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Low-order streams are suggested to dominate the atmospheric CO2 source of all inland waters. Yet, many large-scale stream estimates suffer from methods not designed for gas emission determination and rarely include other greenhouse gases such as CH4. Here, we present a compilation of directly measured CO2 and CH4 concentration data from Swedish low-order streams (> 1600 observations across > 500 streams) covering large climatological and land-use gradients. These data were combined with an empirically derived gas transfer model and the characteristics of a ca. 400,000 km stream network covering the entire country. The total stream CO2 and CH4 emission corresponded to 2.7 Tg C yr(-1) (95% confidence interval: 2.0-3.7) of which the CH4 accounted for 0.7% (0.02 Tg C yr(-1)). The study highlights the importance of low-order streams, as well as the critical need to better represent variability in emissions and stream areal extent to constrain future stream C emission estimates. Funding Agencies|Knut and Alice Wallenberg foundationKnut & Alice Wallenberg Foundation [KAW 2013.0091]; Swedish research council FORMASSwedish Research CouncilSwedish Research Council Formas [214-2009-872]; Swedish research council VRSwedish Research Council [2012-00048, 201604829]; SKB; ERCEuropean Research Council (ERC) [725546]; Carlsberg FoundationCarlsberg Foundation; FORMASSwedish Research Council Formas [2015-1559]; Swedish Research Council (VR)Swedish Research Council; Swedish Environmental Protection Agency; Swedish Agency for Marine and Water Management; Swedish Energy AgencySwedish Energy Agency

Despite the strong quantitative evidence that riparian zones (RZs) are the dominant source of dissolved organic carbon (DOC) to boreal streams, there is still a debate about the potential contribution of upslope areas to fluvial carbon export. To shed new light into this debate, we investigated the molecular composition of dissolved organic matter (DOM) in four upslope-riparian-stream transects in a Northern Swedish forest catchment using absorbance (A(254)/A(365) and SUVA(254)) and fluorescence (fluorescence and freshness indices) metrics. Based on these metrics, our results indicate that stream water DOM molecular composition resembles that of RZs and significantly differs from that of upslope areas. The resemblance between stream and riparian DOM was most apparent for the Dominant Source Layer (DSL), a narrow RZ stratum that, theoretically, contributes the most to solute and water fluxes to streams. Spectroscopic characterization based on traditional interpretations of the metrics suggested that mineral upslope (podzol) DOM is less aromatic, more microbially derived, and more recently produced than organic riparian (histosol) and stream DOM. We conclude that RZs, and specifically DSLs, are the main sources of DOC to boreal headwater streams and potentially to other streams located in low-gradient, organic matter-rich catchments. Plain Language Summary Understanding carbon cycling in natural ecosystems is critical because ongoing climate change can promote the release of previously stored carbon in forest soils to streams and rivers, with the potential to form carbon dioxide, the main greenhouse gas. This is particularly important in boreal ecosystems, which are the largest stores of terrestrial carbon in the world. In this study, we identify the near-stream area, the so-called riparian zone, as the main source of carbon from boreal forest soils to streams. We provide qualitative data to support this, which together with previous quantitative analyses, make up enough evidence to support that the riparian zone is the main source of carbon to streams. As there is still a debate about the potential contribution of other areas in the ecosystem to the fluvial carbon export, our study importantly highlights that the riparian zone should be the focus of scientific assessments and management strategies in relation to carbon exports in surface waters.