Differential effects of biomass burning on carbon and nutrient dynamics in Arctic fluvial ecosystems

The Central Siberia Plateau (CSP) is undergoing rapid climate change resulting in increasing frequency of forest fires, which have uncertain effects on organic matter and nutrient delivery from headwater streams to downstream ecosystems. Across a fire chronosequence (3 to >100 years) underlain by continuous permafrost, we quantified the effects of wildfire on quantity and quality of dissolved organic matter (DOM) and inorganic nutrients in streams. Wildfire decreased DOM concentrations for about 50 years, but elevated nitrate (NO3-) concentrations lasted only 10 years; ammonium and phosphate concentrations were unchanged. This increase in NO3- and decrease in dissolved organic carbon (DOC) results in a wide range of DOC:NO3-, a ratio that is known to regulate NO3- uptake and denitrification in streams. Ultrahigh-resolution mass spectrometry and DOM optical properties showed that the composition of stream DOM changes after fire, with decreased abundance of polyphenols and aliphatic forms of DOM that are typically more biolabile than other forms of OM. Increasing wildfire frequency is thus likely to have major shifts in the metabolism, carbon flux, and nutrient balance of Arctic fluvial systems.