac and Fox were little affected by weeds incorporation and rice straw application.
• Applying weeds increased production and emission but played a minor role in oxidation.
• Soil NH4+-N content and temperature played an important role in CH4 oxidation.
• Weeds incorporation increased CH4 emission by 47–235% in the Jiangsu Province.

Planting winter wheat with rice straw application (WS) after rice harvest is a typical practice in Jiangsu Province, China. However, in the past decades quite a large area of paddy fields appeared to be left in fallow in the winter season, letting abundant weeds incorporated prior to rice transplanting (FSW) during the following rice season. Little is known about the effects of WS and FSW on the process of CH4 emission and on relative contribution of acetate to total CH4 production (Fac) and fraction of CH4 oxidized (Fox). Results show that FSW significantly increased CH4 production potential of paddy soil, CH4 concentration in soil solution, and CH4 emission from the fields (P < 0.01) but WS played a slight role. CH4 oxidation was more influenced by soil NH4+-N content and temperature rather than FSW and WS. The total CH4 emission from paddy fields in the Jiangsu Province during the rice season were estimated to increase by 0.30–0.37 and 0.14–0.21 Tg CH4 in 2008 and 2009, respectively, as a result of FSW. Similar δ13C-values of CH4 production and emission, and CH4 dissolved in soil solution over the season suggest that FSW and WS played a minor role in Fac and Fox. Generally, acetoclastic methanogenesis was dominated (∼80%) at the beginning and the end of the season while Fox was very high (over 65%) throughout most of the rice season. The findings indicate that FSW significantly increased CH4 emission. If this fact is not taken into account seriously, budget of regional CH4 emission from paddy fields will be significantly underestimated.