Lateral Dissolved Organic Carbon Losses Represent ∼10% of Upland Tundra Carbon Losses and Include Seasonal Permafrost Contributions

We investigated lateral dissolved organic carbon (DOC) fluxes from lower-order streams in upland tundra underlain by permafrost to assess their contribution to carbon (C) sink or source strength. The study site, located in Healy, Alaska, is within the Panguingue Creek watershed (66 km²), where permafrost acts as a confining layer for soil pore water. We examined how seasonal hydrology affects DOC export to understand lateral C flux contributions to the site's net ecosystem C balance. Previous estimates of vertical gas exchange (net CO₂ uptake and release) indicated a loss of 52.5 g C m⁻² yr⁻¹ and methane was estimated to lead to an additional loss of 6.4 ± 0.20 CO₂-equivalent (g CO₂–C m⁻² yr⁻¹) suggesting the site is a net C source. We found that a second-order stream exported an additional 5.0 g DOC m⁻² yr⁻¹ laterally, which is approximately 10% of the vertical CO₂ loss, reinforcing the site's source status. Accounting for CO₂, CH₄, and DOC, total C losses were estimated to be 64 g C m⁻² yr⁻¹. Further, we found that shoulder seasons played a key role in C export, with the spring freshet alone accounting for 59% of total DOC flux. Seasonality also influenced DOC age, with older, permafrost-derived DOC exported primarily during spring and fall. These findings underscore the significance of lateral pathways in permafrost C budgets and highlight DOC as a critical, seasonally variable component of C loss.