Major Wildfires Affect Stream Carbon and Nutrient Concentrations in Permafrost Dominated Basins in The Central Siberian Plateau

Abstract:

Arctic soils hold large stores of the planet’s carbon and wildfires are extreme events that have been shown to decrease stream organic carbon exports. Fires combust the organic layer above the permafrost, exposing mineral soils and changing hydrologic pathways reaching aquatic ecosystems, but the effects of fire on stream dissolved organic matter quality and inorganic nutrients are not well understood. Our objective was to characterize water chemistry and dissolved organic matter (DOM) composition across streams in the Central Siberian Plateau that have been influenced by extensive, basin-scale wildfires. Here we quantify dissolved organic carbon (DOC), dissolved organic N (DON), NH₄⁺, NO₃^-, PO₄^3-, major dissolved ions, and DOM composition via FTICR-MS from 15 headwater streams in the Central Siberian Plateau as well as 3 major rivers (Kochechum, Nidym and, Nizhnyaya Tunguska) during the falling limb of the spring freshet in the summer of 2016. These headwater streams comprise a burn gradient spanning from 3 to over 100 years since the last burn, creating an ideal opportunity to study spatial and temporal patterns in water chemistry and DOM composition across a burn gradient. Preliminary data show that DOC concentrations decreased from 23 mg C/L in older burned sites to 12 mg C/L in recently burned sites. NO₃^- and major ion concentrations showed the opposite pattern, with concentrations higher (0.2 mg/L inorganic N) at recently burned sites and lower (0.01 mg/L inorganic N) at older burn sites. DOC and DON concentrations declined with decreasing discharge demonstrating their tight link to surface flow paths. Conversely, inorganic N species and ion concentrations increased at lower flows in the recently burned sites while older burn sites showed less variation in water chemistry. These temporal changes across sites could be due to the subsurface water pathways developed by the exposure of permafrost and changes in active layer depth from recent fires or lack thereof in older burns sites. Our data varies predictably across the fire gradient which could have important implications for stream solute exports to the Arctic Ocean as the potential for fires increases due to climate change.