Linking Soil Nitrogen and Carbon Dynamics to Post-Wildfire Stream Water Quality.

Forest fires create lasting biogeochemical changes with implications for watershed processes and stream water quality. Decreased plant nutrient uptake and increased mineralization of soil organic matter are the mechanisms commonly held responsible for persistent changes. Heating of organic soil layers creates char layers that can persist for many years following wildfire. Our work after the 2002 Hayman Fire shows that increased stream nitrogen (N) is proportional to the extent of a catchment burned, but the specific processes responsible for the patterns are uncertain. To elucidate factors that regulate persistent elevated stream N, as well as post-fire stream carbon (C) export, we examined soil N and C pools and processes across ecotones extending from unburned forests to areas of moderate and high wildfire severity. We analyzed 1-2 cm thick charred organic layers that remain visible 14 years after the fire, underlying mineral soils, and leachate from both layers. The wildfire reduced O horizon total C content from 28% in unburned areas to 18% and 14% in moderate and high severity burned landscapes. Dissolved total N and water soluble inorganic N leached from burned O horizons were 50 to 30% of concentrations found in unburned O horizons with little difference among burn severity classes. On-going work is examining mineralization and nitrification and potential loss of inorganic N from char layers and burned soils. We will also examine the molecular structures of organic C in char layer using pyrolysis-gas chromatography-mass spectrometry and 13C nuclear magnetic resonance, as well as the optical properties and disinfection byproduct formation potential of dissolved organic matter leached from these soil layers. Data to date suggest that elevated soil N pools or turnover may not account for the chronic stream N levels and that in-stream N uptake may contribute the persistent post-fire changes in watershed N retention.