Abstract: Headwater stream plays a vital role in connecting terrestrial ecosystems with inland rivers, and is also a foremost interface of linking the terrestrial carbon sink with oceanic one. With the intensely biogeochemical processes, headwater streams are important contributors to CO₂ emission budget from global inland waters and therefore affecting global carbon cycling. Clarifying the production and emission processes of CO₂ in headwater streams, and illuminating their spatiotemporal characteristics as well as main regulating factors will be beneficial to re-understand carbon processing ability for river systems. Here, we reviewed the primary sources and processes of CO₂ in headwater streams, including in-stream production and external input. We systematically analyzed the spatial and temporal variations at different time scales (i.e., annual, seasonal, daily and diurnal) and spatial scales (i.e., global, regional, watershed, and habitat) of CO₂ emission from headwater streams. We summarized that the meteorological factors, stream structure characteristics, hydro-geomorphological factors, land environmental factors, and in-water physical and chemical factors were key regulators of the spatiotemporal variations in CO₂ emission from headwater streams. Meanwhile, we summarized current methods for CO₂ fluxes monitoring in inland water, and analyzed their applicability and optimization in headwater streams. We constructed a theoretical framework integrating the headstream waters into the carbon budget of headwater catchments, and proposed that the contribution of CO₂ emission from streams to the terrestrial carbon balance cannot be ignored. Finally, based on the disadvantages existing in current research, we proposed several important research fields of CO₂ emission from headstream, including (1) the analysis of dynamic process of CO₂ emission from streams with different geographical environments; (2) the accumulation of observation database for headwater CO₂ emission with high spatiotemporal resolution in different regions; (3) the relationship of CO₂ emission from headwater stream with land landscape pattern as well as the regulatory mechanisms; (4) the response of headstream CO₂ emission to global climate change; (5) the accurate estimation of total CO₂ emission from regional and global headstreams; (6) the establishment and optimization of monitoring and research methods for headstream CO₂ emission.

Key words: headwater stream, CO₂ evasion, spatiotemporal variation, influencing factor, monitoring method.