Export of Greenhouse Gases Across the Soil/Stream Interface in a Boreal Headwater Stream

Abstract:

Lateral export of greenhouse gases (GHGs) from groundwater to surface waters is often poorly quantified in GHG budgets of terrestrial ecosystems, including boreal forests. The importance of lateral CO₂ export through groundwater for net ecosystem carbon balances is becoming increasingly evident. However, riparian zones in the boreal landscape often exhibit redox characteristics that promote production also of N₂O and CH₄, that may reach surface waters and add to the total GHG emissions. We estimate annual and seasonal groundwater export of CO₂, N₂O and CH₄ from a boreal forested catchment (11.9 ha) draining into a 1^st order stream and quantify the total emission of greenhouse gases across the soil/stream interface. We present annual GHG export for two years, one with annual discharge close to the long–term average and one when discharge was twice as high. During the year with normal discharge export of CO₂, N₂O and CH₄ were 12.3, 0.0015, and 0.00014 g m^-2 year^-1, respectively, while the export during the wet year was 14.9, 0.001 and 0.0001 g m^-2 year^-1. In terms of CO₂ equivalents the GHG export was dominated by CO₂, 96% and 91% for the normal and we year, respectively. Corresponding contributions for N₂O were 4% and 9%, respectively, while export of CH₄ was <0.1% during both years. While the export of CO₂ was evenly distributed between the growing season and the non-growing season, most of the N₂O and CH₄ export occurred during the non-growing season. Through measurements of groundwater nitrate concentrations the validity of the existing IPCC emission factor EF5-g for N₂O emissions was tested. It showed that this emission factor underestimates actual groundwater N₂O emissions from the catchment by 2-3 orders of magnitude.