Bridging the gap in riverine greenhouse gas research: From monitoring methods to mechanistic understanding.

Despite their increasing recognition in global greenhouse gas (GHG) budgets, riverine emission estimates face challenges from data gaps and methodological inconsistencies. To evaluate these challenges, this study reviewed 568 publications on riverine emissions published from 2011 to 2024. We analyzed monitoring methods, influencing variables, data characteristics, and emission pathways, using PRISMA protocol. River research represents only 8 % of all aquatic GHG studies, with a strong geographical bias toward large rivers, especially in Asia (52 %). Small rivers remain underrepresented despite their potentially substantial contributions. Current monitoring approaches are dominated by small-scale methods (floating chambers: ∼30 %; headspace method: ∼50 %) that are accurate yet have poor spatiotemporal representation. Large-scale methods utilizing remote sensing and unmanned platforms comprise less than 5 % of applications, leaving critical gaps in regional-scale assessments. Most studies (64 %) span less than one year with low-frequency sampling (39 % one-time snapshots), limiting comprehensive understanding of temporal dynamics. Researchers have focused primarily on instream variables that directly affect local GHG production (90 %), while basin-scale factors have received less attention. We propose a decision tree framework for selecting optimal methods based on spatiotemporal scales, available resources, and emission pathways. Future monitoring approach should integrate small- and large-scale measurements to combine accuracy with broader coverage. Furthermore, isotope and molecular analyses, underutilized in 85 % of studies, can clarify underlying biogeochemical processes and microbial activities. Their integration will thereby reduce uncertainties in global riverine GHG inventories and support more effective mitigation strategies.