Post-fire changes in dissolved organic carbon and nitrate concentration and the effects on nutrient uptake in a boreal forest stream in interior Alaska

Background/Question/Methods

Wildfire frequency and area burned are increasing throughout the North American boreal forest due to warming and drying climate. More frequent fire will have large impacts on carbon and nutrient fluxes in streams, as vegetation in watersheds is lost, soil combusts, and watershed flowpaths are altered. Following a wildfire in 2004 in the Caribou-Poker Creeks Research Watershed located in interior Alaska, stream-water dissolved organic carbon concentration declined, whereas inorganic nitrogen concentration increased. Further, both solutes had strong longitudinal gradients along the stream length, resulting in variation in nutrient stoichiometry. We used the longitudinal gradient to examine how the stoichiometry of dissolved organic carbon to nitrate affects in-stream nutrient uptake by measuring nutrient uptake at four locations along the stream. At each sample location, we measured nutrient uptake using five nutrient additions: ammonium, nitrate, phosphorus, ammonium + phosphorus, and nitrate + phosphorus. 

Results/Conclusions

Preliminary results show that the low dissolved organic carbon:high nitrate reach was less efficient (V_{f_ammonium} = 12mm/min) in retaining ammonium compared to the high dissolved organic carbon:low nitrate reach (V_{f_ammonium} = 16mm/min). When ammonium and phosphorus was added together, the high nitrate stream reach was more efficient in retaining ammonium (V_{f_ammonium} = 14mm/min) compared to when adding only ammonium. These findings suggest that the elevated nitrate concentration observed post-fire was in part due to a reduction in dissolved organic carbon concentration and decline in ammonium uptake and sequestration. These observations may further indicate that future increases in wildfire frequency may increase the amount of nitrogen and decrease the amount of dissolved organic carbon being delivered to the streams, and thus change the stoichiometry of carbon to nutrients within the stream and decrease a streams ability to process nutrients like ammonium. This may in turn decrease a streams ability to retain nutrients such as nitrogen and increase the amount of nitrogen being transported downstream.