Fire severity, time since fire, and site-level characteristics influence streamwater chemistry at baseflow conditions in catchments of the Sierra Nevada, California, USA

Background: Fire plays an important role in controlling the cycling and composition of organic matter and nutrients in terrestrial and aquatic ecosystems. In this study, we investigated the effects of wildfire severity, time since fire, and site-level characteristics on (1) concentration of multiple solutes (dissolved organic carbon, DOC; total dissolved nitrogen, TDN; dissolved organic nitrogen, DON; calcium, Ca²⁺; magnesium, Mg²⁺; potassium, K⁺; sodium, Na⁺; chloride, Cl⁻; nitrate, NO₃ ⁻; ammonium, NH₄ ⁺; sulfate, SO₄ ²⁻; and phosphate, PO₄ ³⁻), and (2) the molecular composition of stream-dissolved organic matter (DOM) across 12 streams sampled under baseflow conditions in Yosemite National Park, California, USA. Samples were collected from low- and high-severity burned stream reaches, as well as an unburned reference stream reach. Results: Fire severity, time since fire, and variability in site-level characteristics emerged as the strongest influences on streamwater chemistry. Results from mixed-effect models indicated that DOC and DON concentrations decreased with time since fire in high-severity burned stream reaches. In low-severity burned stream reaches, DOC concentrations increased, and DON concentrations slightly decreased with time since fire. We also found that declines in aromaticity (expressed as decreased SUVA₂₅₄) and mean molecular weight DOM (expressed as increased E₂:E₃ ratios) with time since fire were associated with high-severity fires. Mixed-effect models also indicated that site-level characteristics played a role in solute responses. Aliphatic structures dominated streamwater DOM composition across fire-impacted catchments, but neither fire severity nor time since fire was a significant predictor of the proportion of aliphatic structures in streamwater DOM. North aspect exhibited the highest concentrations of Ca²⁺, K⁺, and Mg²⁺, whereas the north-northwest aspect exhibited the highest concentrations of Cl⁻ and SO₄ ²⁺. We also observed elevated Ca²⁺, K⁺, and Mg²⁺ in burned (but not reference) stream reaches with pool-riffle versus step-pool bed morphology. Conclusions: Taken together, our findings suggest that the response of stream chemistry to wildfires in the Sierra Nevada, California, can persist for years, varying with both fire severity and site-specific characteristics. These impacts may have important implications for biogeochemical cycles and productivity in aquatic ecosystems in fire-adapted landscapes.