Comparing the variations and influencing factors of CH emissions from paddies and wetlands under CO enrichment: A data synthesis in the last three decades.

Understanding and quantifying the impact of elevated tropospheric carbon dioxide concentration (e [CO]) on methane (CH) globally is important for effectively assessing and mitigating climate warming. Paddies and wetlands are the two important sources of CH emissions. Yet, a quantitative synthetic investigation of the effects of e [CO] on CH emissions from paddies and wetlands on a global scale has not been conducted. Here, we conducted a meta-analysis of 488 observation cases from 40 studies to assess the long-term effects of e [CO] (ambient [CO]+ 53-400 μmol mol) on CH emissions and to identify the relevant key drivers. On aggregate, e [CO] increased CH emissions by 25.7% (p < 0.05) from paddies but did not affect CH emissions from wetlands (-3.29%; p > 0.05). The e [CO] effects on paddy CH emissions were positively related to that on belowground biomass and soil-dissolved CH content. However, these factors under e [CO] resulted in no significant change in CH emissions in wetlands. Particularly, the e [CO]-induced abundance of methanogens increased in paddies but decreased in wetlands. In addition, tillering number of rice and water table levels affected e [CO]-induced CH emissions in paddies and wetlands, respectively. On a global scale, CH emissions changed from an increase (+0.13 and + 0.86 Pg CO-eq yr) under short-term e [CO] into a decrease and no changes (-0.22 and + 0.03 Pg CO-eq yr) under long-term e [CO] in paddies and wetlands, respectively. This suggested that e [CO]-induced CH emissions from paddies and wetlands changed over time. Our results not only shed light on the different stimulative responses of CH emissions to e [CO] from paddy and wetland ecosystems but also suggest that estimates of e [CO]-induced CH emissions from global paddies and wetlands need to account for long-term changes in various regions.