B54C-06
Export of Greenhouse Gases Across the Soil/Stream Interface in a Boreal Headwater Stream

Friday, 18 December 2015: 17:15
2008 (Moscone West)
Mats G Oquist, SLU Swedish University of Agricultural Sciences Umea, Umeå, Sweden
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 1st 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.