Nutrient uptake along a fire gradient in boreal streams of Central Siberia

Fire can transform the boreal forest landscape, thereby leading to potential changes in the loading of organic matter and nutrients to receiving streams and in the retention or transformation of these inputs within the drainage network. We used the Tracer Additions for Spiraling Curve Characterization (TASCC) method to conduct 17 nutrient-addition experiments (9 single additions of NO3- and 8 combined additions of NH4+ and PO43-) in 5 boreal headwater streams underlain by continuous permafrost and draining watersheds with a range of burn histories (4->100 y since last burn) in the Nizhnyaya Tunguska River watershed in Central Siberia. Hydrology, ambient nutrient concentration, and the ratio of dissolved organic C (DOC) to nutrients drove rates of nutrient uptake in the streams. Nutrients were taken up with greater efficiency and magnitude under conditions with high flow and reduced diffusive boundary layer (DBL), regardless of watershed burn history. Ambient molar ratio of DOC: PO43- explained some variation in ambient uptake velocity (upsilon(f)) for NH4+ and PO43-. We also observed tight coupling between ambient rates of NH4+ and PO43- uptake across the watershed burn-history gradient. These data suggest that fire-driven changes in stream chemistry may alter N and P retention and subsequent export of materials to downstream receiving waters. Climate change is likely to enhance the frequency and intensity of boreal forest fires and alter the extent of permafrost. Therefore, understanding the interactions among C, N, and P in these Arctic systems has important implications for global biogeochemical cycling.