Potential for long-term transfer of DOC from riparian zones to streams in boreal catchments

José Ledesma1, Thomas Grabs2, Kevin Bishop2, Sherry Schiff3, Stephan Köhler1
1 Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden
2 Department of Earth Sciences, Uppsala University, Uppsala, Sweden
3 Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Canada

O 7.4 in Controls of dissolved organic matter fluxes in ecosystems

14.07.2014, 12:15-12:35, H19

Boreal regions store large amounts of terrestrial carbon, which can be transferred as dissolved organic carbon (DOC) to inland waters with ramifications for both aquatic ecology and carbon budgets. Riparian zones (RZ), connecting the terrestrial and aquatic compartments, are believed to be the net sources of DOC independent of upslope conditions. Moreover, most important lateral flowpaths are confined to a finite layer in the riparian soil profile, limiting the export of DOC to this ‘dominant source layer’ (DSL). The question that arises is how long boreal RZ can sustain lateral DOC fluxes as the sole source of exported carbon. This study investigates the theoretical turnover time of DOC by comparing the size of the potential sources in relation to the lateral fluxes in the DSL of 13 riparian profiles distributed over a 68 km2 boreal catchment in northern Sweden. The profiles cover a wide range in wetness conditions and soil types and thus are classified in 4 classes: till-wet, till-humid, till-dry, and sediment. The DSL for these 13 profiles was 36 ± 18 (± SD) cm, which, according to our arbitrary definition, implies that, on average, only about one third of the 1 m riparian profile contributed 90 % of the total DOC flux (2008-2009). The 13 RZ exported on average 8.7 ± 6.5 g C m-2 year-1. Significantly higher DOC fluxes were found in till-wet and till-humid RZ in comparison with till-dry and sediment ones. 50% of the variation in DOC fluxes could be explained by median groundwater tables. Moreover, 90% of the variation could be explained by the RZ width, which was mapped in the field. We suggest that lateral DOC fluxes from the entire riparian catchment area could thus be estimated if RZ width was delineated in high resolution soil maps. Specific carbon pools per square meter of RZ (62 ± 27 kg m-2) were similar to those given in the literature for wetlands and histosols but no differences between riparian classes were found. On the other hand, the variability in total carbon pools that account for the organic layer extent was consistent with the variability in DOC fluxes. The estimated turnover times were in the order of hundreds of years for all profiles, meaning that there is, potentially, a long-lasting supply of DOC from these RZ. Rough estimations of the primary production for these types of environments suggest that the export could be maintained without drawing down the riparian carbon pools. This was further supported by measurements of DO14C in some of the streams of the catchment, which indicate that modern carbon is the predominant fraction exported, including streams that have been anthropogenically disturbed by deepening or ditching. We believe that the potential of future climate to increase the fluxes in combination with the potential of RZ to sustain them will have an impact in the future water quality in boreal regions.

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Letzte Änderung 19.06.2014