The Yenisei river passes every type of permafrost regime, from south to north, being characterized by increasing continuity of the permafrost and by decreasing thickness of the active layer. We used that situation to test the hypothesis that amounts and properties of dissolved organic matter (DOM) in small streams draining forested catchments respond to different permafrost regimes. Water samples were taken from eight tributaries along the Yenisei between 67130 0 N and 65149 0 N latitude. The samples were analysed for dissolved organic carbon (DOC) and nitrogen (DON) and DOM was characterized by its chemical composition (XAD-8 fractionation, sugars, lignin phenols, amino acids, protein, UV and fluorescence spectroscopy), and its biodegradability. Most properties of the tributary waters varied depending on latitude. The higher the latitude, the higher were DOC, DON and the proportion of the hydrophobic fraction of DOC. The contribution of hexoses and pentoses to DOC were higher in southern tributaries; on the other hand, phenolic compounds were more abundant in northern tributaries. Mineralizable DOC ranged between 4% and 28% of total DOC. DOM in northern tributaries was significantly (Po0.05) less biodegradable than that in southern tributaries reflecting the differences in the chemical properties of DOM. Our results suggest that the differences in DOM properties are mainly attributed to differences of permafrost regime, affecting depth of active layer, soil organic matter accumulation and vegetation. Soil organic matter and vegetation determine the amount and composition of DOM produced in the catchments while the depth of the active layer likely controls the quantity and quality of DOM exported to streams. Sorptive interactions of DOM with the soil mineral phase typically increase with depth. The results imply that a northern shift of discontinuous permafrost likely will change in the long term the input of DOM into the Yenisei and thus probably into the Kara Sea.
Because of low net production in arctic and subarctic surface water, dissolved organic matter (DOM) discharged from terrestrial settings plays an important role for carbon and nitrogen dynamics in arctic aquatic systems. Sorption, typically controlling the export of DOM from soil, may be influenced by the permafrost regime. To confirm the potential sorptive control on the release of DOM from permafrost soils in central northern Siberia, we examined the sorption of DOM by mineral soils of Gelisols and Inceptisols with varying depth of the active layer. Water-soluble organic matter in the O horizons of the Gelisols was less (338 and 407 mg C kg À1 ) and comprised more dissolved organic carbon (DOC) in the hydrophobic fraction (HoDOC) (63% and 70%) than in the O horizons of the Inceptisols (686 and 706 mg C kg À1 , 45% and 48% HoDOC). All A and B horizons from Gelisols sorbed DOC strongly, with a preference for HoDOC. Almost all horizons of the Inceptisols showed a weaker sorption of DOC than those of the Gelisols. The C horizons of the Inceptisols, having a weak overall DOC sorption, sorbed C in the hydrophilic fraction (HiDOC) stronger than HoDOC. The reason for the poor overall sorption and also the preferential sorption of HiDOC is likely the high pH (pH47.0) of the C horizons and the smaller concentrations of iron oxides. For all soils, the sorption of HoDOC related positively to oxalate-and dithionite-citrate-extractable iron. The A horizons released large amounts of DOC with 46-80% of HiDOC. The released DOC was significantly (r 5 0.78, Po0.05) correlated with the contents of soil organic carbon. From these results, we assume that large concentrations of DOM comprising large shares of HiDOC can pass mineral soils where the active layer is thin (i.e. in Gelisols), and enter streams. Soils with deep active layer (i.e. Inceptisols), may release little DOM because of more frequent infiltration of DOM into their thick mineral horizons despite their smaller contents of reactive, poorly crystalline minerals. The results obtained for the Inceptisols are in agreement with the situation observed for streams connecting to Yenisei at lower latitudes than 65150 0 with continuous to discontinuous permafrost. The smaller sorption of DOM by the Gelisols is in agreement with the larger DOM concentrations in more northern catchments. However, the Gelisols preferentially retained the HoDOC which dominates the DOC in streams towards north. This discrepancy can be explained by additional seepage water from the organic horizons that is discharged into streams without intensive contact with the mineral soil.
Boreal permafrost soils store large amounts of organic carbon (OC). Parts of this carbon (C) might be black carbon (BC) generated during vegetation fires. Rising temperature and permafrost degradation is expected to have different consequences for OC and BC, because BC is considered to be a refractory subfraction of soil organic matter. To get some insight into stocks, variability, and characteristics of BC in permafrost soils, we estimated the benzene polycarboxylic acid (BPCA) method-specific composition and storage of BC, i.e. BPCA-BC, in a 0.44 km(2)-sized catchment at the forest tundra ecotone in northern Siberia. Furthermore, we assessed the BPCA-BC export with the stream draining the catchment. The catchment is composed of various landscape units with south-southwest (SSW) exposed mineral soils characterized by thick active layer or lacking permafrost, north-northeast (NNE) faced mineral soils with thin active layer, and permafrost-affected raised bogs in plateau positions showing in part thermokarst formation. There were indications of vegetation fires at all landscape units. BC was ubiquitous in the catchment soils and BPCA-BC amounted to 0.6-3.0% of OC. This corresponded to a BC storage of 22-3440 g m(-2). The relative contribution of BPCA-BC to OC, as well as the absolute stocks of BPCA-BC were largest in the intact bogs with a shallow active layer followed by mineral soils of the NNE aspects. In both landscape units, a large proportion of BPCA-BC was stored within the permafrost. In contrast, mineral soils with thick active layer or lacking permafrost and organic soils subjected to thermokarst formation stored less BPCA-BC. Permafrost is, hence, not only a crucial factor in the storage of OC but also of BC. In the stream water BPCA-BC amounted on an average to 3.9% of OC, and a yearly export of 0.10 g BPCA-BC m(-2) was calculated, most of it occurring during the period of snow melt with dominance of surface flow. This suggests that BC mobility in dissolved and colloidal phase is an important pathway of BC export from the catchment. Such a transport mechanism may explain the high BC concentrations found in sediments of the Arctic Ocean. [References: 72
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