Riparian Soils Inform Dissolved Organic Matter Delivery in a Norwegian Subarctic River

Critical factors in dissolved organic matter (DOM) cycling are changing in subarctic to arctic systems, with less knowledge than boreal and temperate systems on soils, flowpaths, and biogeochemistry to inform process understanding and catchment modelling. We test the hypothesis that riparian soils accumulate appreciable concentrations of total and labile carbon (C), nitrogen (N), and phosphorus (P) and contribute strongly to subarctic macronutrient cycling across the terrestrial‐to‐aquatic interface. Such subarctic soils are rarely described, especially in terms of combining C, N, and P data together. We sampled hillslope to riparian transitions at four subcatchments (31–61 km 2 ) of the 16,000 km 2 Norwegian River Tana (69° N) to: (i) assess soil C, N, and P concentrations, stocks, soil reactive chemistry, and water soluble macronutrient forms; (ii) understand spatial variability; (iii) consider the role of near‐channel soils in DOM fate across scales in large subarctic rivers, including experimentation on subsoil DOM sorption and soil flowpath conceptualisation. Horizon‐based differences in total C, N, P concentrations and water‐extracted macronutrients showed wetter riparian and stream‐side positions had enhanced total C, N concentrations and DOC concentrations (up to ~200 mgC/L). Stocks of C (2–28 kg/m 2 ), N (0.1–1.2 kg/m 2 ), and P (< 0.1–0.9 kgP/m 2 ) were highly variable, greatest in riparian positions in the plateau tundra sites. Similar P stocks to that of N suggest moderate P and low N supply to ecosystems. Organo‐mineral soil transitions studied show lateral flows through high DOM source layers near‐channel and important hillslope interactions between surface and subsoil pathways capable of retaining (30% DOC removal in column experiments) and altering DOM quality. Our data inform frameworks for DOM cycling in large arctic riverscapes, by: (i) showing strong DOM sources in near‐channel soils highly connected to headwaters, (ii) understanding amounts and quality (absorbance properties and stoichiometry) of potentially transported DOM, and (iii) reactivity and flow routing controlling DOM mobility, sorption and alteration of DOM forms. There is a clear role for combining soil biogeochemistry and hydrology to look inside the catchment ‘box’ to better understand DOM cycling in changing ecosystems.