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Permafrost and fire as regulators of stream chemistry in basins of the Central Siberian Plateau

Lucy M. Parham, Anatoly S. Prokushkin, Oleg. S. Pokrovsky, Sergey V. Titov, Ekaterina Grekova, Liudmila S. Shirokova, William H. McDowell
Biogeochemistry 2013 v.116 no.1-3 pp. 55-68
permafrost, cations, watersheds, dissolved organic carbon, summer, biogeochemical cycles, plateaus, solutes, combustion, runoff, basins, wildfires, climate change, potassium, adsorption, chlorides, organic horizons, spring, streams, growing season
Stream chemistry in permafrost regions is regulated by a variety of drivers that affect hydrologic flowpaths and watershed carbon and nutrient dynamics. Here we examine the extent to which seasonal dynamics of soil active layer thickness and wildfires regulate solute concentration in streams of the continuous permafrost region of the Central Siberian Plateau. Samples were collected from 2006 to 2012 during the frost-free season (May–September) from sixteen watersheds with fire histories ranging from 3 to 120 years. The influence of permafrost was evident through significantly higher dissolved organic carbon (DOC) concentrations in the spring, when only the organic soil horizon was accessible to runoff. As the active layer deepened through the growing season, water was routed deeper through the underlying mineral horizon where DOC underwent adsorption and concentrations decreased. In contrast, mean concentrations of major cations (Ca²⁺ > Na⁺ > Mg²⁺ > K⁺) were significantly higher in the summer, when contact with mineral horizons in the active zone provided a source of cations. Wildfire caused significantly lower concentrations of DOC in more recently burned watersheds, due to removal of a source of DOC through combustion of the organic layer. An opposite trend was observed for dissolved inorganic carbon and major cations in more recently burned watersheds. There was also indication of talik presence in three of the larger watersheds evidenced by Cl⁻concentrations that were ten times higher than those of other watersheds. Because climate change affects both fire recurrence intervals as well as rates of permafrost degradation, delineating their combined effects on solute concentration allows forecasting of the evolution of biogeochemical cycles in this region in the future.