The aim of this work was to assess the biogeochemical role of riparian soils in the High Arctic to determine to what extent these soils may act as sources or sinks of carbon (C) and nitrogen (N). To do so, we compared two riparian areas that varied in riparian vegetation coverage and soil physical perturbation (i.e., thermo‐erosion gully) in NE Greenland (74°N) during late summer. Microbial soil respiration (0.4–3.2 μmol CO2 m−2 s−1) was similar to values previously found across vegetation types in the same area and increased with higher temperatures, soil column depth and soil organic C degradation. Riparian soils had low nitrate concentrations (0.02–0.64 μg N‐NO3− g−1), negligible net nitrification rates and negative net N mineralization rates (−0.58 to 0.33 μg N g−1 day−1), thus indicating efficient microbial N uptake due to low N availability. We did not find any effects of physical perturbation on soil respiration or on N processing, but the dissolved fraction of organic matter in the soil was one order of magnitude lower on the disturbed site. Overall, our results suggest that riparian soils are small N sources to high‐Arctic streams and that a depleted dissolved organic C pool in disturbed soils may decrease exports to the adjacent streams under climate change projection.

Climate change is causing drastic landscape changes in the Arctic, but how these changes modify stream biogeochemistry is not clear yet. We examined how catchment properties influence stream nitrogen (N) and dissolved organic carbon concentrations (DOC) in a high-Arctic environment. We sampled two contrasting headwater streams (10-15 stations over 4.8 and 6.8 km, respectively) in Northeast Greenland (74 N). We characterized the geomorphology (i.e., bedrock, solifluction and alluvial types) and the vegetation (i.e., barren, fell field, grassland and tundra types) cover of each subcatchment area draining into each sampling station and collected water samples for hydrochemistry characterization. The two sampled streams differed in geomorphology and vegetation cover in the catchment. Aucellaelv catchment was mostly covered by a 'bedrock' geomorphology (71%) and 'fellfield' vegetation (51%), whereas Kaeerelv was mostly covered by 'alluvial' geomorphology (65%) and 'grassland' and 'tundra' vegetation (42% and 41% respectively). Hydrochemistry also differed between the two study streams, with higher concentrations of inorganic N forms in Aucellaelv and lower DOC concentrations, compared to Kaerelv. The results from the linear mixed model selection showed that vegetation and geomorphology had contrasting effects on stream hydrochemistry. Subcatchments with higher solifluction sheets and limited vegetation had higher nitrate concentrations but lower DOC concentrations. Interestingly, we also found high variability on the production and removal of nitrate across subcatchments. These results indicate landscape controls to nutrient and organic matter exports via flow paths, soil organic matter stocks and nutrient retention via terrestrial vegetation. Moreover, the results suggest that climate change induced alterations to vegetation cover and soil physical disturbance in high-Arctic catchments will affect stream hydrochemistry, with potential effects in stream productivity, trophic relations as well as change of solute export to downstream coastal areas.

In the Arctic, climate changes contribute to enhanced mobilization of organic matter in streams. Microbial extracellular enzymes are important mediators of stream organic matter processing, but limited information is available on enzyme processes in this remote area. Here, we studied the variability of microbial extracellular enzyme activity in high-Arctic fluvial biofilms. We 2 evaluated twelve stream reaches in North-East Greenland draining areas exhibiting different geomorphological features with contrasting contents of soil organic matter to cover a wide range of environmental conditions. We determined stream nitrogen, phosphorus, and dissolved organic carbon concentrations; quantified algal biomass and bacterial density; and characterized the extracellular enzyme activities involved in catalyzing the cleavage of a range of organic matter compounds (e.g. β-glucosidase, phosphatase, β-xylosidase, cellobiohydrase, and phenol oxidase).We found significant differences in microbial organic matter utilization among the study streams draining contrasting geomorphological features, indicating a strong coupling between terrestrial and stream ecosystems. Phosphatase and phenol oxidase activities were higher in solifluction areas than in alluvial areas. Besides dissolved organic carbon, nitrogen availability was the main driver controlling enzyme activities in the high-Arctic, which suggests enhanced organic matter mineralization at increased nutrient availability. Overall, our study provides novel information on the controls of organic matter usage by high-Arctic stream biofilms, which is of high relevance due to the predicted increase of nutrient availability in high-Arctic streams in global climate change scenarios.

The objective of this study was to evaluate how stream water nutrient concentrations influence biofilm accrual in streams draining mountainous permafrost headwaters. We selected six stream locations in the Zackenberg area (NE Greenland, 74°N) subjected to a gradient in the areal contribution of different geomorphological units in the watersheds and channel stability. We used nutrient diffusing substrates to evaluate biofilm growth (autotrophic and total biomass). We found elevated stream nitrate concentrations in samples from upstream reaches draining larger areas of solifluction sheets and bare rock and with higher channel instability. Nitrate had the highest standardized effect on autotrophic biofilm growth on control disks. However, stream biofilm growth was not nutrient limited as shown by the absence of an increase in biofilm biomass as a response to the experimental nutrient additions. The response to nutrient additions via diffusing substrates depended on the altitude gradient. Overall, our results showed stream nitrogen availability to be one of the main drivers of algal biofilm accrual in high‐Arctic streams, suggesting that the predicted changes in nutrient exports induced by climate change will have strong impacts on the biogeochemistry and ecological functioning of high‐Arctic streams.

No abstract